Mocking of static field that is a class with Mockito

Mocking of static field that is a class with Mockito - java

My Utils class has an init method dependent upon external resources (e.g., a database connection) that I am unsuccessfully mocking with Mockito. Utils seems like it can be share across instance of my application, so is declared as a static (i.e., a class variable) in
public class EmailNotificationWorkItemHandler extends AbstractLogOrThrowWorkItemHandler {
private static Utils utils = new Utils();
public void executeWorkItem(WorkItem workItem, WorkItemManager manager) {
// Throw an error due to many missing parameters on the workitem
String id= (String) workItem.getParameter("ID");
...
try {
RequiredParameterValidator.validate(this.getClass(), workItem);
...
} catch (Throwable e) {
utils.insertErrors(id, errorCode, errorMessage, e.getStackTrace(), -1L); // testing this method called
}
...
}
I realize I could use dependency injection for Utils but am avoiding Spring. Postings about mocking static fields make me feel like I am close:
#Test
public void executeWorkItemMissingParametersTest() {
Utils mockUtils = mock(Utils.class);
WorkItemManager mockWorkItemMgr = mock(WorkItemManager.class);
EmailNotificationWorkItemHandler mockWorkItemHandler = mock(EmailNotificationWorkItemHandler.class);
doAnswer(new Answer<Void>() {
public Void answer(InvocationOnMock invocation) {
Object[] args = invocation.getArguments();
System.out.println("called with arguments: " + Arrays.toString(args));
return null;
}
}).when(mockUtils).insertErrors(any(), any(), any(), any(), anyLong());
try {
doAnswer(new Answer<Void>() { // Unfinished stubbing detected
public Void answer(InvocationOnMock invocation) {
return null;
}
}).when(mockUtils);
Utils.init();
} catch (Exception e) {
System.out.println("In mocking of Utils.init() " + e.getLocalizedMessage());
}
WorkItemImpl workItem = new WorkItemImpl();
workItem.setParameter("ID", "1111-AAAA");
// Lots of required parameters removed to cause an exception and insertErrors to be called
mockWorkItemHandler.executeWorkItem(workItem, mockWorkItemMgr);
verify(mockUtils).insertErrors(any(), any(), contains("RequiredParameterValidator"), any(), anyLong());
}
But a real instance of Utils is being used and called in mockWorkItemHandler - not the mocked Utils - and an "Unfinished stubbing detected" exception occurs where labeled above. My goal is to test the calling of utils.insertErrors as commented in the code and to do this without Utils' side effects. What am I missing in my mocks to (1) use a mocked Utils (without side effects, such as database connections) and (2) to test that mockWorkItemHandler's utils.insertErrors is called to record the missing parameters?
Please note that I have shown what seem to be all the relevant parts of EmailNotificationWorkItemHandler and Utils.

Here is a simple example of mocking construction calls using mockito inline. In order to enable mockito-inline you can refer to this answer since it is turned off by default by mockito. You can use this in your use case and it should fix your problem.
Consumer Class
public class ConsumerClass {
Logger logger = LoggerFactory.getLogger(ConsumerClass.class);
private static final Utils utils = new Utils();
public String callUtilMethod(){
logger.info("ConsumerClass callUtilMethod");
return utils.helloWorld();
}
}
Utils
public class Utils {
Logger logger = LoggerFactory.getLogger(Utils.class);
public String helloWorld() {
logger.info("Utils class Hello World");
return "This is from Utils";
}
}
Testcase
#Test
void testConsumerClass() {
try (MockedConstruction<Utils> mocked = Mockito.mockConstruction(Utils.class, (mock, context) -> {
Mockito.when(mock.helloWorld()).thenReturn("This is from Mock");
})) {
ConsumerClass consumerClass = new ConsumerClass();
Assertions.assertEquals("This is from Mock", consumerClass.callUtilMethod());
}
}

Related

Is it possible to inject mocks through import statements (Mockito)? [duplicate]

I've written a factory to produce java.sql.Connection objects:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return DriverManager.getConnection(...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
}
I'd like to validate the parameters passed to DriverManager.getConnection, but I don't know how to mock a static method. I'm using JUnit 4 and Mockito for my test cases. Is there a good way to mock/verify this specific use-case?

Use PowerMockito on top of Mockito.
Example code:
#RunWith(PowerMockRunner.class)
#PrepareForTest(DriverManager.class)
public class Mocker {
#Test
public void shouldVerifyParameters() throws Exception {
//given
PowerMockito.mockStatic(DriverManager.class);
BDDMockito.given(DriverManager.getConnection(...)).willReturn(...);
//when
sut.execute(); // System Under Test (sut)
//then
PowerMockito.verifyStatic();
DriverManager.getConnection(...);
}
More information:
Why doesn't Mockito mock static methods?

Mocking of static methods in Mockito is possible since Mockito 3.4.0.
For more details see:
https://github.com/mockito/mockito/releases/tag/v3.4.0
https://github.com/mockito/mockito/issues/1013
https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#static_mocks
assertEquals("foo", Foo.method());
try (MockedStatic mocked = mockStatic(Foo.class)) {
mocked.when(Foo::method).thenReturn("bar");
assertEquals("bar", Foo.method());
mocked.verify(Foo::method);
}
assertEquals("foo", Foo.method());
In your case, something like this:
#Test
public void testStaticMockWithVerification() throws SQLException {
try (MockedStatic<DriverManager> dummy = Mockito.mockStatic(DriverManager.class)) {
DatabaseConnectionFactory factory = new MySQLDatabaseConnectionFactory();
dummy.when(() -> DriverManager.getConnection("arg1", "arg2", "arg3"))
.thenReturn(new Connection() {/*...*/});
factory.getConnection();
dummy.verify(() -> DriverManager.getConnection(eq("arg1"), eq("arg2"), eq("arg3")));
}
}
NOTE: mocking STATIC METHODS requires mockito-inline dependency instead of mockito-core.
For JUnit5 also add this:
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-junit-jupiter</artifactId>
<version>${mockito.version}</version>
<scope>test</scope>
</dependency>

The typical strategy for dodging static methods that you have no way of avoiding using, is by creating wrapped objects and using the wrapper objects instead.
The wrapper objects become facades to the real static classes, and you do not test those.
A wrapper object could be something like
public class Slf4jMdcWrapper {
public static final Slf4jMdcWrapper SINGLETON = new Slf4jMdcWrapper();
public String myApisToTheSaticMethodsInSlf4jMdcStaticUtilityClass() {
return MDC.getWhateverIWant();
}
}
Finally, your class under test can use this singleton object by, for example,
having a default constructor for real life use:
public class SomeClassUnderTest {
final Slf4jMdcWrapper myMockableObject;
/** constructor used by CDI or whatever real life use case */
public myClassUnderTestContructor() {
this.myMockableObject = Slf4jMdcWrapper.SINGLETON;
}
/** constructor used in tests*/
myClassUnderTestContructor(Slf4jMdcWrapper myMock) {
this.myMockableObject = myMock;
}
}
And here you have a class that can easily be tested, because you do not directly use a class with static methods.
If you are using CDI and can make use of the #Inject annotation then it is even easier.
Just make your Wrapper bean #ApplicationScoped, get that thing injected as a collaborator (you do not even need messy constructors for testing), and go on with the mocking.

I had a similar issue. The accepted answer did not work for me, until I made the change: #PrepareForTest(TheClassThatContainsStaticMethod.class), according to PowerMock's documentation for mockStatic.
And I don't have to use BDDMockito.
My class:
public class SmokeRouteBuilder {
public static String smokeMessageId() {
try {
return InetAddress.getLocalHost().getHostAddress();
} catch (UnknownHostException e) {
log.error("Exception occurred while fetching localhost address", e);
return UUID.randomUUID().toString();
}
}
}
My test class:
#RunWith(PowerMockRunner.class)
#PrepareForTest(SmokeRouteBuilder.class)
public class SmokeRouteBuilderTest {
#Test
public void testSmokeMessageId_exception() throws UnknownHostException {
UUID id = UUID.randomUUID();
mockStatic(InetAddress.class);
mockStatic(UUID.class);
when(InetAddress.getLocalHost()).thenThrow(UnknownHostException.class);
when(UUID.randomUUID()).thenReturn(id);
assertEquals(id.toString(), SmokeRouteBuilder.smokeMessageId());
}
}

As mentioned before you can not mock static methods with mockito.
If changing your testing framework is not an option you can do the following:
Create an interface for DriverManager, mock this interface, inject it via some kind of dependency injection and verify on that mock.

For those who use JUnit 5, Powermock is not an option. You'll require the following dependencies to successfully mock a static method with just Mockito.
testCompile group: 'org.mockito', name: 'mockito-core', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-junit-jupiter', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-inline', version: '3.6.0'
mockito-junit-jupiter add supports for JUnit 5.
And support for mocking static methods is provided by mockito-inline dependency.
Example:
#Test
void returnUtilTest() {
assertEquals("foo", UtilClass.staticMethod("foo"));
try (MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)) {
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
}
assertEquals("foo", UtilClass.staticMethod("foo"));
}
The try-with-resource block is used to make the static mock remains temporary, so it's mocked only within that scope.
When not using a try block, make sure to close the mock, once you are done with the assertions.
MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
classMock.close();
Mocking void methods:
When mockStatic is called on a class, all the static void methods in that class automatically get mocked to doNothing().

Observation : When you call static method within a static entity, you need to change the class in #PrepareForTest.
For e.g. :
securityAlgo = MessageDigest.getInstance(SECURITY_ALGORITHM);
For the above code if you need to mock MessageDigest class, use
#PrepareForTest(MessageDigest.class)
While if you have something like below :
public class CustomObjectRule {
object = DatatypeConverter.printHexBinary(MessageDigest.getInstance(SECURITY_ALGORITHM)
.digest(message.getBytes(ENCODING)));
}
then, you'd need to prepare the class this code resides in.
#PrepareForTest(CustomObjectRule.class)
And then mock the method :
PowerMockito.mockStatic(MessageDigest.class);
PowerMockito.when(MessageDigest.getInstance(Mockito.anyString()))
.thenThrow(new RuntimeException());

You can do it with a little bit of refactoring:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return _getConnection(...some params...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
//method to forward parameters, enabling mocking, extension, etc
Connection _getConnection(...some params...) throws SQLException {
return DriverManager.getConnection(...some params...);
}
}
Then you can extend your class MySQLDatabaseConnectionFactory to return a mocked connection, do assertions on the parameters, etc.
The extended class can reside within the test case, if it's located in the same package (which I encourage you to do)
public class MockedConnectionFactory extends MySQLDatabaseConnectionFactory {
Connection _getConnection(...some params...) throws SQLException {
if (some param != something) throw new InvalidParameterException();
//consider mocking some methods with when(yourMock.something()).thenReturn(value)
return Mockito.mock(Connection.class);
}
}

I also wrote a combination of Mockito and AspectJ: https://github.com/iirekm/varia/tree/develop/ajmock
Your example becomes:
when(() -> DriverManager.getConnection(...)).thenReturn(...);

Mockito cannot capture static methods, but since Mockito 2.14.0 you can simulate it by creating invocation instances of static methods.
Example (extracted from their tests):
public class StaticMockingExperimentTest extends TestBase {
Foo mock = Mockito.mock(Foo.class);
MockHandler handler = Mockito.mockingDetails(mock).getMockHandler();
Method staticMethod;
InvocationFactory.RealMethodBehavior realMethod = new InvocationFactory.RealMethodBehavior() {
#Override
public Object call() throws Throwable {
return null;
}
};
#Before
public void before() throws Throwable {
staticMethod = Foo.class.getDeclaredMethod("staticMethod", String.class);
}
#Test
public void verify_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
handler.handle(invocation);
//verify staticMethod on mock
//Mockito cannot capture static methods so we will simulate this scenario in 3 steps:
//1. Call standard 'verify' method. Internally, it will add verificationMode to the thread local state.
// Effectively, we indicate to Mockito that right now we are about to verify a method call on this mock.
verify(mock);
//2. Create the invocation instance using the new public API
// Mockito cannot capture static methods but we can create an invocation instance of that static invocation
Invocation verification = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
//3. Make Mockito handle the static method invocation
// Mockito will find verification mode in thread local state and will try verify the invocation
handler.handle(verification);
//verify zero times, method with different argument
verify(mock, times(0));
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
handler.handle(differentArg);
}
#Test
public void stubbing_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"foo");
handler.handle(invocation);
//register stubbing
when(null).thenReturn("hey");
//validate stubbed return value
assertEquals("hey", handler.handle(invocation));
assertEquals("hey", handler.handle(invocation));
//default null value is returned if invoked with different argument
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
assertEquals(null, handler.handle(differentArg));
}
static class Foo {
private final String arg;
public Foo(String arg) {
this.arg = arg;
}
public static String staticMethod(String arg) {
return "";
}
#Override
public String toString() {
return "foo:" + arg;
}
}
}
Their goal is not to directly support static mocking, but to improve its public APIs so that other libraries, like Powermockito, don't have to rely on internal APIs or directly have to duplicate some Mockito code. (source)
Disclaimer: Mockito team thinks that the road to hell is paved with static methods. However, Mockito's job is not to protect your code from static methods. If you don’t like your team doing static mocking, stop using Powermockito in your organization. Mockito needs to evolve as a toolkit with an opinionated vision on how Java tests should be written (e.g. don't mock statics!!!). However, Mockito is not dogmatic. We don't want to block unrecommended use cases like static mocking. It's just not our job.

To mock static method you should use a Powermock look at:
https://github.com/powermock/powermock/wiki/MockStatic.
Mockito doesn't provide this functionality.
You can read nice a article about mockito:
http://refcardz.dzone.com/refcardz/mockito

I found one solution in Mockito. This feature comes with a version only from 3.4.0
https://asolntsev.github.io/en/2020/07/11/mockito-static-methods/
dependency
In your build.gradle replace mockito-core:3.3.3 by mockito-inline:3.4.0:
testImplementation('org.mockito:mockito-inline:3.4.0')
what are we going to mock
class Buddy
{
static String name()
{
return "John";
}
}
Mock the static method
#Test
void lookMomICanMockStaticMethods()
{
assertThat(Buddy.name()).isEqualTo("John");
try (MockedStatic<Buddy> theMock = Mockito.mockStatic(Buddy.class))
{
theMock.when(Buddy::name).thenReturn("Rafael");
assertThat(Buddy.name()).isEqualTo("Rafael");
}
assertThat(Buddy.name()).isEqualTo("John");
}
I think this could help us.

Since that method is static, it already has everything you need to use it, so it defeats the purpose of mocking.
Mocking the static methods is considered to be a bad practice.
If you try to do that, it means there is something wrong with the way you want to perform testing.
Of course you can use PowerMockito or any other framework capable of doing that, but try to rethink your approach.
For example: try to mock/provide the objects, which that static method consumes instead.

When you try to mock the static method, you have to write the test inside the try block. because It's important to note that scoped mocks must be closed by the entity that activates the mock.
try (MockedStatic<Tester> tester = Mockito.mockStatic(Tester.class)) {
tester.when(() -> Tester.testStatic("Testing..")).thenReturn(mock(ReturnObject.class));
//Here you have to write the test cases
}
In the above example, we have to mock the Tester Class testStatic Method with input param as "Testing...". Here, this method will return a ReturnObject class type object. Hence we write mockito when chain like above.
Don't forget to add below dependency in your Gradle/maven
testImplementation 'org.mockito:mockito-inline:4.3.1'

Use JMockit framework. It worked for me. You don't have to write statements for mocking DBConenction.getConnection() method. Just the below code is enough.
#Mock below is mockit.Mock package
Connection jdbcConnection = Mockito.mock(Connection.class);
MockUp<DBConnection> mockUp = new MockUp<DBConnection>() {
DBConnection singleton = new DBConnection();
#Mock
public DBConnection getInstance() {
return singleton;
}
#Mock
public Connection getConnection() {
return jdbcConnection;
}
};

There is an easy solution by using java FunctionalInterface and then add that interface as dependency for the class you are trying to unit test.

For mocking static functions i was able to do it that way:
create a wrapper function in some helper class/object. (using a name variant might be beneficial for keeping things separated and maintainable.)
use this wrapper in your codes. (Yes, codes need to be realized with testing in mind.)
mock the wrapper function.
wrapper code snippet (not really functional, just for illustration)
class myWrapperClass ...
def myWrapperFunction (...) {
return theOriginalFunction (...)
}
of course having multiple such functions accumulated in a single wrapper class might be beneficial in terms of code reuse.

Here I share my mockito MockStatic solution based on an extension as promised in my answer to leokom's solution.
So, why does Mockito choose try-with-resources? Well, simply because they want to keep a tidy ship. That is good programming after all. Try-with-resources allows construction with guaranteed calling of the close method. But in JUnit we already have that in BeforeEach and AfterEach. And one can easily add these for a generic purpose to each test class using an Extension that implements BeforeEachCallback and AfterEachCallback.
So much for the theory. Let's make a static mock for
Instant.now()
I started with an annotation to be able to mark the fields in my test class that I want to use as static mocks.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public #interface StaticMock {
}
This allows me to create a field in my test class for static mocking that I can easily find in my Extension class.
#StaticMock
private MockedStatic<Instant> staticInstantMock;
I added the Extension I created to my test class. You have two options.
Create an Extension for this purpose and add it to the class next to MockitoExtension, which you also need.
Create an Extension and have it inherit from MockitoExtension. Now you can replace MockitoExtension on your test class.
I used the latter of the two.
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
Now we need something to be returned for the static when it's called:
#Mock
private Instant now;
staticInstantMock.when(Instant::now).thenReturn(now);
The whole test class:
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
#StaticMock
private MockedStatic<Instant> staticInstantMock;
#Mock
private Instant now;
#Test
void myTestMethod() {
staticInstantMock.when(Instant::now).thenReturn(now);
assertThat(Instant::now).isSameAs(now); // This would normally happen in the class you are testing...
}
}
Now let's take a look at the Extension class.
import static org.mockito.Mockito.mockStatic;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.List;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.mockito.MockedStatic;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;
public class CompanyMockitoExtension extends MockitoExtension {
#Override
public void beforeEach(ExtensionContext context) {
super.beforeEach(context); // Don't forget to call the super!!
if (context.getTestInstance().isEmpty()) { // Just to be sure...
return;
}
// Get the unit test instance
Object testSubject = context.getTestInstance().get();
initializeStaticMocks(testSubject);
}
private void initializeStaticMocks(Object testSubject) {
// Find all fields that I want to static mock
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> initializeStaticMock(field, testSubject));
}
private void initializeStaticMock(Field field, Object testSubject) {
// Get the type of the static mock. It is within the generic MockedStatic<> class type.
Class<?> typeForStaticMock = (Class<?>) ReflectionHelper.getTypesForGeneric(field)[0];
try {
// Now set the field with the mockStatic method of Mockito.
field.setAccessible(true);
field.set(testSubject, mockStatic(typeForStaticMock));
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to instantiate Static Mock with type: " + typeForStaticMock.getName());
}
}
#Override
public void afterEach(ExtensionContext context) {
super.afterEach(context); // Again, do not forget to call the super.
if (context.getTestInstance().isEmpty()) {
return;
}
Object testSubject = context.getTestInstance().get();
closeStaticMocks(testSubject); // Close all static mocks.
}
private void closeStaticMocks(Object testSubject) {
// Again find all fields we annotated
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> closeStaticMock(field, testSubject));
}
private void closeStaticMock(Field field, Object testSubject) {
// Get the instance and simply call close.
MockedStatic<?> mockedStaticInstance = ReflectionHelper.getFieldInstance(field, testSubject, MockedStatic.class);
mockedStaticInstance.close();
}
}
The nice thing about this extension is that you can add additional mocking stuff. I added verification of no more interactions on all mocks in the AfterEach. This is now automatic when we use this extension. I also added similar behavior for construction mocking as for the static mocking.
As you see, I made my own reflection helper class. I know there are some standard reflection helper classes and those might be better. Here is mine for this purpose.
public class ReflectionHelper {
public static List<Field> getFieldsWithAnnotation(
Object testSubject,
Class<? extends Annotation> annotationType
) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> field.isAnnotationPresent(annotationType))
.collect(toUnmodifiableList());
}
public static List<Field> getCollectionFields(Object testSubject) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> Collection.class.isAssignableFrom(field.getType()))
.collect(toUnmodifiableList());
}
#SuppressWarnings("unchecked")
public static <T> T getFieldInstance(Field field, Object testSubject, Class<T> type) {
return (T) getFieldInstance(field, testSubject);
}
public static Object getFieldInstance(Field field, Object testSubject) {
try {
boolean isStatic = isStatic(field.getModifiers());
Object context = isStatic ? null : testSubject;
field.setAccessible(true);
return field.get(context);
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to get instance of field.");
}
}
public static Type[] getTypesForGeneric(Field field) {
ParameterizedType parameterizedType = (ParameterizedType) field.getGenericType();
return parameterizedType.getActualTypeArguments();
}
}

Refactor it a little bit:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
ConnectionSupplier connectionSupplier = () -> SupplierDriverManager.getConnection();
public void setConnSupplier(ConnectionSupplier supplier) {
this.connectionSupplier = supplier;
}
#Override
public Connection getConnection() {
try {
return connectionSupplier.conn();
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
#FunctionalInterface
interface ConnectionSupplier {
Connection conn();
}
}
Then you can use the mockito:
MySQLDatabaseConnectionFactory.ConnectionSupplier connectionSupplier = mock(MySQLDatabaseConnectionFactory.ConnectionSupplier.class);
when(connectionSupplier.conn()).thenReturn(yourMockObject);
yourConnectionFactory.setConnSupplier(connectionSupplier);

Mocking void method returning a null pointer exception

I am facing an issue when trying to unit test a function call. The call is failing for a void method invocation messageProducer.sendMessage() even though it has been stubbed.
Please find below a simplified snapshot of my code. I am using a doAnswer() stubbing to mock the void method (based on earlier answers on StackOverflow).
I even tried the other options of doThrow() and doNothing() stubbings, but they also fail with the same NPE when calling the stubbed method :(.
Appreciate if someone could suggest a solution/workaround. Many thanks.
Test Class
// Test class
#RunWith(MockitoJUnitRunner.class)
public class RetriggerRequestTest {
    #Mock
    private MessageProducer messageProducer;
 
    #InjectMocks
    private MigrationRequestServiceImpl migrationRequestService;
 
#Before
    public void init() {
     MockitoAnnotations.initMocks(this);
    }
#Test
public void sendRetriggerRequest() throws Exception {
// Below two stubbings also not Work, NPE encountered!
//doNothing().when(messageProducer).sendMessage(any(), anyLong());
//doThrow(new Exception()).doNothing().when(messageProducer).sendMessage(any(), anyLong());
doAnswer(new Answer<Void>() {
public Void answer(InvocationOnMock invocation) {
Object[] args = invocation.getArguments();
System.out.println("called with arguments: " + Arrays.toString(args));
return null;
}
}).when(messageProducer).sendMessage(any(EMSEvent.class), anyLong());
try {
// Gets Null pointer exception
migrationRequestService.retriggerRequest(emsRetriggerRequest);
}
catch (Exception ex) {
fail(ex.getMessage());
}
}
Implementation Class being tested, the stubbed method call from this class throws NPE as indicated in code comments
#Service
#Transactional
public class MigrationRequestServiceImpl implements MigrationRequestService {
#Autowired
MessageProducer messageProducer;
#Override
public void retriggerRequest(EMSRetriggerRequestData emsRetriggerRequestData) throws EMSException {
// Does a bunch of things
submitTaskScheduledEventsToQueue(taskList);
}
private void submitTaskScheduledEventsToQueue(List<Task> taskList) {
System.out.println("Debugging 1...");
taskList.stream().forEach(task -> {
System.out.println("Debugging 2...");
Map<String, Object> detailsMap = new HashMap<String, Object>();
EMSEvent event = new EMSEvent(EMSEventType.TASK_SCHEDULED);
event.setDetails(detailsMap);
LOGGER.info(ContextRetriever.getServiceContext(), ContextRetriever.getRequestContext(), "*** Re-submitting Task: *** " + task.getId());
// ****Gives a null pointer exception here****
messageProducer.sendMessage(event, eventsConfigProperties.getScheduledEventDelay());
});
System.out.println("Debugging 3...");
}
}
Autowired class that is injected into the test class and whose method is throwing the NPE
#Service
public class MessageProducer {
private static final Logger logger = LoggerFactory.getLogger(MessageProducer.class);
private final RabbitTemplate rabbitTemplate;
#Autowired
public MessageProducer(RabbitTemplate rabbitTemplate) {
this.rabbitTemplate = rabbitTemplate;
}
public void sendMessage(EMSEvent emsEvent, Long delay) {
// code to send message to RabbitMQ here
}
}

Do not use doAnswer if you simply want to capture the arguments and process/verify them in some way. Mockito has a defined feature called ArgumentCaptor that is designed just for that. By using it you will not need to haggle around with that void method the way you do:
#Mock private MessageProducer messageProducer;
#Captor private ArgumentCaptor<Event> eventCaptor;
#Captor private ArgumentCaptor<Long> longCaptor;
#InjectMocks
private MigrationRequestServiceImpl migrationRequestService;
#Test
public void sendRetriggerRequest() throws Exception {
// When
migrationRequestService.retriggerRequest(emsRetriggerRequest);
// Then
verify(messageProducer).sendMessage(eventCaptor.capture(), longCaptor.capture());
Event e = eventCaptor().getValue();
Long l = longCaptor().getValue();
}

Thank you Maciej for the answer. Actually I don't want to do anything with the arguments, I just need to skip this method call. I just used doAnswer with some dummy code since doNothing() or doThrow() did not work with this method.
I was able to resolve the issue however. One of the Autowired components (eventsConfigProperties) for the class which was being injected with Mocks (MigrationRequestServiceImpl) was not being mocked in the test class! Thanks to #daniu for pointing this out.
The stack trace from Mockito was not very helpful in debugging the issue, it just gave a null pointer exception right on the method invocation which caused me to think there may be other issues!
Apologize for the mistake, my bad, but thank you and good to know about the ArgumentCaptor, would probably need it for future testing!
Had to add this entry which was autowired into the MigrationRequestService class.
// Test class
#RunWith(MockitoJUnitRunner.class)
public class RetriggerRequestTest {
#Autowired
EventsConfigProperties eventsConfigProperties;
// Other declarations
}

How to Mock Integer.parseInt("Some String") using Mockito [duplicate]

I've written a factory to produce java.sql.Connection objects:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return DriverManager.getConnection(...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
}
I'd like to validate the parameters passed to DriverManager.getConnection, but I don't know how to mock a static method. I'm using JUnit 4 and Mockito for my test cases. Is there a good way to mock/verify this specific use-case?

Use PowerMockito on top of Mockito.
Example code:
#RunWith(PowerMockRunner.class)
#PrepareForTest(DriverManager.class)
public class Mocker {
#Test
public void shouldVerifyParameters() throws Exception {
//given
PowerMockito.mockStatic(DriverManager.class);
BDDMockito.given(DriverManager.getConnection(...)).willReturn(...);
//when
sut.execute(); // System Under Test (sut)
//then
PowerMockito.verifyStatic();
DriverManager.getConnection(...);
}
More information:
Why doesn't Mockito mock static methods?

Mocking of static methods in Mockito is possible since Mockito 3.4.0.
For more details see:
https://github.com/mockito/mockito/releases/tag/v3.4.0
https://github.com/mockito/mockito/issues/1013
https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#static_mocks
assertEquals("foo", Foo.method());
try (MockedStatic mocked = mockStatic(Foo.class)) {
mocked.when(Foo::method).thenReturn("bar");
assertEquals("bar", Foo.method());
mocked.verify(Foo::method);
}
assertEquals("foo", Foo.method());
In your case, something like this:
#Test
public void testStaticMockWithVerification() throws SQLException {
try (MockedStatic<DriverManager> dummy = Mockito.mockStatic(DriverManager.class)) {
DatabaseConnectionFactory factory = new MySQLDatabaseConnectionFactory();
dummy.when(() -> DriverManager.getConnection("arg1", "arg2", "arg3"))
.thenReturn(new Connection() {/*...*/});
factory.getConnection();
dummy.verify(() -> DriverManager.getConnection(eq("arg1"), eq("arg2"), eq("arg3")));
}
}
NOTE: mocking STATIC METHODS requires mockito-inline dependency instead of mockito-core.
For JUnit5 also add this:
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-junit-jupiter</artifactId>
<version>${mockito.version}</version>
<scope>test</scope>
</dependency>

The typical strategy for dodging static methods that you have no way of avoiding using, is by creating wrapped objects and using the wrapper objects instead.
The wrapper objects become facades to the real static classes, and you do not test those.
A wrapper object could be something like
public class Slf4jMdcWrapper {
public static final Slf4jMdcWrapper SINGLETON = new Slf4jMdcWrapper();
public String myApisToTheSaticMethodsInSlf4jMdcStaticUtilityClass() {
return MDC.getWhateverIWant();
}
}
Finally, your class under test can use this singleton object by, for example,
having a default constructor for real life use:
public class SomeClassUnderTest {
final Slf4jMdcWrapper myMockableObject;
/** constructor used by CDI or whatever real life use case */
public myClassUnderTestContructor() {
this.myMockableObject = Slf4jMdcWrapper.SINGLETON;
}
/** constructor used in tests*/
myClassUnderTestContructor(Slf4jMdcWrapper myMock) {
this.myMockableObject = myMock;
}
}
And here you have a class that can easily be tested, because you do not directly use a class with static methods.
If you are using CDI and can make use of the #Inject annotation then it is even easier.
Just make your Wrapper bean #ApplicationScoped, get that thing injected as a collaborator (you do not even need messy constructors for testing), and go on with the mocking.

I had a similar issue. The accepted answer did not work for me, until I made the change: #PrepareForTest(TheClassThatContainsStaticMethod.class), according to PowerMock's documentation for mockStatic.
And I don't have to use BDDMockito.
My class:
public class SmokeRouteBuilder {
public static String smokeMessageId() {
try {
return InetAddress.getLocalHost().getHostAddress();
} catch (UnknownHostException e) {
log.error("Exception occurred while fetching localhost address", e);
return UUID.randomUUID().toString();
}
}
}
My test class:
#RunWith(PowerMockRunner.class)
#PrepareForTest(SmokeRouteBuilder.class)
public class SmokeRouteBuilderTest {
#Test
public void testSmokeMessageId_exception() throws UnknownHostException {
UUID id = UUID.randomUUID();
mockStatic(InetAddress.class);
mockStatic(UUID.class);
when(InetAddress.getLocalHost()).thenThrow(UnknownHostException.class);
when(UUID.randomUUID()).thenReturn(id);
assertEquals(id.toString(), SmokeRouteBuilder.smokeMessageId());
}
}

As mentioned before you can not mock static methods with mockito.
If changing your testing framework is not an option you can do the following:
Create an interface for DriverManager, mock this interface, inject it via some kind of dependency injection and verify on that mock.

For those who use JUnit 5, Powermock is not an option. You'll require the following dependencies to successfully mock a static method with just Mockito.
testCompile group: 'org.mockito', name: 'mockito-core', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-junit-jupiter', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-inline', version: '3.6.0'
mockito-junit-jupiter add supports for JUnit 5.
And support for mocking static methods is provided by mockito-inline dependency.
Example:
#Test
void returnUtilTest() {
assertEquals("foo", UtilClass.staticMethod("foo"));
try (MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)) {
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
}
assertEquals("foo", UtilClass.staticMethod("foo"));
}
The try-with-resource block is used to make the static mock remains temporary, so it's mocked only within that scope.
When not using a try block, make sure to close the mock, once you are done with the assertions.
MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
classMock.close();
Mocking void methods:
When mockStatic is called on a class, all the static void methods in that class automatically get mocked to doNothing().

Observation : When you call static method within a static entity, you need to change the class in #PrepareForTest.
For e.g. :
securityAlgo = MessageDigest.getInstance(SECURITY_ALGORITHM);
For the above code if you need to mock MessageDigest class, use
#PrepareForTest(MessageDigest.class)
While if you have something like below :
public class CustomObjectRule {
object = DatatypeConverter.printHexBinary(MessageDigest.getInstance(SECURITY_ALGORITHM)
.digest(message.getBytes(ENCODING)));
}
then, you'd need to prepare the class this code resides in.
#PrepareForTest(CustomObjectRule.class)
And then mock the method :
PowerMockito.mockStatic(MessageDigest.class);
PowerMockito.when(MessageDigest.getInstance(Mockito.anyString()))
.thenThrow(new RuntimeException());

You can do it with a little bit of refactoring:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return _getConnection(...some params...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
//method to forward parameters, enabling mocking, extension, etc
Connection _getConnection(...some params...) throws SQLException {
return DriverManager.getConnection(...some params...);
}
}
Then you can extend your class MySQLDatabaseConnectionFactory to return a mocked connection, do assertions on the parameters, etc.
The extended class can reside within the test case, if it's located in the same package (which I encourage you to do)
public class MockedConnectionFactory extends MySQLDatabaseConnectionFactory {
Connection _getConnection(...some params...) throws SQLException {
if (some param != something) throw new InvalidParameterException();
//consider mocking some methods with when(yourMock.something()).thenReturn(value)
return Mockito.mock(Connection.class);
}
}

I also wrote a combination of Mockito and AspectJ: https://github.com/iirekm/varia/tree/develop/ajmock
Your example becomes:
when(() -> DriverManager.getConnection(...)).thenReturn(...);

Mockito cannot capture static methods, but since Mockito 2.14.0 you can simulate it by creating invocation instances of static methods.
Example (extracted from their tests):
public class StaticMockingExperimentTest extends TestBase {
Foo mock = Mockito.mock(Foo.class);
MockHandler handler = Mockito.mockingDetails(mock).getMockHandler();
Method staticMethod;
InvocationFactory.RealMethodBehavior realMethod = new InvocationFactory.RealMethodBehavior() {
#Override
public Object call() throws Throwable {
return null;
}
};
#Before
public void before() throws Throwable {
staticMethod = Foo.class.getDeclaredMethod("staticMethod", String.class);
}
#Test
public void verify_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
handler.handle(invocation);
//verify staticMethod on mock
//Mockito cannot capture static methods so we will simulate this scenario in 3 steps:
//1. Call standard 'verify' method. Internally, it will add verificationMode to the thread local state.
// Effectively, we indicate to Mockito that right now we are about to verify a method call on this mock.
verify(mock);
//2. Create the invocation instance using the new public API
// Mockito cannot capture static methods but we can create an invocation instance of that static invocation
Invocation verification = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
//3. Make Mockito handle the static method invocation
// Mockito will find verification mode in thread local state and will try verify the invocation
handler.handle(verification);
//verify zero times, method with different argument
verify(mock, times(0));
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
handler.handle(differentArg);
}
#Test
public void stubbing_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"foo");
handler.handle(invocation);
//register stubbing
when(null).thenReturn("hey");
//validate stubbed return value
assertEquals("hey", handler.handle(invocation));
assertEquals("hey", handler.handle(invocation));
//default null value is returned if invoked with different argument
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
assertEquals(null, handler.handle(differentArg));
}
static class Foo {
private final String arg;
public Foo(String arg) {
this.arg = arg;
}
public static String staticMethod(String arg) {
return "";
}
#Override
public String toString() {
return "foo:" + arg;
}
}
}
Their goal is not to directly support static mocking, but to improve its public APIs so that other libraries, like Powermockito, don't have to rely on internal APIs or directly have to duplicate some Mockito code. (source)
Disclaimer: Mockito team thinks that the road to hell is paved with static methods. However, Mockito's job is not to protect your code from static methods. If you don’t like your team doing static mocking, stop using Powermockito in your organization. Mockito needs to evolve as a toolkit with an opinionated vision on how Java tests should be written (e.g. don't mock statics!!!). However, Mockito is not dogmatic. We don't want to block unrecommended use cases like static mocking. It's just not our job.

To mock static method you should use a Powermock look at:
https://github.com/powermock/powermock/wiki/MockStatic.
Mockito doesn't provide this functionality.
You can read nice a article about mockito:
http://refcardz.dzone.com/refcardz/mockito

I found one solution in Mockito. This feature comes with a version only from 3.4.0
https://asolntsev.github.io/en/2020/07/11/mockito-static-methods/
dependency
In your build.gradle replace mockito-core:3.3.3 by mockito-inline:3.4.0:
testImplementation('org.mockito:mockito-inline:3.4.0')
what are we going to mock
class Buddy
{
static String name()
{
return "John";
}
}
Mock the static method
#Test
void lookMomICanMockStaticMethods()
{
assertThat(Buddy.name()).isEqualTo("John");
try (MockedStatic<Buddy> theMock = Mockito.mockStatic(Buddy.class))
{
theMock.when(Buddy::name).thenReturn("Rafael");
assertThat(Buddy.name()).isEqualTo("Rafael");
}
assertThat(Buddy.name()).isEqualTo("John");
}
I think this could help us.

Since that method is static, it already has everything you need to use it, so it defeats the purpose of mocking.
Mocking the static methods is considered to be a bad practice.
If you try to do that, it means there is something wrong with the way you want to perform testing.
Of course you can use PowerMockito or any other framework capable of doing that, but try to rethink your approach.
For example: try to mock/provide the objects, which that static method consumes instead.

When you try to mock the static method, you have to write the test inside the try block. because It's important to note that scoped mocks must be closed by the entity that activates the mock.
try (MockedStatic<Tester> tester = Mockito.mockStatic(Tester.class)) {
tester.when(() -> Tester.testStatic("Testing..")).thenReturn(mock(ReturnObject.class));
//Here you have to write the test cases
}
In the above example, we have to mock the Tester Class testStatic Method with input param as "Testing...". Here, this method will return a ReturnObject class type object. Hence we write mockito when chain like above.
Don't forget to add below dependency in your Gradle/maven
testImplementation 'org.mockito:mockito-inline:4.3.1'

Use JMockit framework. It worked for me. You don't have to write statements for mocking DBConenction.getConnection() method. Just the below code is enough.
#Mock below is mockit.Mock package
Connection jdbcConnection = Mockito.mock(Connection.class);
MockUp<DBConnection> mockUp = new MockUp<DBConnection>() {
DBConnection singleton = new DBConnection();
#Mock
public DBConnection getInstance() {
return singleton;
}
#Mock
public Connection getConnection() {
return jdbcConnection;
}
};

There is an easy solution by using java FunctionalInterface and then add that interface as dependency for the class you are trying to unit test.

For mocking static functions i was able to do it that way:
create a wrapper function in some helper class/object. (using a name variant might be beneficial for keeping things separated and maintainable.)
use this wrapper in your codes. (Yes, codes need to be realized with testing in mind.)
mock the wrapper function.
wrapper code snippet (not really functional, just for illustration)
class myWrapperClass ...
def myWrapperFunction (...) {
return theOriginalFunction (...)
}
of course having multiple such functions accumulated in a single wrapper class might be beneficial in terms of code reuse.

Here I share my mockito MockStatic solution based on an extension as promised in my answer to leokom's solution.
So, why does Mockito choose try-with-resources? Well, simply because they want to keep a tidy ship. That is good programming after all. Try-with-resources allows construction with guaranteed calling of the close method. But in JUnit we already have that in BeforeEach and AfterEach. And one can easily add these for a generic purpose to each test class using an Extension that implements BeforeEachCallback and AfterEachCallback.
So much for the theory. Let's make a static mock for
Instant.now()
I started with an annotation to be able to mark the fields in my test class that I want to use as static mocks.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public #interface StaticMock {
}
This allows me to create a field in my test class for static mocking that I can easily find in my Extension class.
#StaticMock
private MockedStatic<Instant> staticInstantMock;
I added the Extension I created to my test class. You have two options.
Create an Extension for this purpose and add it to the class next to MockitoExtension, which you also need.
Create an Extension and have it inherit from MockitoExtension. Now you can replace MockitoExtension on your test class.
I used the latter of the two.
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
Now we need something to be returned for the static when it's called:
#Mock
private Instant now;
staticInstantMock.when(Instant::now).thenReturn(now);
The whole test class:
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
#StaticMock
private MockedStatic<Instant> staticInstantMock;
#Mock
private Instant now;
#Test
void myTestMethod() {
staticInstantMock.when(Instant::now).thenReturn(now);
assertThat(Instant::now).isSameAs(now); // This would normally happen in the class you are testing...
}
}
Now let's take a look at the Extension class.
import static org.mockito.Mockito.mockStatic;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.List;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.mockito.MockedStatic;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;
public class CompanyMockitoExtension extends MockitoExtension {
#Override
public void beforeEach(ExtensionContext context) {
super.beforeEach(context); // Don't forget to call the super!!
if (context.getTestInstance().isEmpty()) { // Just to be sure...
return;
}
// Get the unit test instance
Object testSubject = context.getTestInstance().get();
initializeStaticMocks(testSubject);
}
private void initializeStaticMocks(Object testSubject) {
// Find all fields that I want to static mock
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> initializeStaticMock(field, testSubject));
}
private void initializeStaticMock(Field field, Object testSubject) {
// Get the type of the static mock. It is within the generic MockedStatic<> class type.
Class<?> typeForStaticMock = (Class<?>) ReflectionHelper.getTypesForGeneric(field)[0];
try {
// Now set the field with the mockStatic method of Mockito.
field.setAccessible(true);
field.set(testSubject, mockStatic(typeForStaticMock));
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to instantiate Static Mock with type: " + typeForStaticMock.getName());
}
}
#Override
public void afterEach(ExtensionContext context) {
super.afterEach(context); // Again, do not forget to call the super.
if (context.getTestInstance().isEmpty()) {
return;
}
Object testSubject = context.getTestInstance().get();
closeStaticMocks(testSubject); // Close all static mocks.
}
private void closeStaticMocks(Object testSubject) {
// Again find all fields we annotated
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> closeStaticMock(field, testSubject));
}
private void closeStaticMock(Field field, Object testSubject) {
// Get the instance and simply call close.
MockedStatic<?> mockedStaticInstance = ReflectionHelper.getFieldInstance(field, testSubject, MockedStatic.class);
mockedStaticInstance.close();
}
}
The nice thing about this extension is that you can add additional mocking stuff. I added verification of no more interactions on all mocks in the AfterEach. This is now automatic when we use this extension. I also added similar behavior for construction mocking as for the static mocking.
As you see, I made my own reflection helper class. I know there are some standard reflection helper classes and those might be better. Here is mine for this purpose.
public class ReflectionHelper {
public static List<Field> getFieldsWithAnnotation(
Object testSubject,
Class<? extends Annotation> annotationType
) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> field.isAnnotationPresent(annotationType))
.collect(toUnmodifiableList());
}
public static List<Field> getCollectionFields(Object testSubject) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> Collection.class.isAssignableFrom(field.getType()))
.collect(toUnmodifiableList());
}
#SuppressWarnings("unchecked")
public static <T> T getFieldInstance(Field field, Object testSubject, Class<T> type) {
return (T) getFieldInstance(field, testSubject);
}
public static Object getFieldInstance(Field field, Object testSubject) {
try {
boolean isStatic = isStatic(field.getModifiers());
Object context = isStatic ? null : testSubject;
field.setAccessible(true);
return field.get(context);
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to get instance of field.");
}
}
public static Type[] getTypesForGeneric(Field field) {
ParameterizedType parameterizedType = (ParameterizedType) field.getGenericType();
return parameterizedType.getActualTypeArguments();
}
}

Refactor it a little bit:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
ConnectionSupplier connectionSupplier = () -> SupplierDriverManager.getConnection();
public void setConnSupplier(ConnectionSupplier supplier) {
this.connectionSupplier = supplier;
}
#Override
public Connection getConnection() {
try {
return connectionSupplier.conn();
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
#FunctionalInterface
interface ConnectionSupplier {
Connection conn();
}
}
Then you can use the mockito:
MySQLDatabaseConnectionFactory.ConnectionSupplier connectionSupplier = mock(MySQLDatabaseConnectionFactory.ConnectionSupplier.class);
when(connectionSupplier.conn()).thenReturn(yourMockObject);
yourConnectionFactory.setConnSupplier(connectionSupplier);

Java Mockito Test Static property assign with function [duplicate]

I've written a factory to produce java.sql.Connection objects:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return DriverManager.getConnection(...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
}
I'd like to validate the parameters passed to DriverManager.getConnection, but I don't know how to mock a static method. I'm using JUnit 4 and Mockito for my test cases. Is there a good way to mock/verify this specific use-case?

Use PowerMockito on top of Mockito.
Example code:
#RunWith(PowerMockRunner.class)
#PrepareForTest(DriverManager.class)
public class Mocker {
#Test
public void shouldVerifyParameters() throws Exception {
//given
PowerMockito.mockStatic(DriverManager.class);
BDDMockito.given(DriverManager.getConnection(...)).willReturn(...);
//when
sut.execute(); // System Under Test (sut)
//then
PowerMockito.verifyStatic();
DriverManager.getConnection(...);
}
More information:
Why doesn't Mockito mock static methods?

Mocking of static methods in Mockito is possible since Mockito 3.4.0.
For more details see:
https://github.com/mockito/mockito/releases/tag/v3.4.0
https://github.com/mockito/mockito/issues/1013
https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#static_mocks
assertEquals("foo", Foo.method());
try (MockedStatic mocked = mockStatic(Foo.class)) {
mocked.when(Foo::method).thenReturn("bar");
assertEquals("bar", Foo.method());
mocked.verify(Foo::method);
}
assertEquals("foo", Foo.method());
In your case, something like this:
#Test
public void testStaticMockWithVerification() throws SQLException {
try (MockedStatic<DriverManager> dummy = Mockito.mockStatic(DriverManager.class)) {
DatabaseConnectionFactory factory = new MySQLDatabaseConnectionFactory();
dummy.when(() -> DriverManager.getConnection("arg1", "arg2", "arg3"))
.thenReturn(new Connection() {/*...*/});
factory.getConnection();
dummy.verify(() -> DriverManager.getConnection(eq("arg1"), eq("arg2"), eq("arg3")));
}
}
NOTE: mocking STATIC METHODS requires mockito-inline dependency instead of mockito-core.
For JUnit5 also add this:
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-junit-jupiter</artifactId>
<version>${mockito.version}</version>
<scope>test</scope>
</dependency>

The typical strategy for dodging static methods that you have no way of avoiding using, is by creating wrapped objects and using the wrapper objects instead.
The wrapper objects become facades to the real static classes, and you do not test those.
A wrapper object could be something like
public class Slf4jMdcWrapper {
public static final Slf4jMdcWrapper SINGLETON = new Slf4jMdcWrapper();
public String myApisToTheSaticMethodsInSlf4jMdcStaticUtilityClass() {
return MDC.getWhateverIWant();
}
}
Finally, your class under test can use this singleton object by, for example,
having a default constructor for real life use:
public class SomeClassUnderTest {
final Slf4jMdcWrapper myMockableObject;
/** constructor used by CDI or whatever real life use case */
public myClassUnderTestContructor() {
this.myMockableObject = Slf4jMdcWrapper.SINGLETON;
}
/** constructor used in tests*/
myClassUnderTestContructor(Slf4jMdcWrapper myMock) {
this.myMockableObject = myMock;
}
}
And here you have a class that can easily be tested, because you do not directly use a class with static methods.
If you are using CDI and can make use of the #Inject annotation then it is even easier.
Just make your Wrapper bean #ApplicationScoped, get that thing injected as a collaborator (you do not even need messy constructors for testing), and go on with the mocking.

I had a similar issue. The accepted answer did not work for me, until I made the change: #PrepareForTest(TheClassThatContainsStaticMethod.class), according to PowerMock's documentation for mockStatic.
And I don't have to use BDDMockito.
My class:
public class SmokeRouteBuilder {
public static String smokeMessageId() {
try {
return InetAddress.getLocalHost().getHostAddress();
} catch (UnknownHostException e) {
log.error("Exception occurred while fetching localhost address", e);
return UUID.randomUUID().toString();
}
}
}
My test class:
#RunWith(PowerMockRunner.class)
#PrepareForTest(SmokeRouteBuilder.class)
public class SmokeRouteBuilderTest {
#Test
public void testSmokeMessageId_exception() throws UnknownHostException {
UUID id = UUID.randomUUID();
mockStatic(InetAddress.class);
mockStatic(UUID.class);
when(InetAddress.getLocalHost()).thenThrow(UnknownHostException.class);
when(UUID.randomUUID()).thenReturn(id);
assertEquals(id.toString(), SmokeRouteBuilder.smokeMessageId());
}
}

As mentioned before you can not mock static methods with mockito.
If changing your testing framework is not an option you can do the following:
Create an interface for DriverManager, mock this interface, inject it via some kind of dependency injection and verify on that mock.

For those who use JUnit 5, Powermock is not an option. You'll require the following dependencies to successfully mock a static method with just Mockito.
testCompile group: 'org.mockito', name: 'mockito-core', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-junit-jupiter', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-inline', version: '3.6.0'
mockito-junit-jupiter add supports for JUnit 5.
And support for mocking static methods is provided by mockito-inline dependency.
Example:
#Test
void returnUtilTest() {
assertEquals("foo", UtilClass.staticMethod("foo"));
try (MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)) {
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
}
assertEquals("foo", UtilClass.staticMethod("foo"));
}
The try-with-resource block is used to make the static mock remains temporary, so it's mocked only within that scope.
When not using a try block, make sure to close the mock, once you are done with the assertions.
MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
classMock.close();
Mocking void methods:
When mockStatic is called on a class, all the static void methods in that class automatically get mocked to doNothing().

Observation : When you call static method within a static entity, you need to change the class in #PrepareForTest.
For e.g. :
securityAlgo = MessageDigest.getInstance(SECURITY_ALGORITHM);
For the above code if you need to mock MessageDigest class, use
#PrepareForTest(MessageDigest.class)
While if you have something like below :
public class CustomObjectRule {
object = DatatypeConverter.printHexBinary(MessageDigest.getInstance(SECURITY_ALGORITHM)
.digest(message.getBytes(ENCODING)));
}
then, you'd need to prepare the class this code resides in.
#PrepareForTest(CustomObjectRule.class)
And then mock the method :
PowerMockito.mockStatic(MessageDigest.class);
PowerMockito.when(MessageDigest.getInstance(Mockito.anyString()))
.thenThrow(new RuntimeException());

You can do it with a little bit of refactoring:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return _getConnection(...some params...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
//method to forward parameters, enabling mocking, extension, etc
Connection _getConnection(...some params...) throws SQLException {
return DriverManager.getConnection(...some params...);
}
}
Then you can extend your class MySQLDatabaseConnectionFactory to return a mocked connection, do assertions on the parameters, etc.
The extended class can reside within the test case, if it's located in the same package (which I encourage you to do)
public class MockedConnectionFactory extends MySQLDatabaseConnectionFactory {
Connection _getConnection(...some params...) throws SQLException {
if (some param != something) throw new InvalidParameterException();
//consider mocking some methods with when(yourMock.something()).thenReturn(value)
return Mockito.mock(Connection.class);
}
}

I also wrote a combination of Mockito and AspectJ: https://github.com/iirekm/varia/tree/develop/ajmock
Your example becomes:
when(() -> DriverManager.getConnection(...)).thenReturn(...);

Mockito cannot capture static methods, but since Mockito 2.14.0 you can simulate it by creating invocation instances of static methods.
Example (extracted from their tests):
public class StaticMockingExperimentTest extends TestBase {
Foo mock = Mockito.mock(Foo.class);
MockHandler handler = Mockito.mockingDetails(mock).getMockHandler();
Method staticMethod;
InvocationFactory.RealMethodBehavior realMethod = new InvocationFactory.RealMethodBehavior() {
#Override
public Object call() throws Throwable {
return null;
}
};
#Before
public void before() throws Throwable {
staticMethod = Foo.class.getDeclaredMethod("staticMethod", String.class);
}
#Test
public void verify_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
handler.handle(invocation);
//verify staticMethod on mock
//Mockito cannot capture static methods so we will simulate this scenario in 3 steps:
//1. Call standard 'verify' method. Internally, it will add verificationMode to the thread local state.
// Effectively, we indicate to Mockito that right now we are about to verify a method call on this mock.
verify(mock);
//2. Create the invocation instance using the new public API
// Mockito cannot capture static methods but we can create an invocation instance of that static invocation
Invocation verification = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
//3. Make Mockito handle the static method invocation
// Mockito will find verification mode in thread local state and will try verify the invocation
handler.handle(verification);
//verify zero times, method with different argument
verify(mock, times(0));
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
handler.handle(differentArg);
}
#Test
public void stubbing_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"foo");
handler.handle(invocation);
//register stubbing
when(null).thenReturn("hey");
//validate stubbed return value
assertEquals("hey", handler.handle(invocation));
assertEquals("hey", handler.handle(invocation));
//default null value is returned if invoked with different argument
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
assertEquals(null, handler.handle(differentArg));
}
static class Foo {
private final String arg;
public Foo(String arg) {
this.arg = arg;
}
public static String staticMethod(String arg) {
return "";
}
#Override
public String toString() {
return "foo:" + arg;
}
}
}
Their goal is not to directly support static mocking, but to improve its public APIs so that other libraries, like Powermockito, don't have to rely on internal APIs or directly have to duplicate some Mockito code. (source)
Disclaimer: Mockito team thinks that the road to hell is paved with static methods. However, Mockito's job is not to protect your code from static methods. If you don’t like your team doing static mocking, stop using Powermockito in your organization. Mockito needs to evolve as a toolkit with an opinionated vision on how Java tests should be written (e.g. don't mock statics!!!). However, Mockito is not dogmatic. We don't want to block unrecommended use cases like static mocking. It's just not our job.

To mock static method you should use a Powermock look at:
https://github.com/powermock/powermock/wiki/MockStatic.
Mockito doesn't provide this functionality.
You can read nice a article about mockito:
http://refcardz.dzone.com/refcardz/mockito

I found one solution in Mockito. This feature comes with a version only from 3.4.0
https://asolntsev.github.io/en/2020/07/11/mockito-static-methods/
dependency
In your build.gradle replace mockito-core:3.3.3 by mockito-inline:3.4.0:
testImplementation('org.mockito:mockito-inline:3.4.0')
what are we going to mock
class Buddy
{
static String name()
{
return "John";
}
}
Mock the static method
#Test
void lookMomICanMockStaticMethods()
{
assertThat(Buddy.name()).isEqualTo("John");
try (MockedStatic<Buddy> theMock = Mockito.mockStatic(Buddy.class))
{
theMock.when(Buddy::name).thenReturn("Rafael");
assertThat(Buddy.name()).isEqualTo("Rafael");
}
assertThat(Buddy.name()).isEqualTo("John");
}
I think this could help us.

Since that method is static, it already has everything you need to use it, so it defeats the purpose of mocking.
Mocking the static methods is considered to be a bad practice.
If you try to do that, it means there is something wrong with the way you want to perform testing.
Of course you can use PowerMockito or any other framework capable of doing that, but try to rethink your approach.
For example: try to mock/provide the objects, which that static method consumes instead.

When you try to mock the static method, you have to write the test inside the try block. because It's important to note that scoped mocks must be closed by the entity that activates the mock.
try (MockedStatic<Tester> tester = Mockito.mockStatic(Tester.class)) {
tester.when(() -> Tester.testStatic("Testing..")).thenReturn(mock(ReturnObject.class));
//Here you have to write the test cases
}
In the above example, we have to mock the Tester Class testStatic Method with input param as "Testing...". Here, this method will return a ReturnObject class type object. Hence we write mockito when chain like above.
Don't forget to add below dependency in your Gradle/maven
testImplementation 'org.mockito:mockito-inline:4.3.1'

Use JMockit framework. It worked for me. You don't have to write statements for mocking DBConenction.getConnection() method. Just the below code is enough.
#Mock below is mockit.Mock package
Connection jdbcConnection = Mockito.mock(Connection.class);
MockUp<DBConnection> mockUp = new MockUp<DBConnection>() {
DBConnection singleton = new DBConnection();
#Mock
public DBConnection getInstance() {
return singleton;
}
#Mock
public Connection getConnection() {
return jdbcConnection;
}
};

There is an easy solution by using java FunctionalInterface and then add that interface as dependency for the class you are trying to unit test.

For mocking static functions i was able to do it that way:
create a wrapper function in some helper class/object. (using a name variant might be beneficial for keeping things separated and maintainable.)
use this wrapper in your codes. (Yes, codes need to be realized with testing in mind.)
mock the wrapper function.
wrapper code snippet (not really functional, just for illustration)
class myWrapperClass ...
def myWrapperFunction (...) {
return theOriginalFunction (...)
}
of course having multiple such functions accumulated in a single wrapper class might be beneficial in terms of code reuse.

Here I share my mockito MockStatic solution based on an extension as promised in my answer to leokom's solution.
So, why does Mockito choose try-with-resources? Well, simply because they want to keep a tidy ship. That is good programming after all. Try-with-resources allows construction with guaranteed calling of the close method. But in JUnit we already have that in BeforeEach and AfterEach. And one can easily add these for a generic purpose to each test class using an Extension that implements BeforeEachCallback and AfterEachCallback.
So much for the theory. Let's make a static mock for
Instant.now()
I started with an annotation to be able to mark the fields in my test class that I want to use as static mocks.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public #interface StaticMock {
}
This allows me to create a field in my test class for static mocking that I can easily find in my Extension class.
#StaticMock
private MockedStatic<Instant> staticInstantMock;
I added the Extension I created to my test class. You have two options.
Create an Extension for this purpose and add it to the class next to MockitoExtension, which you also need.
Create an Extension and have it inherit from MockitoExtension. Now you can replace MockitoExtension on your test class.
I used the latter of the two.
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
Now we need something to be returned for the static when it's called:
#Mock
private Instant now;
staticInstantMock.when(Instant::now).thenReturn(now);
The whole test class:
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
#StaticMock
private MockedStatic<Instant> staticInstantMock;
#Mock
private Instant now;
#Test
void myTestMethod() {
staticInstantMock.when(Instant::now).thenReturn(now);
assertThat(Instant::now).isSameAs(now); // This would normally happen in the class you are testing...
}
}
Now let's take a look at the Extension class.
import static org.mockito.Mockito.mockStatic;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.List;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.mockito.MockedStatic;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;
public class CompanyMockitoExtension extends MockitoExtension {
#Override
public void beforeEach(ExtensionContext context) {
super.beforeEach(context); // Don't forget to call the super!!
if (context.getTestInstance().isEmpty()) { // Just to be sure...
return;
}
// Get the unit test instance
Object testSubject = context.getTestInstance().get();
initializeStaticMocks(testSubject);
}
private void initializeStaticMocks(Object testSubject) {
// Find all fields that I want to static mock
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> initializeStaticMock(field, testSubject));
}
private void initializeStaticMock(Field field, Object testSubject) {
// Get the type of the static mock. It is within the generic MockedStatic<> class type.
Class<?> typeForStaticMock = (Class<?>) ReflectionHelper.getTypesForGeneric(field)[0];
try {
// Now set the field with the mockStatic method of Mockito.
field.setAccessible(true);
field.set(testSubject, mockStatic(typeForStaticMock));
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to instantiate Static Mock with type: " + typeForStaticMock.getName());
}
}
#Override
public void afterEach(ExtensionContext context) {
super.afterEach(context); // Again, do not forget to call the super.
if (context.getTestInstance().isEmpty()) {
return;
}
Object testSubject = context.getTestInstance().get();
closeStaticMocks(testSubject); // Close all static mocks.
}
private void closeStaticMocks(Object testSubject) {
// Again find all fields we annotated
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> closeStaticMock(field, testSubject));
}
private void closeStaticMock(Field field, Object testSubject) {
// Get the instance and simply call close.
MockedStatic<?> mockedStaticInstance = ReflectionHelper.getFieldInstance(field, testSubject, MockedStatic.class);
mockedStaticInstance.close();
}
}
The nice thing about this extension is that you can add additional mocking stuff. I added verification of no more interactions on all mocks in the AfterEach. This is now automatic when we use this extension. I also added similar behavior for construction mocking as for the static mocking.
As you see, I made my own reflection helper class. I know there are some standard reflection helper classes and those might be better. Here is mine for this purpose.
public class ReflectionHelper {
public static List<Field> getFieldsWithAnnotation(
Object testSubject,
Class<? extends Annotation> annotationType
) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> field.isAnnotationPresent(annotationType))
.collect(toUnmodifiableList());
}
public static List<Field> getCollectionFields(Object testSubject) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> Collection.class.isAssignableFrom(field.getType()))
.collect(toUnmodifiableList());
}
#SuppressWarnings("unchecked")
public static <T> T getFieldInstance(Field field, Object testSubject, Class<T> type) {
return (T) getFieldInstance(field, testSubject);
}
public static Object getFieldInstance(Field field, Object testSubject) {
try {
boolean isStatic = isStatic(field.getModifiers());
Object context = isStatic ? null : testSubject;
field.setAccessible(true);
return field.get(context);
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to get instance of field.");
}
}
public static Type[] getTypesForGeneric(Field field) {
ParameterizedType parameterizedType = (ParameterizedType) field.getGenericType();
return parameterizedType.getActualTypeArguments();
}
}

Refactor it a little bit:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
ConnectionSupplier connectionSupplier = () -> SupplierDriverManager.getConnection();
public void setConnSupplier(ConnectionSupplier supplier) {
this.connectionSupplier = supplier;
}
#Override
public Connection getConnection() {
try {
return connectionSupplier.conn();
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
#FunctionalInterface
interface ConnectionSupplier {
Connection conn();
}
}
Then you can use the mockito:
MySQLDatabaseConnectionFactory.ConnectionSupplier connectionSupplier = mock(MySQLDatabaseConnectionFactory.ConnectionSupplier.class);
when(connectionSupplier.conn()).thenReturn(yourMockObject);
yourConnectionFactory.setConnSupplier(connectionSupplier);

Mocking static methods with Mockito

I've written a factory to produce java.sql.Connection objects:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return DriverManager.getConnection(...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
}
I'd like to validate the parameters passed to DriverManager.getConnection, but I don't know how to mock a static method. I'm using JUnit 4 and Mockito for my test cases. Is there a good way to mock/verify this specific use-case?

Use PowerMockito on top of Mockito.
Example code:
#RunWith(PowerMockRunner.class)
#PrepareForTest(DriverManager.class)
public class Mocker {
#Test
public void shouldVerifyParameters() throws Exception {
//given
PowerMockito.mockStatic(DriverManager.class);
BDDMockito.given(DriverManager.getConnection(...)).willReturn(...);
//when
sut.execute(); // System Under Test (sut)
//then
PowerMockito.verifyStatic();
DriverManager.getConnection(...);
}
More information:
Why doesn't Mockito mock static methods?

Mocking of static methods in Mockito is possible since Mockito 3.4.0.
For more details see:
https://github.com/mockito/mockito/releases/tag/v3.4.0
https://github.com/mockito/mockito/issues/1013
https://javadoc.io/doc/org.mockito/mockito-core/latest/org/mockito/Mockito.html#static_mocks
assertEquals("foo", Foo.method());
try (MockedStatic mocked = mockStatic(Foo.class)) {
mocked.when(Foo::method).thenReturn("bar");
assertEquals("bar", Foo.method());
mocked.verify(Foo::method);
}
assertEquals("foo", Foo.method());
In your case, something like this:
#Test
public void testStaticMockWithVerification() throws SQLException {
try (MockedStatic<DriverManager> dummy = Mockito.mockStatic(DriverManager.class)) {
DatabaseConnectionFactory factory = new MySQLDatabaseConnectionFactory();
dummy.when(() -> DriverManager.getConnection("arg1", "arg2", "arg3"))
.thenReturn(new Connection() {/*...*/});
factory.getConnection();
dummy.verify(() -> DriverManager.getConnection(eq("arg1"), eq("arg2"), eq("arg3")));
}
}
NOTE: mocking STATIC METHODS requires mockito-inline dependency instead of mockito-core.
For JUnit5 also add this:
<dependency>
<groupId>org.mockito</groupId>
<artifactId>mockito-junit-jupiter</artifactId>
<version>${mockito.version}</version>
<scope>test</scope>
</dependency>

The typical strategy for dodging static methods that you have no way of avoiding using, is by creating wrapped objects and using the wrapper objects instead.
The wrapper objects become facades to the real static classes, and you do not test those.
A wrapper object could be something like
public class Slf4jMdcWrapper {
public static final Slf4jMdcWrapper SINGLETON = new Slf4jMdcWrapper();
public String myApisToTheSaticMethodsInSlf4jMdcStaticUtilityClass() {
return MDC.getWhateverIWant();
}
}
Finally, your class under test can use this singleton object by, for example,
having a default constructor for real life use:
public class SomeClassUnderTest {
final Slf4jMdcWrapper myMockableObject;
/** constructor used by CDI or whatever real life use case */
public myClassUnderTestContructor() {
this.myMockableObject = Slf4jMdcWrapper.SINGLETON;
}
/** constructor used in tests*/
myClassUnderTestContructor(Slf4jMdcWrapper myMock) {
this.myMockableObject = myMock;
}
}
And here you have a class that can easily be tested, because you do not directly use a class with static methods.
If you are using CDI and can make use of the #Inject annotation then it is even easier.
Just make your Wrapper bean #ApplicationScoped, get that thing injected as a collaborator (you do not even need messy constructors for testing), and go on with the mocking.

I had a similar issue. The accepted answer did not work for me, until I made the change: #PrepareForTest(TheClassThatContainsStaticMethod.class), according to PowerMock's documentation for mockStatic.
And I don't have to use BDDMockito.
My class:
public class SmokeRouteBuilder {
public static String smokeMessageId() {
try {
return InetAddress.getLocalHost().getHostAddress();
} catch (UnknownHostException e) {
log.error("Exception occurred while fetching localhost address", e);
return UUID.randomUUID().toString();
}
}
}
My test class:
#RunWith(PowerMockRunner.class)
#PrepareForTest(SmokeRouteBuilder.class)
public class SmokeRouteBuilderTest {
#Test
public void testSmokeMessageId_exception() throws UnknownHostException {
UUID id = UUID.randomUUID();
mockStatic(InetAddress.class);
mockStatic(UUID.class);
when(InetAddress.getLocalHost()).thenThrow(UnknownHostException.class);
when(UUID.randomUUID()).thenReturn(id);
assertEquals(id.toString(), SmokeRouteBuilder.smokeMessageId());
}
}

As mentioned before you can not mock static methods with mockito.
If changing your testing framework is not an option you can do the following:
Create an interface for DriverManager, mock this interface, inject it via some kind of dependency injection and verify on that mock.

For those who use JUnit 5, Powermock is not an option. You'll require the following dependencies to successfully mock a static method with just Mockito.
testCompile group: 'org.mockito', name: 'mockito-core', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-junit-jupiter', version: '3.6.0'
testCompile group: 'org.mockito', name: 'mockito-inline', version: '3.6.0'
mockito-junit-jupiter add supports for JUnit 5.
And support for mocking static methods is provided by mockito-inline dependency.
Example:
#Test
void returnUtilTest() {
assertEquals("foo", UtilClass.staticMethod("foo"));
try (MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)) {
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
}
assertEquals("foo", UtilClass.staticMethod("foo"));
}
The try-with-resource block is used to make the static mock remains temporary, so it's mocked only within that scope.
When not using a try block, make sure to close the mock, once you are done with the assertions.
MockedStatic<UtilClass> classMock = mockStatic(UtilClass.class)
classMock.when(() -> UtilClass.staticMethod("foo")).thenReturn("bar");
assertEquals("bar", UtilClass.staticMethod("foo"));
classMock.close();
Mocking void methods:
When mockStatic is called on a class, all the static void methods in that class automatically get mocked to doNothing().

Observation : When you call static method within a static entity, you need to change the class in #PrepareForTest.
For e.g. :
securityAlgo = MessageDigest.getInstance(SECURITY_ALGORITHM);
For the above code if you need to mock MessageDigest class, use
#PrepareForTest(MessageDigest.class)
While if you have something like below :
public class CustomObjectRule {
object = DatatypeConverter.printHexBinary(MessageDigest.getInstance(SECURITY_ALGORITHM)
.digest(message.getBytes(ENCODING)));
}
then, you'd need to prepare the class this code resides in.
#PrepareForTest(CustomObjectRule.class)
And then mock the method :
PowerMockito.mockStatic(MessageDigest.class);
PowerMockito.when(MessageDigest.getInstance(Mockito.anyString()))
.thenThrow(new RuntimeException());

You can do it with a little bit of refactoring:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
#Override public Connection getConnection() {
try {
return _getConnection(...some params...);
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
//method to forward parameters, enabling mocking, extension, etc
Connection _getConnection(...some params...) throws SQLException {
return DriverManager.getConnection(...some params...);
}
}
Then you can extend your class MySQLDatabaseConnectionFactory to return a mocked connection, do assertions on the parameters, etc.
The extended class can reside within the test case, if it's located in the same package (which I encourage you to do)
public class MockedConnectionFactory extends MySQLDatabaseConnectionFactory {
Connection _getConnection(...some params...) throws SQLException {
if (some param != something) throw new InvalidParameterException();
//consider mocking some methods with when(yourMock.something()).thenReturn(value)
return Mockito.mock(Connection.class);
}
}

I also wrote a combination of Mockito and AspectJ: https://github.com/iirekm/varia/tree/develop/ajmock
Your example becomes:
when(() -> DriverManager.getConnection(...)).thenReturn(...);

Mockito cannot capture static methods, but since Mockito 2.14.0 you can simulate it by creating invocation instances of static methods.
Example (extracted from their tests):
public class StaticMockingExperimentTest extends TestBase {
Foo mock = Mockito.mock(Foo.class);
MockHandler handler = Mockito.mockingDetails(mock).getMockHandler();
Method staticMethod;
InvocationFactory.RealMethodBehavior realMethod = new InvocationFactory.RealMethodBehavior() {
#Override
public Object call() throws Throwable {
return null;
}
};
#Before
public void before() throws Throwable {
staticMethod = Foo.class.getDeclaredMethod("staticMethod", String.class);
}
#Test
public void verify_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
handler.handle(invocation);
//verify staticMethod on mock
//Mockito cannot capture static methods so we will simulate this scenario in 3 steps:
//1. Call standard 'verify' method. Internally, it will add verificationMode to the thread local state.
// Effectively, we indicate to Mockito that right now we are about to verify a method call on this mock.
verify(mock);
//2. Create the invocation instance using the new public API
// Mockito cannot capture static methods but we can create an invocation instance of that static invocation
Invocation verification = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"some arg");
//3. Make Mockito handle the static method invocation
// Mockito will find verification mode in thread local state and will try verify the invocation
handler.handle(verification);
//verify zero times, method with different argument
verify(mock, times(0));
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
handler.handle(differentArg);
}
#Test
public void stubbing_static_method() throws Throwable {
//register staticMethod call on mock
Invocation invocation = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"foo");
handler.handle(invocation);
//register stubbing
when(null).thenReturn("hey");
//validate stubbed return value
assertEquals("hey", handler.handle(invocation));
assertEquals("hey", handler.handle(invocation));
//default null value is returned if invoked with different argument
Invocation differentArg = Mockito.framework().getInvocationFactory().createInvocation(mock, withSettings().build(Foo.class), staticMethod, realMethod,
"different arg");
assertEquals(null, handler.handle(differentArg));
}
static class Foo {
private final String arg;
public Foo(String arg) {
this.arg = arg;
}
public static String staticMethod(String arg) {
return "";
}
#Override
public String toString() {
return "foo:" + arg;
}
}
}
Their goal is not to directly support static mocking, but to improve its public APIs so that other libraries, like Powermockito, don't have to rely on internal APIs or directly have to duplicate some Mockito code. (source)
Disclaimer: Mockito team thinks that the road to hell is paved with static methods. However, Mockito's job is not to protect your code from static methods. If you don’t like your team doing static mocking, stop using Powermockito in your organization. Mockito needs to evolve as a toolkit with an opinionated vision on how Java tests should be written (e.g. don't mock statics!!!). However, Mockito is not dogmatic. We don't want to block unrecommended use cases like static mocking. It's just not our job.

To mock static method you should use a Powermock look at:
https://github.com/powermock/powermock/wiki/MockStatic.
Mockito doesn't provide this functionality.
You can read nice a article about mockito:
http://refcardz.dzone.com/refcardz/mockito

I found one solution in Mockito. This feature comes with a version only from 3.4.0
https://asolntsev.github.io/en/2020/07/11/mockito-static-methods/
dependency
In your build.gradle replace mockito-core:3.3.3 by mockito-inline:3.4.0:
testImplementation('org.mockito:mockito-inline:3.4.0')
what are we going to mock
class Buddy
{
static String name()
{
return "John";
}
}
Mock the static method
#Test
void lookMomICanMockStaticMethods()
{
assertThat(Buddy.name()).isEqualTo("John");
try (MockedStatic<Buddy> theMock = Mockito.mockStatic(Buddy.class))
{
theMock.when(Buddy::name).thenReturn("Rafael");
assertThat(Buddy.name()).isEqualTo("Rafael");
}
assertThat(Buddy.name()).isEqualTo("John");
}
I think this could help us.

Since that method is static, it already has everything you need to use it, so it defeats the purpose of mocking.
Mocking the static methods is considered to be a bad practice.
If you try to do that, it means there is something wrong with the way you want to perform testing.
Of course you can use PowerMockito or any other framework capable of doing that, but try to rethink your approach.
For example: try to mock/provide the objects, which that static method consumes instead.

When you try to mock the static method, you have to write the test inside the try block. because It's important to note that scoped mocks must be closed by the entity that activates the mock.
try (MockedStatic<Tester> tester = Mockito.mockStatic(Tester.class)) {
tester.when(() -> Tester.testStatic("Testing..")).thenReturn(mock(ReturnObject.class));
//Here you have to write the test cases
}
In the above example, we have to mock the Tester Class testStatic Method with input param as "Testing...". Here, this method will return a ReturnObject class type object. Hence we write mockito when chain like above.
Don't forget to add below dependency in your Gradle/maven
testImplementation 'org.mockito:mockito-inline:4.3.1'

Use JMockit framework. It worked for me. You don't have to write statements for mocking DBConenction.getConnection() method. Just the below code is enough.
#Mock below is mockit.Mock package
Connection jdbcConnection = Mockito.mock(Connection.class);
MockUp<DBConnection> mockUp = new MockUp<DBConnection>() {
DBConnection singleton = new DBConnection();
#Mock
public DBConnection getInstance() {
return singleton;
}
#Mock
public Connection getConnection() {
return jdbcConnection;
}
};

There is an easy solution by using java FunctionalInterface and then add that interface as dependency for the class you are trying to unit test.

For mocking static functions i was able to do it that way:
create a wrapper function in some helper class/object. (using a name variant might be beneficial for keeping things separated and maintainable.)
use this wrapper in your codes. (Yes, codes need to be realized with testing in mind.)
mock the wrapper function.
wrapper code snippet (not really functional, just for illustration)
class myWrapperClass ...
def myWrapperFunction (...) {
return theOriginalFunction (...)
}
of course having multiple such functions accumulated in a single wrapper class might be beneficial in terms of code reuse.

Here I share my mockito MockStatic solution based on an extension as promised in my answer to leokom's solution.
So, why does Mockito choose try-with-resources? Well, simply because they want to keep a tidy ship. That is good programming after all. Try-with-resources allows construction with guaranteed calling of the close method. But in JUnit we already have that in BeforeEach and AfterEach. And one can easily add these for a generic purpose to each test class using an Extension that implements BeforeEachCallback and AfterEachCallback.
So much for the theory. Let's make a static mock for
Instant.now()
I started with an annotation to be able to mark the fields in my test class that I want to use as static mocks.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public #interface StaticMock {
}
This allows me to create a field in my test class for static mocking that I can easily find in my Extension class.
#StaticMock
private MockedStatic<Instant> staticInstantMock;
I added the Extension I created to my test class. You have two options.
Create an Extension for this purpose and add it to the class next to MockitoExtension, which you also need.
Create an Extension and have it inherit from MockitoExtension. Now you can replace MockitoExtension on your test class.
I used the latter of the two.
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
Now we need something to be returned for the static when it's called:
#Mock
private Instant now;
staticInstantMock.when(Instant::now).thenReturn(now);
The whole test class:
#ExtendWith({CompanyMockitoExtension.class})
class MyExtendedTestClass {
#StaticMock
private MockedStatic<Instant> staticInstantMock;
#Mock
private Instant now;
#Test
void myTestMethod() {
staticInstantMock.when(Instant::now).thenReturn(now);
assertThat(Instant::now).isSameAs(now); // This would normally happen in the class you are testing...
}
}
Now let's take a look at the Extension class.
import static org.mockito.Mockito.mockStatic;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.List;
import org.junit.jupiter.api.extension.ExtensionContext;
import org.mockito.MockedStatic;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;
public class CompanyMockitoExtension extends MockitoExtension {
#Override
public void beforeEach(ExtensionContext context) {
super.beforeEach(context); // Don't forget to call the super!!
if (context.getTestInstance().isEmpty()) { // Just to be sure...
return;
}
// Get the unit test instance
Object testSubject = context.getTestInstance().get();
initializeStaticMocks(testSubject);
}
private void initializeStaticMocks(Object testSubject) {
// Find all fields that I want to static mock
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> initializeStaticMock(field, testSubject));
}
private void initializeStaticMock(Field field, Object testSubject) {
// Get the type of the static mock. It is within the generic MockedStatic<> class type.
Class<?> typeForStaticMock = (Class<?>) ReflectionHelper.getTypesForGeneric(field)[0];
try {
// Now set the field with the mockStatic method of Mockito.
field.setAccessible(true);
field.set(testSubject, mockStatic(typeForStaticMock));
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to instantiate Static Mock with type: " + typeForStaticMock.getName());
}
}
#Override
public void afterEach(ExtensionContext context) {
super.afterEach(context); // Again, do not forget to call the super.
if (context.getTestInstance().isEmpty()) {
return;
}
Object testSubject = context.getTestInstance().get();
closeStaticMocks(testSubject); // Close all static mocks.
}
private void closeStaticMocks(Object testSubject) {
// Again find all fields we annotated
List<Field> staticMockFields = ReflectionHelper.getFieldsWithAnnotation(testSubject, StaticMock.class);
staticMockFields.forEach(field -> closeStaticMock(field, testSubject));
}
private void closeStaticMock(Field field, Object testSubject) {
// Get the instance and simply call close.
MockedStatic<?> mockedStaticInstance = ReflectionHelper.getFieldInstance(field, testSubject, MockedStatic.class);
mockedStaticInstance.close();
}
}
The nice thing about this extension is that you can add additional mocking stuff. I added verification of no more interactions on all mocks in the AfterEach. This is now automatic when we use this extension. I also added similar behavior for construction mocking as for the static mocking.
As you see, I made my own reflection helper class. I know there are some standard reflection helper classes and those might be better. Here is mine for this purpose.
public class ReflectionHelper {
public static List<Field> getFieldsWithAnnotation(
Object testSubject,
Class<? extends Annotation> annotationType
) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> field.isAnnotationPresent(annotationType))
.collect(toUnmodifiableList());
}
public static List<Field> getCollectionFields(Object testSubject) {
Class<?> testSubjectClass = testSubject.getClass();
return Arrays.stream(testSubjectClass.getDeclaredFields())
.filter(field -> Collection.class.isAssignableFrom(field.getType()))
.collect(toUnmodifiableList());
}
#SuppressWarnings("unchecked")
public static <T> T getFieldInstance(Field field, Object testSubject, Class<T> type) {
return (T) getFieldInstance(field, testSubject);
}
public static Object getFieldInstance(Field field, Object testSubject) {
try {
boolean isStatic = isStatic(field.getModifiers());
Object context = isStatic ? null : testSubject;
field.setAccessible(true);
return field.get(context);
} catch (IllegalAccessException e) {
throw new RuntimeException("Failed to get instance of field.");
}
}
public static Type[] getTypesForGeneric(Field field) {
ParameterizedType parameterizedType = (ParameterizedType) field.getGenericType();
return parameterizedType.getActualTypeArguments();
}
}

Refactor it a little bit:
public class MySQLDatabaseConnectionFactory implements DatabaseConnectionFactory {
ConnectionSupplier connectionSupplier = () -> SupplierDriverManager.getConnection();
public void setConnSupplier(ConnectionSupplier supplier) {
this.connectionSupplier = supplier;
}
#Override
public Connection getConnection() {
try {
return connectionSupplier.conn();
} catch (SQLException e) {
throw new RuntimeException(e);
}
}
#FunctionalInterface
interface ConnectionSupplier {
Connection conn();
}
}
Then you can use the mockito:
MySQLDatabaseConnectionFactory.ConnectionSupplier connectionSupplier = mock(MySQLDatabaseConnectionFactory.ConnectionSupplier.class);
when(connectionSupplier.conn()).thenReturn(yourMockObject);
yourConnectionFactory.setConnSupplier(connectionSupplier);

Develop Reference

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Mocking of static field that is a class with Mockito - java

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