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Is it possible to mock System.identityHashCode?

Good day.
I have a tool which instruments objects in project by adding certain bytecode-generated toString implementation to each. The logic of generated toString is irrelevant but what important is that its return value depends on System.identityHashCode invocations, made inside.
Now I want to cover the generated logic with automated tests. I created a class for testing, generated toString for it and want to assert over its toString output. It's obvious that such test won't be reproducible, since System.identityHashCode gives different results between test runs.
Is it possible somehow to mock System.identityHashCode or stabilize the result it is given?

It is possible to do this using Powermock, as described here: How to Mock System.getProperty using Mockito
However, the better approach would be to create a class that wraps System and then inject the wrapper into the class you are trying to test.
Then in your unit test you can inject a test implementation of the wrapper and control the output from System.

I would introduce a testing seam:
#Override public String toString() {
return toString(System.identityHashCode(object));
}
/** Visible for testing. */
String toString(int hashCode) {
// Your implementation here.
}
This way, you can call toString(int) as many times as you want from adjacent tests without concerning yourself with PowerMock or even Mockito.
As an alternative, if System.identityHashCode is used elsewhere in your class such that results need to be consistent, you can take a replacement ToIntFunction in your constructor, and have the default implementation pass System.identityHashCode as a reference to a static method.
public YourWrapperClass(Object object) {
this(object, System::identityHashCode);
}
/** Visible for testing. */
YourWrapperClass(Object object, ToIntFunction<Object> hashCodeFunction) {
this.object = object;
this.hashCodeFunction = hashCodeFunction;
}

How do I get Mockito mocks to cause a failure when unexpected calls are made?

I have some mock objects that are probably going to get passed around a bit and might end up being fairly complex.
I'd like to either have Mockito output a log for each call made to a mock or I'd like it to fail whenever an unexpected call is made so I can iterate through those calls and set up appropriate responses.
How can I accomplish this?

The most-idiomatic way of doing this is with verifyNoMoreInteractions, as in Mockito docs #8:
//interactions
mock.doSomething();
mock.doSomethingUnexpected();
//verification
verify(mock).doSomething();
//following will fail because 'doSomethingUnexpected()' is unexpected
verifyNoMoreInteractions(mock);
I say "most-idiomatic" above because that method has its own warning label, which links to the blog post "Should I worry about the unexpected?" by Mockito originator Szczepan Faber.
verifyNoMoreInteractions() is not recommended to use in every test method. verifyNoMoreInteractions() is a handy assertion from the interaction testing toolkit. Use it only when it's relevant. Abusing it leads to overspecified, less maintainable tests.
In short, you should have a very clear reason to check what your dependency is not doing or what your system-under-test is not calling, as opposed to what they are doing and calling. You might use verifyNoMoreInteractions for an RPC object, if you want to avoid unnecessary RPC calls, but not (say) a calculator with no side effects. Even better is to specify your exact requirements with never() or times(int) as parameters to verify.
That said, there are two even-less-idiomatic ways of doing this:
You can take an overall log of calls made using mockingDetails(Object) and iterating through getInvocations(). That should reflectively give you a whole list of the invocations. I have a hard time imagining how this would be useful in a test, but it might be useful in cleaning up a nebulous or poorly-documented existing system.
You can make the mock's default action to throw an exception, which means that if anyone calls something that you haven't stubbed, the test will immediately fail.
// untested pseudocode
YourObject yourObject = Mockito.mock(YourObject.class, withSettings()
.defaultAnswer(invocation -> {
throw new UnsupportedOperationException(invocation.toString());
}));
Sure, that'd work, but you'd not only be violating one of Mockito's core principles (mocks are nice by default, using EasyMock's definition of "nice"), but you'd also force yourself to only stub using doVerb (doReturn, doAnswer, etc) because calls to when(yourObject.doAnything()) would necessarily throw that exception before the call to when would even run.
Developers who are familiar with Mockito would likely say that this exception-prone cure is worse than the disease, and may be useful only for temporarily diagnosing the most tangled legacy code.

I was just asking myself the same question and... The solution using ReturnsSmartNulls will return SmartNulls instead of null... So it is meaningful for non-void methods only right ? What about void methods, the ones having side effects ?
In my opinion, if you want to make sure that your test fails when a method of your mock is called without your explicit behavior definition of it (doXXX(...).when(...) mockito methods) you can initialize your mocks with a custom default answer that will throw an exception, or better... fail your test.
For example you can add the following class inside your test class (or outside if you intend to use it elsewhere, or even use a MockitoConfiguration class as previously mentionned depending on what you want):
static class FailAnswer implements Answer<Object> {
#Override
public Object answer(InvocationOnMock invocation) {
String methodName = invocation.getMethod().getName();
String className = invocation.getMethod().getDeclaringClass().getSimpleName();
return fail(String.format("%s#%s should not have been called", className, methodName));
}
}
Then init your mock with this fake answer in your setUp method :
#BeforeEach
void setUp() {
delegateService = mock(DelegateService.class, new FailAnswer());
classUnderTest = new ClassUnderTest(delegateService);
}
Unfortunately, this solution is not compatible with #Mock annotation which only takes native predefined answers from org.mockito.Answers enum as argument. So that forces you to manually init every mock, spy, captor within the setUp method (RIP MockitoAnnotations.initMocks(this))
Benefit :
you get rid of default behavior of mockito mocks, sometimes hidding unintentionnal uses of mocks for specifics use cases (does is really matter ?)
=> You must define everything you use (inside tests or tests fixtures)
=> you don't have to make verification to make sure your test have not invoked methods it shouldn't have.
Drawbacks :
This is an unusual usage of mockito, so this makes your test less affordable
You give up on MockitoAnnotations feature
As you override mockito default stubbing, you must use the stubbing form do().when() instead of when(...).do(....), the latter providing au type-checking unlike the former.
Warning : This solution doesn't garantee your mock is called, it just garantees that the method you don't stub won't be called. It doesn't come as replacement for counting methods invocations neither.

The best answer I found is to configure Mockito to return SmartNulls.
https://static.javadoc.io/org.mockito/mockito-core/2.6.9/org/mockito/Mockito.html#RETURNS_SMART_NULLS
This implementation can be helpful when working with legacy code. Unstubbed methods often return null. If your code uses the object returned by an unstubbed call you get a NullPointerException. This implementation of Answer returns SmartNull instead of null. SmartNull gives nicer exception message than NPE because it points out the line where unstubbed method was called. You just click on the stack trace.
You can do it by mock or by default (might cause problems with other frameworks like Spring).
Manually
Writer writerMock = mock(Writer.class, RETURNS_SMART_NULLS);
Annotation
#Mock(answer = Answers.RETURNS_SMART_NULLS)
Set as Global Default
Configuration class must be in exactly this package. This might lead to strange failures with Spring.
package org.mockito.configuration;
import org.mockito.internal.stubbing.defaultanswers.ReturnsSmartNulls;
import org.mockito.stubbing.Answer;
public class MockitoConfiguration extends DefaultMockitoConfiguration {
public Answer<Object> getDefaultAnswer() {
return new ReturnsSmartNulls();
}
}
See: https://solidsoft.wordpress.com/2012/07/02/beyond-the-mockito-refcard-part-1-a-better-error-message-on-npe-with-globally-configured-smartnull/
I had problems with SpringBootRepositories and #MockBean when enabling the global default:
java.lang.ClassCastException: org.mockito.codegen.Object$MockitoMock$191495750 cannot be cast to xxx.xxx.MyObject
Example of error output
org.junit.ComparisonFailure: expected:<[DataRecordType{id=null, name='SomeRecord', pathTemplate='SomeTemplate'}]> but was:<[SmartNull returned by this unstubbed method call on a mock: dataRecordTypeRepository bean.getById(1L);]>

If you are trying to track the flow, you can use Mockito verify to check if certain call has been made.
verify(yourMockedObject).yourMethod();
you can also use times to verify if certain call has to be made exactly some number of times.
verify(yourMockedObject, times(4)).yourMethod();
It is not a good practice to make your unit test complex. Try to test only small unit of your code at a time.

Unit testing method parameter verification best practice

What is the best practice for verifying a method call with complex parameters when unit testing?
Say I'm testing a function like this:
class ClassA {
ClassB dependency;
void someFunction(SomeInputForA input) {
// do some thing
dependency.anotherFunction(differentInput);
}
}
The two options that I can think of for verifying that someFunction calls anotherFunction with the proper input are:
A) do a verify on the mock of dependency for calling anotherFunction
unitUnderTest.dependency = mockClassB;
InputClass expectedDifferentInput = ... ;
verify(mockClassB).anotherFunction(expectedDifferentInput);
B) do an argument captor on the call of anotherFunction and assert the properties
unitUnderTest.dependency = mockClassB;
ArgumentCaptor<InputClass> captor = ArgumentCaptor.for(InputClass.class);
verify(mockClassB).anotherFunction(captor.capture());
InputClass capturedInput = captor.getValue();
assertEquals(value, capturedInput.param1);
// and more asserts afterwards
Is there a suggested path here? I'd lean towards the captor method because it feels more rigorous and is not relying on objects equals implementations being proper.
Thoughts?

Is differentInput computated off input?
If so then your B) is the better way to go as you are saying for Input A, you expect ClassA to change this to expectedDifferentInput and want to verify the delegating class (ClassB) is called. You are verifying the transformation of the input and delegating logic of ClassA.
If differentInput has no relation to input then you don't need to use the captor as really you are just checking delegation.
Any public caller to someFunction on ClassA shouldn't need to know about ClassB so it can be said both methods A) and B) are actually white box testing, in this case and so you might as well use the captors anyway. As you vary your input to someFunction, captors may also help you to identify edge cases if differentInput is computed off input.

You can always use matchers on the object passed into mockClassB.anotherFunction(). For example, if you want to compare fields on an object, you can write:
Matcher<YourClass> yourMatcher = Matchers.hasProperty("yourProperty", Matchers.equals(whatever));
verify(mockClassB).anotherFunction(argThat(yourMatcher));
I prefer this way, since you can share syntax for the when and the verify for the matchers and you can combine any combination of matchers. You just need to include the latest mockito and hamcrest libraries to get this to work.

I've used argument captors but VERY sparingly. The biggest issue you run into is that this route creates fragile unit tests. And no one is happy when they make a small change to a class and then find themselves struggling with unit tests in calling classes that shouldn't have been affected.
That being said, absolutely you have to eventually ensure that correct calls are made. But if you rely on an equals override working, then you are relying on that class having an equals method that works, and this is then part of that class's contract (and unit tested in that class) which is reasonable.
So that's why I'd vote for keeping it simple and just using verify. Same thing in the end, but your unit test is just less fragile.

unit test best practice for method with mocks in Mockito

Lets say we have method to test in class A that calls method from class B. To test it we created mock for B and then verify if it was called. Is verify(...) enough for unit test or I need assert actual result of tested method?
Below is simplified example to clarify my concern:
public class StringWriterATest {
StringWriterB b = mock(StringWriterB.class);
#Test
public void stringWriterATest() {
StringBuffer sb = new StringBuffer();
StringWriterA a = new StringWriterA();
a.stringWriterB=b;
a.append(sb);
ArgumentCaptor<StringBuffer> argument = ArgumentCaptor.forClass(StringBuffer.class);
verify(b).append(argument.capture());
assertEquals("StringWriterA", ((StringBuffer)argument.getValue()).toString());
//do we really need this or above is enough for proper unit test of method a.append(sb);
//assertEquals("StringWriterA_StringWriterB", sb);
}
}
public class StringWriterA {
public StringWriterB stringWriterB;
public void append(StringBuffer sb) {
sb.append("StringWriterA");
stringWriterB.append(sb);
}
}
class StringWriterB {
public void append(StringBuffer sb) {
sb.append("StringWriterB");
}
}
Regards,
Max

There is never a need to mock a return value and verify an object at the same time.
Consider this:
StringWriterA is the class under test. Therefore you'll definitely want to use assertions to verify the behavior of this class. In order to do this, you mock out a dependency, StringWriterB.
You do not want to test StringWriterB in your test of StringWriterA, therefore any assertions of StringWriterB interactions in your test are in the wrong place.
You must assume that StringWriterB is behaving as expected. You either want to verify that StringWriterA called StringWriterB correctly (using verify()) or you want to mock its expected behavior and mock the return values.
If you mock, then the verify is implicit since the mocked return value will not be returned if the method is not called.
In your case, StringWriterA.append() does not return any value, so only a verify is even possible. That StringWriterB.append() also works should have a similar verify test in a stringWriterBTest of its own.
Note: It's nice to be explicit with tests. Since test methods are never called outside of a framework, there is never a need to type them out, so you can have much longer method names than in production code methods. A nice convention is:
<underTest>Should<Expected>[When]<Condition>()
i.e.
stringWriterAShouldAppendConstantAndDelegateToStringWriterB()
stringWriterAShouldThrowNullPointerExceptionWhenNullArgument()
When you have test failures in your build (continuous integration), then you don't have to hunt down what went wrong, the method name appears right by the failure and you can read it to know exactly what behavior must be fixed.

In your example, StringWriterB stores no state and the append method could easily be static. In that case then the call is purely a side effect and does not need to be tested.
However, I suspect your real code is much more complex. If there is a of another object accessing StringWriterB then you maye want to mock it out in case there are unexpected calls to it. You also may want to add the verify of B if you expect it to be expanded in the future -- possibly storing state from the append call and having accessors.
One thing to consider is what the purpose of the call to StringWriterA.append() is. If it's job is to append the string StringWriterAStringWriterB then that is what you should be testing and a mock is not necessary. How StringWriterA accomplishes that task is immaterial. If, however, part of its job is to also call the StringWriterB.append() method then a mock may will be necessary unless you want to test StringWriterB in A's test.
My rule of thumb WRT mocks is to use real objects until the wiring for the objects I'm not directly testing gets too hairy or too brittle. If I feel like a good portion of my tests are in actuality testing other objects then mocks would be a good idea.

First of all try to show somebody the test you wrote. It is hard to read in my opinion. You can't really tell from it what behaviour you are testing. A brief intro for you how to make it a bit more readable can be found here How to Write Clean, Testable Code .
Using argument captors is also a smell. Some examples how to avoid it can be found on this tdd blog.
To answer you question, verify is used to verify interactions between classes. It is used to drive the design of your code. The result (if needed) should be specified by a when or given at the beginning of your test.
Further information how to drive your design with mocks (when, given, verify, ...) and how mocks are different to stubs can be found here: Mocks are not stubs. That example uses JMock not Mockito for defining mocks, but it is very similar (it is about the concepts, not the details of implementation and libraries you use).

Automatic stubbing in java word. What to use?

I have huge class that I need to build stub for.
To give you picture it is Messages class of the GWT. Often this is class with dozens of methods that return String.
With JMock I can do stubbing, but I will end with allowing each method... This is not something that I would like to see.
Is there something that will automatically build the stubs for each method? I need this methods to return something predefined, like empty string, but I will be happy with any suggestions.

In JMock you can allow the methods you care about with explicit results and then allow any other method of the messages object with an allowing statement that does not include a method. E.g.:
allowing(m).getBlah("something");
will(returnValue("foo"));
allowing(m); // matches anything else, will return some default value if called
But...
If you are just stubbing a bunch of getter methods, a mock object framework is the wrong tool to use. Mock objects are used for testing that the object under test sends the correct commands to neighbouring objects to effect change in its environment.
It's often easier to create a stub class if the interface that only contains getters. Or you can use Usurper to generate stubs automatically.

For an interface you can use the functionality of java.lang.reflect.Proxy. Assuming that you want to stub answers for the MessageConstants class, the code will look similar to:
InvocationHandler handler = new InvocationHandler() {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if (String.class.equals(method.getReturnType()))
return "proxy generated method return";
throw new AbstractMethodError("Not implemented");
}
};
Class<?> proxyClass = Proxy.getProxyClass(MessageConstants.class.getClassLoader(), new Class[] { MessageConstants.class });
MessageConstants messageConstants = (MessageConstants) proxyClass.getConstructor(new Class[] {InvocationHandler.class}).newInstance(new Object[] { handler });
System.out.println(messageConstants.description());
messageConstants.getBoolean("someBoolean");
and will result in
proxy generated method return
Exception in thread "main" java.lang.Error: Not implemented
at xxx.Application$1.invoke(Application.java:48)
at $Proxy0.getBoolean(Unknown Source)
at xxx.Application.main(Application.java:64)
The InvocationHandler drives the stubbing, while the rest is just plumbing.

Glad to see you found an answer. Just for further info, jMock allows quite flexible specifications of how you match methods, see http://www.jmock.org/match-object-or-method.html. For example, you can do this sort of thing:
allowing (any(Object.class)).method("get.*").withNoArguments();
to match any getter.
S

If you use EasyMock, you only need to specify behavior/expectations/stubs for the methods you actually expect to be called and used.
After using both JMock and EasyMock, I have to say that EasyMock's API is about 10x easier to use, and since the interface is mostly statically-typed, it's refactoring safe as well (you are using Strings for method names, etc.).