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Calling static method of class loaded via different ClassLoader isn't working. Is this expected?

I have the following class which uses a simple customer ClassLoader to load a class and then call a static method directly on it:
public class App {
public static void main(String[] args) throws IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException {
MyCustClassLoader loader = new MyCustClassLoader();
Class<?> c = loader.findClass("classloading.FooObservable");
Object ob = c.newInstance();
Method md = c.getDeclaredMethod("addListener", Listener.class);
FooListener fooListener = new FooListener("app class loader");
// md.invoke(ob, fooListener); <<< works it I uncomment this.
FooObservable.addListener(fooListener);
md = c.getMethod("fooDidSomething");
md.invoke(ob);
md.invoke(ob);
md.invoke(ob);
}
}
My custom class loader looks like this:
package classloading;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
public class MyCustClassLoader extends ClassLoader {
#Override
public Class<?> findClass(String name) {
byte[] bt = loadClassData(name);
return defineClass(name, bt, 0, bt.length);
}
private byte[] loadClassData(String className) {
//read class
InputStream is = getClass().getClassLoader().getResourceAsStream(className.replace(".", "/")+".class");
ByteArrayOutputStream byteSt = new ByteArrayOutputStream();
//write into byte
int len =0;
try {
while((len=is.read())!=-1){
byteSt.write(len);
}
} catch (IOException e) {
e.printStackTrace();
}
//convert into byte array
return byteSt.toByteArray();
}
}
The static method in question is defined in class FooObservable which contains a list of listeners and a notify method that can be called to notify listeners that something has happened:
package classloading;
import java.util.ArrayList;
import java.util.List;
public class FooObservable {
private static List<Listener> listeners = new ArrayList<Listener>();
int x = 0;
public static void addListener(Listener l) {
listeners.add(l);
}
public void fooDidSomething() {
x++;
for (Listener l : listeners) {
l.notify(x);
}
}
}
It doesn't seem to work, in that nothing is printed. However, when I uncomment the line md.invoke class App above and comment out the static call FooObservable.addListener, it works and I see:
foo listener (app class loader) notified: 1
foo listener (app class loader) notified: 2
foo listener (app class loader) notified: 3
Why is this happening? Is it down to the fact that when the line is uncommented, the static call addListener is invoked against FooObservable which is loaded using the Java AppClassLoader and as such is reloaded with an empty list of listeners?

This line:
FooObservable.addListener(fooListener);
is creating a different instance of FooObservable, created by the default classloader. Because it's a different classloader, it's a totally different object. Calls against the md.<> are against the FooObservable from your class loader. Those are two different instances of the same static class (that's part of the point of classloaders - you can have different instances of the same class).
If instead of calling md.invoke(ob) you called FooObservable.fooDidSomething(), you would see a successful result (from the parent classloader instance).
Without calling md.addListener (the commented out line), the version loaded by your classloader has no listeners.

Javassist API with Junit

I am attempting to change some third party class definitions, before each test, to simulate different results. I have to use something like javassist because extending the classes, sometimes, is just not possible due to the access modifiers. Here is an example of what I am attempting to do with javassist and junit combined:
public class SimulatedSession extends SomeThirdParty {
private boolean isJoe = false;
public SimulatedSession(final boolean isJoe) {
this.isJoe = isJoe;
}
#Override
public void performThis() {
final ClassPool classPool = ClassPool.getDefault();
final CtClass internalClass = classPool.get("some.package.Class");
final CtMethod callMethod = internalClass.getDeclaredMethod("doThis");
if (isJoe) {
callMethod.setBody("{System.out.println(\"Joe\");}");
} else {
callMethod.setBody("{System.out.println(\"mik\");}");
}
internalClass.toClass();
}
}
#Test
public void firstTest() {
SimulatedSession toUse = new SimulatedSession(false);
// do something with this object and this flow
}
#Test
public void nextTest() {
SimulatedSession toUse = new SimulatedSession(true);
// do something with this object and this flow
}
if I run each test individually, I can run the code just fine. When I run them using the unit suite, one test after the other, I get a "frozen class issue". To get around this, I am looking at this post, however, I must admit I am unsure as to how one can use a different class pool to solve the issue.

Your current code will try to load twice the same class into the same ClassLoader which is forbidden, you can only load once a class for a given ClassLoader.
To make your unit tests pass, I had to:
Create my own temporary ClassLoader that will be able to load some.package.Class (that I replaced by javassist.MyClass for testing purpose) and that will be implemented in such way that it will first try to load the class from it before the parent's CL.
Set my own ClassLoader as context ClassLoader.
Change the code of SimulatedSession#performThis() to be able to get the class instance created by this method and to call internalClass.defrost() to prevent the "frozen class issue".
Invoke by reflection the method doThis() to make sure that I have different output by using the class instance returned by SimulatedSession#performThis() to make sure that the class used has been loaded with my ClassLoader.
So assuming that my class javassist.MyClass is:
package javassist;
public class MyClass {
public void doThis() {
}
}
The method SimulatedSession#performThis() with the modifications:
public Class<?> performThis() throws Exception {
final ClassPool classPool = ClassPool.getDefault();
final CtClass internalClass = classPool.get("javassist.MyClass");
// Prevent the "frozen class issue"
internalClass.defrost();
...
return internalClass.toClass();
}
The unit tests:
// The custom CL
private URLClassLoader cl;
// The previous context CL
private ClassLoader old;
#Before
public void init() throws Exception {
// Provide the URL corresponding to the folder that contains the class
// `javassist.MyClass`
this.cl = new URLClassLoader(new URL[]{new File("target/classes").toURI().toURL()}){
protected Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException {
try {
// Try to find the class for this CL
return findClass(name);
} catch( ClassNotFoundException e ) {
// Could not find the class so load it from the parent
return super.loadClass(name, resolve);
}
}
};
// Get the current context CL and store it into old
this.old = Thread.currentThread().getContextClassLoader();
// Set the custom CL as new context CL
Thread.currentThread().setContextClassLoader(cl);
}
#After
public void restore() throws Exception {
// Restore the context CL
Thread.currentThread().setContextClassLoader(old);
// Close the custom CL
cl.close();
}
#Test
public void firstTest() throws Exception {
SimulatedSession toUse = new SimulatedSession(false);
Class<?> c = toUse.performThis();
// Invoke doThis() by reflection
Object o2 = c.newInstance();
c.getMethod("doThis").invoke(o2);
}
#Test
public void nextTest() throws Exception {
SimulatedSession toUse = new SimulatedSession(true);
Class<?> c = toUse.performThis();
// Invoke doThis() by reflection
Object o2 = c.newInstance();
c.getMethod("doThis").invoke(o2);
}
Output:
mik
Joe

Take a look at retransformer. It's a Javassist based lib I wrote for running tests just like this. It's a bit more terse than using raw Javassist.

Maybe another approach. We had a similar problem as we once mocked a dependency - we could not reset it. So we did the following: Before each test we replace the 'live' instance with our mock. After the tests, we restore the live instance. So I propose that you replace the modified instance of your third party code for each test.
#Before
public void setup()
{
this.liveBeanImpl = (LiveBean) ReflectionTools.getFieldValue(this.beanToTest, "liveBean");
ReflectionTools.setFieldValue(this.beanToTest, "liveBean", new TestStub());
}
#After
public void cleanup()
{
ReflectionTools.setFieldValue(this.beanToTest, "liveBean", his.liveBeanImpl);
}
The setFieldValue looks like this:
public static void setFieldValue(Object instanceToModify, String fieldName, Object valueToSet)
{
try
{
Field declaredFieldToSet = instanceToModify.getClass().getDeclaredField(fieldName);
declaredFieldToSet.setAccessible(true);
declaredFieldToSet.set(instanceToModify, valueToSet);
declaredFieldToSet.setAccessible(false);
}
catch (Exception exception)
{
String className = exception.getClass().getCanonicalName();
String message = exception.getMessage();
String errorFormat = "\n\t'%s' caught when setting value of field '%s': %s";
String error = String.format(errorFormat, className, fieldName, message);
Assert.fail(error);
}
}
So maybe your tests will pass if you reset your implementation for each test. Do you get the idea?

How to make Java use my classloader for class resolution

Test case below. Output:
custom loading of: pkg.TestRun
ran.
wish this would run in my custom classloader
What I want to happen is to see "custom loading of: pkg.TestRun" show up between the second and third lines of the output.
How can I get it to load the dependencies of a class from my custom classloader, even when the first class is loaded from the parent? Note, that in my real case, I get a class not found exception because the equivalent of OtherClass is not known to the parent classloader.
I know one solution is to have the custom class loader explicitly load TestRun. However, how to load TestRun is already known to the parent classloader and I don't want to have to manage finding it separately since it's already done and it might be tricky for me to somehow figure that out when it's already being managed without me doing anything. And I've tried to do something like super.getResource (returns null) or findClass (already sets parent as classloader for it) but neither worked.
So, can I let the parent find the class, but the custom loader define it? Or, is there just a way to make it so that it will always use my custom loader to look for dependencies?
package pkg;
public class TestCL {
static class MyCL extends ClassLoader {
MyCL(ClassLoader parent) {
super(parent);
}
#Override
public Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
System.out.println("custom loading of: " + name);
return getParent().loadClass(name);
}
}
public static void main(String[] args) throws Exception {
MyCL cl = new MyCL(Thread.currentThread().getContextClassLoader());
Thread.currentThread().setContextClassLoader(cl);
cl.loadClass("pkg.TestRun").getMethod("run", new Class[] {}).invoke(null);
}
}
class TestRun {
public static void run() {
System.out.println("ran.");
OtherClass.runAlso();
}
}
class OtherClass {
public static void runAlso() {
System.out.println("wish this would run in my custom classloader");
}
}

The problem was you were not loading anything using your custom classloader
It asks it parent to load the class, so everything was being loaded by the main classloader.
Try this modified code:
package stackoverflow.june2012.classloader;
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
public class TestCL {
static class MyCL extends URLClassLoader {
MyCL(URL[] urls) {
// NOTE: No parent classloader!
super(urls, null);
}
#Override
public Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
System.out.println("custom loading of: " + name);
return super.loadClass(name, resolve);
}
#Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
System.out.println("findClass: " + name);
System.out.println("NOTE: Only called if this classloader does NOT have a parent");
return super.findClass(name);
}
}
public static void main(String[] args) throws Exception {
URL url = new File("./bin").toURI().toURL();
System.out.println("url to search for classes: " + url);
MyCL cl = new MyCL(new URL[] {url});
Thread.currentThread().setContextClassLoader(cl);
Class loadClass = cl.loadClass("stackoverflow.june2012.classloader.TestRun");
System.out.println("Loaded TestRun using classloader: " + loadClass.getClassLoader());
loadClass.getMethod("run", new Class[] {}).invoke(null);
}
}
And I had to move your other 2 classes into a separate file, otherwise it cant be accessed as it was package-private in a different classloader:
package stackoverflow.june2012.classloader;
public class TestRun {
public static void run() {
System.out.println("ran.");
OtherClass.runAlso();
}
}
class OtherClass {
public static void runAlso() {
System.out.println("wish this would run in my custom classloader");
}
}

One solution is to use the parent classloader (or the one that already knows where it is) to read the bytes of the class, then define it in the custom class loader.
So, something like this:
public static byte[] loadBytesForClass(ClassLoader loader, String fqn) throws IOException {
InputStream input = loader.getResourceAsStream(fqn.replace(".", "/") + ".class");
if (input == null) {
System.out.println("Could not load bytes for class: " + fqn);
}
ByteArrayOutputStream output = new ByteArrayOutputStream();
StringUtil.copy(input, output);
return output.toByteArray();
}
Then you can call defineClass(name, bytes, 0, bytes.length) with those bytes to define it in the custom class loader.

How to test a ClassFileTransformer / javaagent?

I implemented a ClassFileTransformer for a javaagent using ASM. Because it has some bugs, I want to write a JUnit test case for it. How do I do this?
Using pseudo-code I thought along the lines:
// Have a test class as subject
public static class Subject {
public void doSomething(){...}
}
// Manually load and transform the subject
...?
// Normally execute some now transformed methods of the subject
new Subject().doSomething();
// Check the result of the call (i.e. whether the correct attached methods were called)
Assert.assertTrue(MyClassFileTransformer.wasCalled());
Now the question is: How do I manually load and transform the subject and make the JVM/Classloader use my manipulated version of it? Or do I completely miss something?

I got it. One needs to implement an own ClassLoader that does the same transformation with the test subject as the ClassFileTransformer (e.g. calls it). And of course the subject class may not already be loaded, so there may not be any direct usage of it. So I used Java reflection API to execute the methods of the subject class.
In a separate file:
public static class Subject {
public void doSomething(){...}
}
In the test:
private static class TransformingClassLoader extends ClassLoader {
private final String className;
public TransformingClassLoader(String className) {
super();
this.className = className;
}
#Override
public Class<?> loadClass(String name) throws ClassNotFoundException {
if (name.equals(className)) {
byte[] byteBuffer = instrumentByteCode(fullyQualifiedSubjectClass);
return defineClass(className, byteBuffer, 0, byteBuffer.length);
}
return super.loadClass(name);
}
}
#Test
public void testSubject(){
ClassLoader classLoader = new TransformingClassLoader(fullyQualifiedSubjectClass);
Class<?> subjectClass = classLoader.loadClass(fullyQualifiedSubjectClass);
Constructor<?> constructor = subjectClass.getConstructor();
Object subject = constructor.newInstance();
Method doSomething = subjectClass.getMethod("doSomething");
doSomething.invoke(subject);
Assert.assertTrue(MyClassFileTransformer.wasCalled());
}

$0 (Program Name) in Java? Discover main class?

Is there a way to find the name of the program that is running in Java? The class of the main method would be good enough.

Try this:
StackTraceElement[] stack = Thread.currentThread ().getStackTrace ();
StackTraceElement main = stack[stack.length - 1];
String mainClass = main.getClassName ();
Of course, this only works if you're running from the main thread. Unfortunately I don't think there's a system property you can query to find this out.
Edit: Pulling in #John Meagher's comment, which is a great idea:
To expand on #jodonnell you can also
get all stack traces in the system
using Thread.getAllStackTraces(). From
this you can search all the stack
traces for the "main" Thread to
determine what the main class is. This
will work even if your class is not
running in the main thread.

System.getProperty("sun.java.command")

To expand on #jodonnell you can also get all stack traces in the system using Thread.getAllStackTraces(). From this you can search all the stack traces for the main Thread to determine what the main class is. This will work even if your class is not running in the main thread.

This is the code I came up with when using the combined responses of jodonnell and John Meagher. It stores the main class in a static variable to reduce overhead of repeated calls:
private static Class<?> mainClass;
public static Class<?> getMainClass() {
if (mainClass != null)
return mainClass;
Collection<StackTraceElement[]> stacks = Thread.getAllStackTraces().values();
for (StackTraceElement[] currStack : stacks) {
if (currStack.length==0)
continue;
StackTraceElement lastElem = currStack[currStack.length - 1];
if (lastElem.getMethodName().equals("main")) {
try {
String mainClassName = lastElem.getClassName();
mainClass = Class.forName(mainClassName);
return mainClass;
} catch (ClassNotFoundException e) {
// bad class name in line containing main?!
// shouldn't happen
e.printStackTrace();
}
}
}
return null;
}

Also from the command line you could run the jps tool. Sounds like a
jps -l
will get you what you want.

For access to the class objects when you are in a static context
public final class ClassUtils {
public static final Class[] getClassContext() {
return new SecurityManager() {
protected Class[] getClassContext(){return super.getClassContext();}
}.getClassContext();
};
private ClassUtils() {};
public static final Class getMyClass() { return getClassContext()[2];}
public static final Class getCallingClass() { return getClassContext()[3];}
public static final Class getMainClass() {
Class[] c = getClassContext();
return c[c.length-1];
}
public static final void main(final String[] arg) {
System.out.println(getMyClass());
System.out.println(getCallingClass());
System.out.println(getMainClass());
}
}
Obviously here all 3 calls will return
class ClassUtils
but you get the picture;
classcontext[0] is the securitymanager
classcontext[1] is the anonymous securitymanager
classcontext[2] is the class with this funky getclasscontext method
classcontext[3] is the calling class
classcontext[last entry] is the root class of this thread.

Try this :
Java classes have static instance of their own class (java.lang.Class type).
That means if we have a class named Main.
Then we can get its class instance by
Main.class
If you're interested in name only then,
String className = Main.class.getName();

Or you could just use getClass(). You can do something like:
public class Foo
{
public static final String PROGNAME = new Foo().getClass().getName();
}
And then PROGNAME will be available anywhere inside Foo. If you're not in a static context, it gets easier as you could use this:
String myProgramName = this.getClass().getName();