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Mockito + PowerMock LinkageError while mocking system class

I've got such a code snippet: 

@RunWith(PowerMockRunner.class)
@PrepareForTest({Thread.class})
public class AllMeasuresDataTest {

@Before
public void setUp() throws Exception {
}

@Test
public void testGetMeasures() {
    AllMeasuresData measure = new AllMeasuresData();
    assertEquals(measure.getMeasures(), null);
    HashMap<String, Measure> map = new HashMap<String, Measure>();
    measure.setMeasures(map);
    assertEquals(measure.getMeasures(), map);
    measure.setMeasures(null);
    assertEquals(measure.getMeasures(), null);
}

@Test
public void testAllMeasuresData() throws IOException {
    ClassLoader loader = PowerMockito.mock(ClassLoader.class);
    Thread threadMock = PowerMockito.mock(Thread.class);
    Vector<URL> vec = new Vector<URL>();
    Mockito.when(loader.getResources("measure")).thenReturn(vec.elements());
    Mockito.when(threadMock.getContextClassLoader()).thenReturn(loader);
    PowerMockito.mockStatic(Thread.class);
    Mockito.when(Thread.currentThread()).thenReturn(threadMock);
        ...
    }
}

While running this tests I got: 

java.lang.LinkageError: loader constraint violation: loader (instance of org/powermock/core/classloader/MockClassLoader) previously initiated loading for a different type with name "javax/management/MBeanServer"
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:791)
at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
at org.powermock.core.classloader.MockClassLoader.loadUnmockedClass(MockClassLoader.java:201)
at org.powermock.core.classloader.MockClassLoader.loadModifiedClass(MockClassLoader.java:149)
at org.powermock.core.classloader.DeferSupportingClassLoader.loadClass(DeferSupportingClassLoader.java:67)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at org.codecover.instrumentation.java.measurement.ProtocolImpl.initializeMBean(ProtocolImpl.java:247)
at org.codecover.instrumentation.java.measurement.ProtocolImpl.<init>(ProtocolImpl.java:237)
at org.codecover.instrumentation.java.measurement.ProtocolImpl.getInstance(ProtocolImpl.java:185)
at measure.CodeCoverCoverageCounter$6ya5ud0ow79ijrr1dvjrp4nxx60qhxeua02ta2fzpmb1d.<clinit>(MeasureCalculatorsHolder.java:146)
at measure.MeasureCalculatorsHolder.<clinit>(MeasureCalculatorsHolder.java:17)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:188)
at javassist.runtime.Desc.getClassObject(Desc.java:43)
at javassist.runtime.Desc.getClassType(Desc.java:152)
at javassist.runtime.Desc.getType(Desc.java:122)
at javassist.runtime.Desc.getType(Desc.java:78)
at algorithm.AllMeasuresDataTest.testGetMeasures(AllMeasuresDataTest.java:26)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.junit.internal.runners.TestMethod.invoke(TestMethod.java:66)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:312)
at org.junit.internal.runners.MethodRoadie$2.run(MethodRoadie.java:86)
at org.junit.internal.runners.MethodRoadie.runBeforesThenTestThenAfters(MethodRoadie.java:94)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.executeTest(PowerMockJUnit44RunnerDelegateImpl.java:296)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit49RunnerDelegateImpl$PowerMockJUnit49MethodRunner.executeTestInSuper(PowerMockJUnit49RunnerDelegateImpl.java:116)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit49RunnerDelegateImpl$PowerMockJUnit49MethodRunner.executeTest(PowerMockJUnit49RunnerDelegateImpl.java:77)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$PowerMockJUnit44MethodRunner.runBeforesThenTestThenAfters(PowerMockJUnit44RunnerDelegateImpl.java:284)
at org.junit.internal.runners.MethodRoadie.runTest(MethodRoadie.java:84)
at org.junit.internal.runners.MethodRoadie.run(MethodRoadie.java:49)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.invokeTestMethod(PowerMockJUnit44RunnerDelegateImpl.java:209)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.runMethods(PowerMockJUnit44RunnerDelegateImpl.java:148)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl$1.run(PowerMockJUnit44RunnerDelegateImpl.java:122)
at org.junit.internal.runners.ClassRoadie.runUnprotected(ClassRoadie.java:34)
at org.junit.internal.runners.ClassRoadie.runProtected(ClassRoadie.java:44)
at org.powermock.modules.junit4.internal.impl.PowerMockJUnit44RunnerDelegateImpl.run(PowerMockJUnit44RunnerDelegateImpl.java:120)
at org.powermock.modules.junit4.common.internal.impl.JUnit4TestSuiteChunkerImpl.run(JUnit4TestSuiteChunkerImpl.java:101)
at org.powermock.modules.junit4.common.internal.impl.AbstractCommonPowerMockRunner.run(AbstractCommonPowerMockRunner.java:53)
at org.powermock.modules.junit4.PowerMockRunner.run(PowerMockRunner.java:53)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.codecover.juniteclipse.runner.EclipseTestRunner.main(EclipseTestRunner.java:40)

Do you know how can I prevent this? I maybe there is another way to mock such a piece of code: 

ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
...
Enumeration<URL> resources = classLoader.getResources(path);
What is it you're trying to mock? And why?
First test is getters and setters test, I'm calling constructor there (and there the exception occurs). The second one is constructor test. I want to get control of what resources enumeration contains in third code snippet.
First of all, it looks to me that your tests are very tightly coupled to your implementation. By experience, this will lead to fragile tests. Preferrably, you want to think "black box" when writing your tests. "What is this piece of code supposed to do", rather than "How is this piece of code doing it". Second, I think you'd be better off just creating a set of resources and let the Java runtime deal with the classloading itself.
It is possible to create various sets of resources, as they where test cases?
Sure. The easiest for you is probably to parametrize the name of the resources. Then you can pass different resource names into your tests.

c
crandrad

Try adding this annotation to your Test class:

@PowerMockIgnore("javax.management.*")

Worked for me.

precision *"to your Test Class". Simple and usefull answer !
Can this be done by code or config as well? I could not find any way to do this. We have hundreds of tests ... i can not adjust them all.
@FredericLeitenberger see my answer below
Can you please also explain the intution and meaning of this fix? What instruction are we giving to PowerMockito using that line?
J
Jason D

Similar to the accepted response here, I ended up having to exclude all of the SSL related classes: 

@PowerMockIgnore({"javax.management.*", "org.apache.http.conn.ssl.*", "com.amazonaws.http.conn.ssl.*", "javax.net.ssl.*"})

Adding that to the top of my class resolved the error.

Still needed to add some more paths but you saved my life man! @PowerMockIgnore({"javax.management.*", "org.apache.http.conn.ssl.*", "com.amazonaws.*", "javax.net.ssl.*","com.sun.*"})
Good to know about com.sun too.
I needed the following: @PowerMockIgnore({"javax.management.*", "javax.crypto.*"})
This one saved me: @PowerMockIgnore({"javax.management.*", "org.apache.http.*", "com.amazonaws.http.conn.ssl.*", "javax.net.ssl.*", "com.sun.*", "javax.xml.*", "javax.crypto.*"})
L
Logan Wayne

Classloader conflict, use this: @PowerMockIgnore("javax.management.*")

Let mock classloader do not load javax.*. It works.

After using @PowerMockIgnore("javax.management.*"), the test class works well singly. But running as Junit test on that package got Failed to load ApplicationContext error. org.apache.catalina.LifecycleException: A child container failed during start and so on.
R
Rens Groenveld

This may be a bit of an old topic, but I have also ran into this problem. Turns out that some of the java versions cannot handle powermockito when powermock finds out there are 2 classes with the same name in the same package (over different dependencies).

With any version higher than Java 7_25 it gives this error.

"With any version higher than Java 7_25 it gives this error.", This is informative.
What it means: "cannot handle powermockito"? Is there any way to deal with it besides ignoring by annotation?
It's a long time ago, but I think we sorted it out by making sure there are not 2 classes with the same name in the same kind of package. Of course, if you have 2 libraries you depend on, and they reside in there... it is going to be difficult. I don't know if this issue has been fixed in the meantime.
u
user3474985

In PowerMock 1.7.0 a user-defined global configuration can be added to your project's classpath. PowerMockConfig 

org/powermock/extensions/configuration.properties

Simply add a line in the properties file like: 

powermock.global-ignore=javax.management.*

This will resolve the error for all the test classes in your project.

Note, spaces and double quotes are not allowed in the configuration.properties file. See: github.com/powermock/powermock/issues/989
t
try-catch-finally

In order to mock system classes, prepare the class that is the target of the test, not Thread.class. There's no way PowerMock will be able to instrument Thread.class because it is required during JVM startup - well before PowerMock can instrument.

The way instrumentation works, once a class is loaded, it can no longer be intstrumented.

See the PowerMock wiki.

D
David Georg Reichelt

Depending on your individual setup, it may be necessary to add more methods to @PowerMockIgnore. I stumbled over this with slf4j, for using PowerMock and slf4j together, you'll need 

@PowerMockIgnore({ "com.sun.org.apache.xerces.*", "javax.xml.*", "org.xml.*", "javax.management.*", "org.w3c.dom.*" })
Compared to other examples of @PowerMockIgnore , this one worked for me, because of the "org.xml.*" entry. Others with this issue should check, which signature issues arise from PowerMock and Ignore those packages with the annotation.

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