当对间谍使用doThrow时，Mockito不会引发正确的异常

Question

I'm using PowerMock (Mockito) to mock a subcall to another method in the same class. More specifically I have something like this:

public class myClass{
    public void MyMethod1(){
        //do something
        try{
            myMethod2();
        } catch (MyExceptionType e) {
            //do something
            throw e;
        }
    }

    public int MyMethod2() throws MyExceptionType {...}
}

Now in my unit tests, I'm able to mock responses of MyMethod2 using a spy, and doing something like doReturn(1).when(myClass).myMethod2() . However, something strange happens when I do something like this: doThrow(myExeptionType).when(myClass).myMethod2() . When I call myClass.myMethod1() during my test, it throws a NullPointerException, but the strange bit is that if I use a debugger and inspect throw e , e is the correct exception of type MyExceptionType.

Here's the stack trace of that NullPointerException:

java.lang.NullPointerException
    at java.util.Arrays$ArrayList.<init>(Arrays.java:2842)
    at java.util.Arrays.asList(Arrays.java:2828)
    at org.mockito.internal.exceptions.stacktrace.StackTraceFilter.filter(StackTraceFilter.java:31)
    at org.mockito.internal.exceptions.stacktrace.ConditionalStackTraceFilter.filter(ConditionalStackTraceFilter.java:23)
    at org.mockito.internal.invocation.realmethod.FilteredCGLIBProxyRealMethod.invoke(FilteredCGLIBProxyRealMethod.java:29)
    at org.mockito.internal.invocation.InvocationImpl.callRealMethod(InvocationImpl.java:108)
    at org.mockito.internal.stubbing.answers.CallsRealMethods.answer(CallsRealMethods.java:36)
    at org.mockito.internal.handler.MockHandlerImpl.handle(MockHandlerImpl.java:93)
    at org.mockito.internal.handler.NullResultGuardian.handle(NullResultGuardian.java:29)
    at org.mockito.internal.handler.InvocationNotifierHandler.handle(InvocationNotifierHandler.java:38)
    at org.mockito.internal.creation.MethodInterceptorFilter.intercept(MethodInterceptorFilter.java:51)
    at com.amazon.inventory.workflow.common.wrapper.FCContainerServiceWrapper$$EnhancerByMockitoWithCGLIB$$a0f00456.getContainerHierarchyDown(<generated>)
    at com.amazon.inventory.workflow.common.wrapper.containerservice.GetContainerHierarchyDownTest.runTest(GetContainerHierarchyDownTest.java:50)
    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:606)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.powermock.modules.junit4.rule.PowerMockStatement.evaluate(PowerMockRule.java:49)
    at org.junit.rules.ExpectedException$ExpectedExceptionStatement.evaluate(ExpectedException.java:110)
    at org.junit.rules.RunRules.evaluate(RunRules.java:18)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
    at org.junit.runners.Suite.runChild(Suite.java:128)
    at org.junit.runners.Suite.runChild(Suite.java:24)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:148)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:77)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:195)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:63)
    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:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Hopefully my question is not too confusing, thanks!

Answer 1

Your comments and subsequent answer have revealed the problem. You're trying to mock your exception object. Mockito was not designed to be able to do this. The reason is that exceptions are generally considered to be value objects. They carry around information - a message, a stack trace, sometimes a reference to a second exception; but as a general rule, they don't actually have any functionality.

The purpose of mocking any class is to get an object that has none of its own functionality, that is, none of its methods do anything, except where explicitly implemented within the test. But an exception already fits that criterion, so there is nothing to be gained by mocking it. The advice at http://www.mockobjects.com/2007/04/test-smell-everything-is-mocked.html is good advice indeed.

So, you have a couple of options, both of which will solve your problem nicely.

(1) Create a real exception and use that in your test. Depending on what constructors MyException has, this might look like this.

MyException toThrow = new MyException("testing");
doThrow(toThrow).when(someMock).someMethod();

(2) Let Mockito create the exception object for you, by just specifying its class in the doThrow call.

doThrow(MyException.class).when(someMock).someMethod();

Answer 2

I've found that the issue resided in the fact that mockito tries to filter out the stack trace of the exception thrown to remove the "EnhancedByMockito" strings appended to mocked class names. So basically I was doing this:

MyClass mySpy = Mockito.spy(MyClass.class);
MyException mockedException = Mockito.mock(MyException.class);
doThrow(mockedException).when(mySpy).someMethod();

Of course, in this example, mockedException.getStackTrace() would return null, which would then generate a null pointer exception when Mockito tried to filter the stack trace.

Hopefully this clarifies my question and could end up being useful to someone else.

To solve the issue, I simply mocked a stack trace for my exception like so:

throwableException = (Exception) mock(Class.forName(exceptionToThrow));
StackTraceElement[] mockedStackTrace = new StackTraceElement[0];
when(throwableException.getStackTrace()).thenReturn(mockedStackTrace);

Answer 3

Remember to assert your exception in your test method. Like by using the JUnit annotation:

@Test(expected = MyExceptionType.class)