Java – EasyMock expectations with void methods

easymockjavaunit testing

I'm using EasyMock to do some unit tests and I don't understand the usage of EasyMock.expectLastCall(). As you can see in my code below, I have an object with a method that returns void getting called in some other object's method. I would think that I have to make EasyMock expect that method call, but I tried commenting out the expectLastCall() invocation and it still works. Is it because I passed EasyMock.anyObject()) that it registered it as an expected call or is there something else going on?

MyObject obj = EasyMock.createMock(MyObject.class);
MySomething something = EasyMock.createMock(MySomething.class);
EasyMock.expect(obj.methodThatReturnsSomething()).andReturn(something);

obj.methodThatReturnsVoid(EasyMock.<String>anyObject());

// whether I comment this out or not, it works
EasyMock.expectLastCall();

EasyMock.replay(obj);

// This method calls the obj.methodThatReturnsVoid()
someOtherObject.method(obj);

The API doc for EasyMock says this about expectLastCall():

Returns the expectation setter for the last expected invocation in the current thread. This method is used for expected invocations on void methods.

Best Answer

This method returns you the handle of expectation through IExpectationSetters; which gives you ability to validate(assert) that your void method was called or not and related behaviors e.g.

EasyMock.expectLastCall().once();
EasyMock.expectLastCall().atLeastOnce();
EasyMock.expectLastCall().anyTimes();

Detailed API of the IExpectationSetters is here.

In your example you are just getting the handle and not doing anything with it hence you don't see any impact of having or removing the statement. It's very same as you call some getter method or declare some variable and don't use it.

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Java – EasyMock: Void Methods

If I understand what you want to do correctly, you should be able to use andAnswer():

mockObject.someMethod(eq(param1), eq(param2));
expectLastCall().andAnswer(new IAnswer() {
    public Object answer() {
        //supply your mock implementation here...
        SomeClass arg1 = (SomeClass) getCurrentArguments()[0];
        AnotherClass arg2 = (AnotherClass) getCurrentArguments()[1];
        arg1.doSomething(blah);
        //return the value to be returned by the method (null for void)
        return null;
    }
});

The EasyMock User Guide explains:

Creating Return Values or Exceptions

Sometimes we would like our mock object to return a value or throw an exception that is created at the time of the actual call. Since EasyMock 2.2, the object returned by expectLastCall() and expect(T value) provides the method andAnswer(IAnswer answer) which allows [you] to specify an implementation of the interface IAnswer that is used to create the return value or exception.

Inside an IAnswer callback, the arguments passed to the mock call are available via EasyMock.getCurrentArguments(). If you use these, refactorings like reordering parameters may break your tests. You have been warned.

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Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Java – EasyMock: Void Methods

Creating Return Values or Exceptions

Related Topic