Mocking a class object using Mockito and PowerMockito

Question

Is it possible to mock a class object using Mockito and/or PowerMockito?

Something like:

Class<Runnable> mockRunnableClass = mock(Class<Runnable>.class);

Answer 1

First, as stated in the comments, you would need to do:

Class<Runnable> mockRunnableaClass = (Class<Runnable>)mock(Class.class);

But that won't work in the usual way because of a limitation with PowerMock. You cannot simply mock classes in from java.lang, java.net, java.io or other system classes because they're loaded by Java's bootstrap classloader and cannot be byte-code manipulated by PowerMock's classloader. (See PowerMock FAQ #4.) As of PowerMock 1.2.5, you can work around this. If the class you wanted to test was this:

public class ClassToTest {
  private Class<Runnable> runnableClass;

  public void setRunnableClass(Class<Runnable> runnableClass) {
    this.runnableClass = runnableClass;
  }

  public Runnable foo() {
    return runnableClass.newInstance();
  }
}

Then you would do this:

@RunWith(PowerMockRunner.class)
@PrepareForTest({ ClassToTest.class }) // Prepare the calling class for test
public class SystemClassUserTest {

  @Test
  public void testFoo() throws Exception {
    Class<Runnable> mockClass = (Class<Runnable>) mock(Class.class);
    Runnable mockRunnable = mock(Runnable.class);

    ClassToTest objectUT = new ClassToTest();
    objectUT.setRunnableClass(mockClass);

    when(mockClass.newInstance()).thenReturn(mockRunnable);
    assertThat(objectUT.foo(), is(sameInstance(mockRunnable);
  }
}

Answer 2

why not using an agent if you can't refactor the code there isn't many options, as @jherics mentionned, java system classes are loaded by the bootstrap classloader and powermock can't redefine their bytecode.

However Powermock now coms with an agent, that will allow system classes mock. Check here for complete explanation.

The main idea is to modify your java command and add :

-javaagent: path/to/powermock-module-javaagent-1.4.12.jar

The basic thing this agent is doing is to definalize classes, to allow future mocking in a specific test, that's why you'll need to use specific types to communicate with the agent, for example with JUnit :

@Rule PowerMockRule rule = new PowerMockRule(); // found in the junit4 rule agent jar

TestNG is also supported. Just check the wiki page for more information.

Hope that helps.