RevCommit JGit mock

Question

My question is about mocking a RevCommit object from the JGit library. When simply mocking an object of this type and defining it's behaviour, I get an error. For example:

RevCommit revCommitMock = mock(RevCommit.class);        
Mockito.when(revCommitMock.getShortMessage()).thenReturn("ExampleMessage");

This will result in a NullPointerException.

Probably, the way to do it correctly is calling method:

parse(RevWalk rw, byte[] raw) 

on an instance of RevCommit object, but how to do it properly? I get NullPointerException parsing mocked object of type RevWalk. Thanks for any help in advance.

Answer 1

To avoid the NullPointerException, you would need to fully simulate the behaviour of a real RevCommit. To get its behaviour right in all aspects, you would end up re-building large parts of the actual RevCommit implementation in JGit.

This is why I recommend to run tests against a real repository. For said reasons, it is usually not advisable to mock types that you don't own (see https://github.com/mockito/mockito/wiki/How-to-write-good-tests).

And with JGit it is very simple to create test-repositories:

public class JGitTest {

  @Rule
  public TemporaryFolder tempFolder = new TemporaryFolder();
  private Git git;

  @Before
  public void setUp() throws GitAPIException {
    git = Git.init().setDirectory(tempFolder.getRoot()).call();
  }

  @After
  public void tearDown() {
    git.getRepository().close();
  }

  @Test
  public void testFirst() {
    // with 'git' you have access to a fully functional repository
  }
}

See here for another example that initializes a repository and uses a TemporaryFolder to clean up: https://gist.github.com/rherrmann/5341e735ce197f306949fc58e9aed141