无法使用 Spock 模拟 Java 方法调用

Question

I am trying to mock my following java method using Spock.

        public List<DiffEntry> listDifferences(String oldCommit, String newCommit, Git git)
          throws GitAPIException, RevisionSyntaxException, AmbiguousObjectException,
          IncorrectObjectTypeException, IOException {

        logger.info(
            "Inside DiffCommits#listDifferences to calculating difference commits refs {}  and {} ",
            oldCommit, newCommit);

        ObjectId oldTree = git.getRepository().resolve(oldCommit);
        ObjectId newTree = git.getRepository().resolve(newCommit);

        if (oldTree == null || newTree == null) {
          logger.warn(
              "Could not resolve either old {} or new commits {}, difference cant not be calculated",
              oldCommit, newCommit);
          throw new RefNotFoundException("Unable to resolve tag reference. Invalid tag provided");
        }

        ObjectReader reader = git.getRepository().newObjectReader();
        CanonicalTreeParser oldTreeIter = new CanonicalTreeParser();
        oldTreeIter.reset(reader, oldTree);

        CanonicalTreeParser newTreeIter = new CanonicalTreeParser();
        newTreeIter.reset(reader, newTree);

        DiffFormatter df = new DiffFormatter(new ByteArrayOutputStream());

        df.setRepository(git.getRepository());
        List<DiffEntry> entries;
        entries = df.scan(newTreeIter, oldTreeIter);
        df.close();

        if (logger.isDebugEnabled()) {
          for (int i = 0; i < entries.size(); i++) {
            logger.debug("Entry: " + entries.get(i));
          }
        }

        return entries;
      }

Everything is working fine but the mocking of following code get failed

DiffFormatter df = new DiffFormatter(new ByteArrayOutputStream());

And getting error at

df.setRepository(git.getRepository());

The error that I am getting is

> org.eclipse.jgit.lib.StoredConfig$$EnhancerByCGLIB$$9a2f8398 cannot be
> cast to org.eclipse.jgit.diff.DiffConfig java.lang.ClassCastException:
> org.eclipse.jgit.lib.StoredConfig$$EnhancerByCGLIB$$9a2f8398 cannot be
> cast to org.eclipse.jgit.diff.DiffConfig  at
> org.eclipse.jgit.diff.DiffFormatter.setReader(DiffFormatter.java:201)
>   at
> org.eclipse.jgit.diff.DiffFormatter.setRepository(DiffFormatter.java:180)
>   at
> com.sf.bt.mdm.subscription.scmdiff.DiffCommits.listDifferences(DiffCommits.java:65)
>   at com.sf.bt.mdm.subscription.service.DiffCommitSpec.test list
> differences(DiffCommitSpec.groovy:59)

Any kind of help will be appreciated

Answer 1

DiffFormatter df = new DiffFormatter(new ByteArrayOutputStream());

This code is problematic from the point of view of unit testing. Its not a dependency so it can't be injected in the class. So basically you're trying to mock a constructor ( new ) object.

Technically the only way you can try is using Global Mocks and create spies wrapping the real object: Read the documentation . These are not regular spock's Mocks and Stubs.

However I believe there are other approaches. After all the link I've sent you starts with "think twice before using..." for a reason ;)

1. You've by yourself decided to put this object creation inside the method, which is ok. However this means that you do not treat this DiffFormatter as a real dependency. So maybe even in unit test you should let it run, concentrating on mocking its input parameters themselves? Just the same holds also for ByteArrayOutputStream . This plays nicely in general with the concept that the code should be written in a way that unit tests should not really care about the internal implementation of the methods. Always prefer blackbox testing to white-box testing.

2. Alternatively if you think its a dependency, then maybe you should make it a data field. Then You could inject it into the test with a regular mocks. If this class should be created per method invocation, you can inject Supplier<DiffFormatter> and in method just call it's get method, it will act as a factory.