How to show changes between commits with JGit

To obtain the tree of the head commit, call

git.getRepository().resolve( "HEAD^{tree}" )

and to obtain the tree of the parent of the HEAD commit, call

git.getRepository().resolve( "HEAD~1^{tree}" )

Search for 'Git caret and tilde' if you are interested in more details.

To summarize, here goes a snippet that computes the diff of two commits:

File file = new File( git.getRepository().getWorkTree(), "file.txt" );
writeFile( file, "first version" );
RevCommit newCommit = commitChanges();
writeFile( file, "second version" );
RevCommit oldCommit = commitChanges();

ObjectReader reader = git.getRepository().newObjectReader();
CanonicalTreeParser oldTreeIter = new CanonicalTreeParser();
ObjectId oldTree = git.getRepository().resolve( "HEAD^{tree}" ); // equals newCommit.getTree()
oldTreeIter.reset( reader, oldTree );
CanonicalTreeParser newTreeIter = new CanonicalTreeParser();
ObjectId newTree = git.getRepository().resolve( "HEAD~1^{tree}" ); // equals oldCommit.getTree()
newTreeIter.reset( reader, newTree );

DiffFormatter df = new DiffFormatter( new ByteArrayOutputStream() ); // use NullOutputStream.INSTANCE if you don't need the diff output
df.setRepository( git.getRepository() );
List<DiffEntry> entries = df.scan( oldTreeIter, newTreeIter );

for( DiffEntry entry : entries ) {
  System.out.println( entry );
}

private RevCommit commitChanges() throws GitAPIException {
  git.add().addFilepattern( "." ).call();
  return git.commit().setMessage( "commit message" ).call();
}

private static void writeFile( File file, String content ) throws IOException {
  FileOutputStream outputStream = new FileOutputStream( file );
  outputStream.write( content.getBytes( "UTF-8" ) );
  outputStream.close();
}

For further considerations about showing changes between commits, you may want to read this in-depth discussion of JGit's diff APIs that can be found here: http://www.codeaffine.com/2016/06/16/jgit-diff/

I am using the following code to print diffs between two commits. Using DiffFormatter.scan seems simpler than the other solutions I have seen on SO.

It might be that I am missing some case(s) where this is not supported, but it works fine for our use case.

public static void main(String[] args) throws Exception {
    Repository repository = new FileRepositoryBuilder()
            .setGitDir(new File("c:/temp/jgit-test/.git")).build();
    // Here we get the head commit and it's first parent.
    // Adjust to your needs to locate the proper commits.
    RevCommit headCommit = getHeadCommit(repository);
    RevCommit diffWith = headCommit.getParent(0);
    FileOutputStream stdout = new FileOutputStream(FileDescriptor.out);
    try (DiffFormatter diffFormatter = new DiffFormatter(stdout)) {
        diffFormatter.setRepository(repository);
        for (DiffEntry entry : diffFormatter.scan(diffWith, headCommit)) {
            diffFormatter.format(diffFormatter.toFileHeader(entry));
        }
    }
}

private static RevCommit getHeadCommit(Repository repository) throws Exception {
    try (Git git = new Git(repository)) {
        Iterable<RevCommit> history = git.log().setMaxCount(1).call();
        return history.iterator().next();
    }
}