如何使用 JGit API 从父提交中获取树？

Question

For a given commit I want to get the parent(s) commit tree so I can go on to compare the changes. I am finding that getTree() on the parent RevCommit objects always returns null.

    ObjectId lastCommitId = repository.resolve(Constants.HEAD);

    RevWalk revWalk = new RevWalk(repository);
    RevCommit commit = revWalk.parseCommit(lastCommitId);

    List<RevCommit> parents = new ArrayList<>();
    for(RevCommit parent : commit.getParents()) {
        parents.add(parent);
    }

    if ( parents.get(0).getTree() == null ) {
        System.out.println("first parent tree was null");
    }

Am i going about this the wrong way? Are the parent objects a shallow copy and I have to do something to populate the tree property?

I did get it to work as follows but would still like to know if this is the right way to do it.

    ObjectId lastCommitId = repository.resolve(Constants.HEAD);

    RevWalk revWalk = new RevWalk(repository);
    RevCommit commit = revWalk.parseCommit(lastCommitId);

    List<RevCommit> parents = new ArrayList<>();
    for(RevCommit parent : commit.getParents()) {

        RevCommit deepCopy = revWalk.parseCommit(parent.getId());

        parents.add(deepCopy);

    }

    if ( parents.get(0).getTree() != null ) {
        System.out.println(parents.get(0).getTree().toString());
    } else {
        System.out.println("first parent tree was null");
    }

Answer 1

Your second approach is right. commit.getParents() returns incomplete RevCommit s. While their ID attribute is set, all other attributes (tree, message, author, committer, etc.) are not. Hence the NullPointerException . In order to actually use the parent commit you need to first parse the commit header, either with parseCommit like you did

parentCommit = revWalk.parseCommit(parentCommit.getId());

or with parseHeaders

revWalk.parseHeaders(parentCommit);