JGit：在分支中的提交时读取文件的内容

Question

I want to read the content of a file at a certain commit in a certain branch: I am currently, using this code to read the content of a file at a certain commit ignoring the branch.

    public static String readContentOfFileAtCommit(String commitStr, String fileName)
        throws IOException {

    String content = null;
    ObjectId lastCommitId = currentRepo.resolve(commitStr);

    try (RevWalk revWalk = new RevWalk(currentRepo)) {
        RevCommit commit = revWalk.parseCommit(lastCommitId);
        RevTree tree = commit.getTree();

        try (TreeWalk treeWalk = new TreeWalk(currentRepo)) {
            treeWalk.addTree(tree);
            treeWalk.setRecursive(true);
            treeWalk.setFilter(PathFilter.create(fileName));
            if (!treeWalk.next()) {
                throw new IllegalStateException("Did not find expected file:" + fileName);
            }

            ObjectId objectId = treeWalk.getObjectId(0);
            ObjectLoader loader = currentRepo.open(objectId);
            content = new String(loader.getBytes());
        }

        revWalk.dispose();
    }

    return content;
}

My goal is to get the content of a file at a certain commit that is done on a certain branch.

Answer 1

Different from most older VCS tools branch, a branch in Git is simply a lightweight movable pointer to one of these commits. It does not "contain" any commits itself. In other words, it's only a simple file that contains the 40 character SHA-1 checksum of the commit it points to. There's also a very illustrative example in Git - Branches in a Nutshell :

As stated by Rüdiger Herrmann, while a commit is usually created on a branch, you cannot say that a commit "was done on a branch" after the fact. Branches may be added, deleted, renamed, or updated there after. For example, even if the branch or tag v1.0 is removed, the commit 98ca9 , 34ac2 , f30ab are still there, and reachable by master . I would suggest you to read the Pro Git (2nd Edition), Chapter 3.1 Git - Branches in a Nutshell for more detail.

As for JGit, here's my implementation of reading path for a specific commit:

private String getContent(RevCommit commit, String path) throws IOException {
  try (TreeWalk treeWalk = TreeWalk.forPath(git.getRepository(), path, commit.getTree())) {
    ObjectId blobId = treeWalk.getObjectId(0);
    try (ObjectReader objectReader = repo.newObjectReader()) {
      ObjectLoader objectLoader = objectReader.open(blobId);
      byte[] bytes = objectLoader.getBytes();
      return new String(bytes, StandardCharsets.UTF_8);
    }
  }
}