Jgit 提取出两个版本之间的差异文件

要提取出两个版本之间的差异文件，可以使用 JGit 库中的 `DiffCommand` 类。以下是一个简单的示例代码，可以提取出两个版本之间的差异文件：

Git git = new Git(repository);
ObjectId oldVersion = ObjectId.fromString("commit_id_1");
ObjectId newVersion = ObjectId.fromString("commit_id_2");

DiffCommand diffCommand = git.diff();
diffCommand.setOldTree(prepareTreeParser(repository, oldVersion));
diffCommand.setNewTree(prepareTreeParser(repository, newVersion));
List<DiffEntry> diffs = diffCommand.call();

for (DiffEntry diff : diffs) {
    System.out.println("Diff: " + diff.getChangeType() + ": " + diff.getOldPath() + " -> " + diff.getNewPath());
    // 获取差异文件的内容
    ObjectReader reader = repository.newObjectReader();
    CanonicalTreeParser oldTreeIter = new CanonicalTreeParser();
    oldTreeIter.reset(reader, diff.getOldId());
    CanonicalTreeParser newTreeIter = new CanonicalTreeParser();
    newTreeIter.reset(reader, diff.getNewId());
    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    DiffFormatter formatter = new DiffFormatter(outputStream);
    formatter.setRepository(repository);
    formatter.format(diff);
    System.out.println("Content: " + outputStream.toString());
}

其中，`prepareTreeParser` 方法可以用来准备一个树对象的解析器。示例代码如下：

private static AbstractTreeIterator prepareTreeParser(Repository repository, ObjectId objectId) throws IOException {
    try (RevWalk walk = new RevWalk(repository)) {
        RevCommit commit = walk.parseCommit(objectId);
        RevTree tree = walk.parseTree(commit.getTree().getId());
        CanonicalTreeParser treeParser = new CanonicalTreeParser();
        try (ObjectReader reader = repository.newObjectReader()) {
            treeParser.reset(reader, tree.getId());
        }
        walk.dispose();
        return treeParser;
    }
}

注意，在使用 JGit 库之前，需要先打开一个 Git 仓库的对象。可以使用以下代码来打开 Git 仓库对象：

FileRepositoryBuilder builder = new FileRepositoryBuilder();
Repository repository = builder.setGitDir(new File("/path/to/.git"))
        .readEnvironment()
        .findGitDir()
        .build();

其中 `/path/to/.git` 是 Git 仓库的目录。