Clone a git repository into an InMemoryRepository with JGit

There is a proper solution without any files on disk, even temporary:

    // sample usage
    private void loadFromGit() throws Exception {
        ObjectLoader loader = loadRemote("https://github.com/msangel/promisified-resource-loader", "master", "README.md");
        loader.copyTo(System.out);
    }

    private ObjectLoader loadRemote(String uri, String branch, String filename) throws Exception {
        DfsRepositoryDescription repoDesc = new DfsRepositoryDescription();
        InMemoryRepository repo = new InMemoryRepository(repoDesc);
        Git git = new Git(repo);
        git.fetch()
                .setRemote(uri)
                .setRefSpecs(new RefSpec("+refs/heads/*:refs/heads/*"))
                .call();
        repo.getObjectDatabase();
        ObjectId lastCommitId = repo.resolve("refs/heads/"+ branch);
        RevWalk revWalk = new RevWalk(repo);
        RevCommit commit = revWalk.parseCommit(lastCommitId);
        RevTree tree = commit.getTree();
        TreeWalk treeWalk = new TreeWalk(repo);
        treeWalk.addTree(tree);
        treeWalk.setRecursive(true);
        treeWalk.setFilter(PathFilter.create(filename));
        if (!treeWalk.next()) {
            return null;
        }
        ObjectId objectId = treeWalk.getObjectId(0);
        ObjectLoader loader = repo.open(objectId);
        return loader;
    }

So, actually you have TreeWalk here (which is default to recursive), but you can change this and manually iterate all from repo.

The CloneCommand will always create a file-based repository. The lines from your post that create an InMemoryRepository have no effect on the clone command.

I suggest to clone into a temporary location if you only need the repository to make a change and push the result.

For example:

Git.cloneRepository().setURI( ... ).setDirectory( new File("/path/to/empty/dir" ) ).call();  

An InMemoryRepository in JGit still requires a work directory for operations like checkout, etc. Only the object database which is usually found in the .git directory is stored in memory