Download only a single file from a remote git branch using Jgit

Question

I am trying to download a single file(myFile.xml) from a remote git branch without cloning all the repository, i found this code but it doesent work the error is in getCO;ponent function exctly in RevCommit commit = revWalk.parseCommit(lastCommitId);

public void getComponent() throws IOException
      {
        File repoDir = new File("https://**.git");
           // open the repository
           Repository repository = new FileRepository(repoDir);
           // find the HEAD
           ObjectId lastCommitId = repository.resolve("47100a898d1c76558e49d3f292b8e7a6d052fe51");
           System.out.println("lastcommit to string "+lastCommitId.toString());

           // now we have to get the commit
           RevWalk revWalk = new RevWalk(repository);
           RevCommit commit = revWalk.parseCommit(lastCommitId);
           // and using commit's tree find the path
           RevTree tree = commit.getTree();
           TreeWalk treeWalk = new TreeWalk(repository);
           treeWalk.addTree(tree);
           treeWalk.setRecursive(true);
           treeWalk.setFilter(PathFilter.create("abcd/myFile.xml"));
           if (!treeWalk.next()) {
               throw new IllegalStateException("Unable to download file.");
               }
           ObjectId objectId = treeWalk.getObjectId(0);
           ObjectLoader loader = repository.open(objectId);

           // and then one can use either
           InputStream in = loader.openStream();
           // or
           loader.copyTo(System.out);
  }  

Exception in thread "main" org.eclipse.jgit.errors.MissingObjectException: Missing unknown 47100a898d1c76558e49d3f292b8e7a6d052fe51
    at org.eclipse.jgit.internal.storage.file.WindowCursor.open(WindowCursor.java:168)
    at org.eclipse.jgit.lib.ObjectReader.open(ObjectReader.java:236)
    at org.eclipse.jgit.revwalk.RevWalk.parseAny(RevWalk.java:890)
    at org.eclipse.jgit.revwalk.RevWalk.parseCommit(RevWalk.java:800)
    at testGit.Authenticate.getComponent(Authenticate.java:138)
    at testGit.Authenticate.main(Authenticate.java:90)

Answer 1

Git protocols do not allow to do this directly, access to remote repositories is limited by design. One option with JGit would be to clone the repository in-memory only via InMemoryRepository, however this is probably only useful for smaller repositories as it might use up lots of memory otherwise.

See also the full snippet in the jgit-cookbook