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Rudiger Herrmann

Getting Started with JGit

If you ever wondered how basic Git commands like git init, git checkout and so on are executed in JGit, read on.

This tutorial gives an overview of the most commonly used git commands and their counterparts in JGit. It walks through the steps to create a repository, fetch contents from a remote, add and remove files to/from the history, inspect the history, and finally push back the changes to the originating repository.

JGit provides an API that is similar to the Git high-level commands. Instead of

git commit -m "My first commit"

on the command line, you would write

git.commit().setMessage( "My first commit" ).call();

in JGit.

All JGit commands have a call() method that, after setting up the command is used to actually execute it. The classes are named after the respective Git command with the suffix Command. While some commands offer a public constructor, it is recommended to use the Git factory class to create command instances like shown in the above example.

Getting the Library

But before diving further into the JGit API, let’s get hold of the library first. The most common way to get JGit is probably from the Maven repository. But if you prefer OSGi bundles, then there is also a p2 repository for you. The download page lists the necessary information to integrate the library.

For the scope of this article it is sufficient to integrate what is referred to as the core library in project/bundle org.eclipse.jgit. If you are interested what else the is in the JGit source code repository, I recommand reading the Introduction to the JGit Sources

Creating a Repository

To start with, we need a repository. And to get hold of such thing we can either initialize a new repository or clone an existing one.

The InitCommand lets us create an empty repository. The following line

Git git = Git.init().setDirectory( "/path/to/repo" ).call();

will create a repository with a work directory at the location given to setDirectory(). The .git directory will be directly underneath in /path/to/repo/.git. For a detailed explanation of the InitCommand please read the Initializing Git Repositories with JGit article.

An existing repository can be cloned with the CloneCommand

Git git = Git.cloneRepository()
  .setURI( "https://github.com/eclipse/jgit.git" )
  .setDirectory( "/path/to/repo" )
  .call();

The code above will clone the JGit repository into the local directory ‘path/to/repo’. All options of the CloneCommand are expained in depth in How to Clone Git Repositories with JGit.

If you happen to have an existing local repository already that you wish to use, you can do so as described in How to Access a Git Repository with JGit.

Close Git When Done

Note that commands that return an instance of Git like the InitCommand or CloneCommand may leak file handles if they are not explicitly closed (git.close()) when no longer needed.

Fortunately, Git implements AutoCloseable so that you can use the try-with-resources statement.

Populating a Repository

Now that we have a repository, we can start populating its history. But in order to commit a file we first need to add it to the socalled index (aka staging area). Only files that were added (or removed from) the index will be considered by the commit command.

The JGit command therefore is – you guess it – the AddCommand.

DirCache index = git.add().addFilePattern( "readme.txt" ).call();

Consequently the above line adds the file readme.txt to the index. It is noteworthy that the actual contents of the file are copied to the index. This means that later modifications to the file will not be contained in the index, unless they are added again.

The path given to addFilePattern() must be relative to the work directory root. If a path does not point to an existing file, it is simply ignored.

Though the method name suggests that also patterns are accepted, the JGit support herefore is limited. Passing a ‘.’ will add all files within the work directory recursively. But fileglobs (e.g. *.java) as they are available in native Git are not yet supported.

The index returned by call(), in JGit named DirCache, can be examined to verify that it actually contains what we expect. Its getEntryCount() method returns the total number of files and getEntry() returns the entry at the specified position.

Now everything is prepared to use the CommitCommand in order to store the changes in the repository.

RevCommit commit = git.commit().setMessage( "Create readme file" ).call();

At least the message must be specified, otherwise call() will complain with a NoMessageException. An empty message, however is allowed. The author and committer are taken from the configuration if not denoted with the accordingly labelled methods.

The returned RevCommit describes the commit with its message, author, committer, time stamp, and of course a pointer to the tree of files and directories that constitute this commit.

In the same way that new or changed files need to be added, deleted files need to be removed explicitly. The RmCommand is the counterpart of the AddCommand and can be used in the same way (with contrary result of course).

DirCache index = git.rm().addFilepattern( "readme.txt" ).call();

The above line will remove the given file again. Since it is the only file within the repository, the returned index will return zero when asked for the number of entries in it.

Unless setCached( true ) was specified, the file will also be deleted from the work directory. Because Git does not track directories the RmCommand also deletes empty parent directories of the given files.

An attempt to remove a non-existing file is ignored. But unlike the AddCommand, the RmCommand does not accept wildcards in its addFilepattern() method. All files to be removed need to be specified individually.

And with the next commit these changes will be stored in the repository. Note that it is perfectly legal to create an empty commit, i.e. one that hasn’t had files added or removed before executed. Though I’m not aware of a decent use case.

State of a Repository

The status command lists files that have differences between either the index and the current HEAD commit or the work directory and the index or files that are not tracked by Git.

In its simplest form, the StatusCommand collects the status of all files that belong to the repository:

Status status = git.status().call();

The getters of the Status object should be self-explaining. They return the set of file names which are in the state that the method name describes. For example, after the readme.txt file was added to the index like shown previously, status.getAdded() would return a set that contains the path to the just added file.

If there are no differences at all and no untracked files either, Status.isClean() will return true. And as its name implies, returns Status.hasUncommittedChanges() true if there are uncommitted changes.

With addPath(), the StatusCommand can be configured to show only the status of certain files. The given path must either name a file or a directory. Non-existing paths are ignored and regular expressions or wildcards are not supported.

Status status = git.status().addPath( "documentation" ).call();

In the above example, the status of all files recursively underneath the ‘documentation’ directory will be computed.

Exploring a Repository

Now that the repository has a (small) history we will look into the command to list existing commits.

The simplest form of the git log counterpart of JGit allows to list all commits that are reachable from current HEAD.

Iterable<RevCommit> iterable = git.log().call();

The returned iterator can be used to loop over all commits that are found by the LogCommand.

For more advanced use cases I recommend to directly use the RevWalk API, the same class that is also used by the LogCommand. Apart from providing more flexibility it also avoids a possible resource leak that occurs because the RevWalk that is used internally by the LogCommand is never closed.

For example, its markStart() method can be used to also list commits that are reachable from other branches (or more generally speaking from other refs).

Unfortunately, only ObjectIds are accepted and therefore the desired refs need to be resolved first. An ObjectId in JGit encapsulates a SHA-1 hash that points to an object in Gits object database. Here, ObjectIds that point to commits are required and resolving in this context means to obtain the ObjectId that a particular ref points to.

Putting it all together, it looks like the snippet below:

Repository repository = git.getRepository()
try( RevWalk revWalk = new RevWalk( repository ) ) {
  ObjectId commitId = repository.resolve( "refs/heads/side-branch" );
  revWalk.markStart( revWalk.parseCommit( commitId ) );
  for( RevCommit commit : revWalk ) {
    System.out.println( commit.getFullMessage );
  }
}

The commit id to which the branch ‘side-branch’ points is obtained and then the RevWalk is instructed to start iterating over the history from there. Because markStart() requires a RevCommit, RevWalk’s parseCommit() is used to resolve the commit id into an actual commit.

Once the RevWalk is set up, the snippet loops over the commits to print the message of each commit.
The try-with-resource statement ensures that the RevWalk will be closed when done. Note that it is legal to call markStart() multiple times to include multiple refs into the traversal.

A RevWalk can also be configured to filter commits, either by matching attributes of the commit object itself or by matching paths of the directory tree that it represents. If known in advance, uninteresting commits and their ancestry chain can be excluded from the output. And of course the output can be sorted, for example by date or topologically (all children before parents). But these features are outside of the scope of this article but may be covered in a future article of its own.

Exchanging with a Remote Repository

Often a local repository was cloned from a remote repository. And the changes that were made locally should ultimately be published to the originating repository. To accomplish this there is the PushCommand, the counterpart of git push.

The simplest form will push the current branch to its corresponding remote branch.

Iterable<PushResult> iterable = local.push().call();
PushResult pushResult = iterable.iterator().next();
Status status 
  = pushResult.getRemoteUpdate( "refs/heads/master" ).getStatus();

The command returns an iterable of PushResults. In the above case the iterable holds a single element. To verify that the push succeeded, the pushResult can be asked to return a RemoteRefUpdate for a given branch.

A RemoteRefUpdate describes in detail what was updated and how it was updated. But it also has a status attribute that summarizes the outcome. And if the status returns OK, we can rest assured that the operation succeeded.

Even though the command works without giving any advice, it has plenty of options where in the following only the more commonly used are listed. By default the command pushes to the default remote called ‘origin’. Use setRemote() to specify the URL or name of a different remote repository. If other branches than the current one should be pushed refspecs can be specified with setRefSpec(). Whether tags should also be transferred can be controlled with setPushTags(). And finally, if you are uncertain whether the outcome is desired, there is a dry-run option that allows to simulate a push operation.

Now that we have seen how to transfer local objects to a remote repository we will look a how the opposite direction works. The FetchCommand can be used much like its push counterpart and also succeeds with its default settings.

FetchResult fetchResult = local.fetch().call();
TrackingRefUpdate refUpdate 
  = fetchResult.getTrackingRefUpdate( "refs/remotes/origin/master" );
Result result = refUpdate.getResult();

Without further configuration, the command fetches changes from the branch that corresponds to the current branch on the default remote.

The FetchResult provides detailed information about the outcome of the operation. For each affected branch, a TrackingRefUpdate instance can be obtained. Most interesting probably is the return value of getResult() that summarizes how the update turned out. In addition it holds information about which local ref (getLocalBame()) was updated with which remote ref (getRemoteName()) and to which object id the local ref pointed before and after the update (getOldObjectId() and getNewObjectid()).

If the remote repository requires authentication, the PushCommand and FetchCommand can be prepared in the same way as all commands that communicate with remote repositories.

Concluding Getting Started with JGit

Now it is your turn to tkae JGit for a spin. The high-level JGit API isn’t hard to understand. If you konw what git command to use, you can easily guess which classes and methods to use in JGit.

While not all subtleties of the of the Git command line are available there is solid support for the most often used functionalities. And if there is something crucial missing, you can often resort to the lower-level APIs of JGit to work around the limitation.

The snippets shown throughout the article are excerpts of a collection of learning tests.

If you still have difficulties or questions, please leave a comment or ask the friendly and helpful JGit community for assistance.

Reference: Getting Started with JGit from our JCG partner Rudiger Herrmann at the Code Affine blog.
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