Embedding JGitAlex Blewitt

@alblue

Level Zero

• JGit is a command line-program

• java -jar jgit.sh ...

• ./jgit.sh ...

• System.getRuntime().exec(“java -jar jgit.sh”)

JGit.sh is a shell script with an additional

!Not ‘embedded’ - but useful for memory constrained or GC sensitive applications

Level Zero• Advantages

• You already know how to use this

• No new commands needed

• Simple

• Useful if in-process memory is limited

• Disadvantages

• No re-use between runs

• Spawns a new JVM each time

• Have to parse the results via text stream

Level One

• The jgit command line processor is in the ‘pgm’ jar and can be invoked directly

• org.eclipse.jgit.pgm.Main. main(new String[] {...})

• --git-dir /path/to/.git ls-tree HEAD

• --git-dir /path/to/.git show HEAD

Level One• add• archive• blame

• branch• checkout• clone

• commit• config

• daemon• diff• diff-tree

• fetch• gc• glog

• init• log• ls-remote

• ls-tree• merge• push

• reflog• reset

• rev-list• rev-parse• rm

• show• show-ref• status

Level One• Advantages

• Easy to remember

• Uses existing commands

• In-process allows for repeated runs

• Disadvantages

• High level

• Have to parse output

• Does not allow for optimisations between runs

Level Two

• Create/use Git and built-in porcelain commands

• Git git = org.eclipse.jgit.api.Git. open(new File(“/path/to/.git”))

• git.clean()

• git.log()

• git.lsRemote()

Level Two

• Commands use builder pattern

• git.clean().setCleanDirectories(true). setIgnore(true).call()

• git.lsRemote().setRemote().setTags(true). setHeads(true).call()

i Builder pattern allows for new ‘arguments’ to be added over time

Level Two• Advantages

• Allows commands to be compile-time checked

• Does not involve text processing

• Can interpret/process results

• Can invoke many commands on repo

• Disadvantages

• Required arguments may be missing

• Limited to provided command API

• May be more optimal to go deeper in some cases

Level Three

• Work directly with the repository

• repository = FileRepositoryBuilder.create( new File(“...”))

• builder also handles cases like GIT_ environment variables and .git in parent directories

• Repository provides object and ref databases

Level Three

• repository.getTags()

• repository.getAllRefs()

• repository.getBranch() (current branch)

• repository.getRef(...)

• HEAD = repository.getRef(“HEAD”)

• repository.open(HEAD.getObjectId()). copyTo(System.out)

Level Three• Advantages

• Can use caches like RepositoryCache

• Can work with/update references directly

• Disadvantages

• Limited direct API on repository

• Have to work with lower level APIs

Level Four

• Repositories are processed by walkers

• Git repository content

• References point to Commits (and tags,refs)

• Commits point to Commits and Trees

• Trees point to Trees and Blobs

• Think of it as a Commit Iterator (RevWalk) or Directory/File Iterator (TreeWalk)

Level FourRevWalk rw = new RevWalk(repository);

HEAD = repository.resolve(“HEAD”)

rw.markStart(rw.parseCommit(HEAD))

Iterator<RevCommit> it = rw.iterator()

while(it.hasNext()) RevCommit commit = it.next() System.out.println( commit.abbreivate(6) .name() + “ ” + commit.getShortMessage())

Level FourTreeWalk tw = new TreeWalk(repository);

tree = repository.resolve(“HEAD^{tree}”)

tw.addTree(tree) // tree ‘0’

tw.setRecursive(true)

tw.setFilter(PathFilter.create(“some/file”))

while(tw.next()) id = tw.getObjectId(0) repository.open(id).copyTo(System.out)

Level Four• Advantages

• Can construct complex filters

• Can walk commits between ranges

• Can walk multiple trees at once (e.g. for diffing)

• Disadvantages

• Lacks a simple API to ‘get this file’

• Seems confusing at first

• Dispose to release resources before re-use

!Walkers are not thread safe, so create

separate ones if needed

Level Five

• ObjectInserter and ObjectReader are used to put and get data from repositories

• id = repository.newObjectInserter( Constants.OJB_BLOB, “hello world”.getBytes(“UTF-8”))

• repository.newObjectReader().open(id). copyTo(System.out)

Level Five• Advantages

• Ultimate flexibility

• Can store any content needed

• Use a ‘notes-like’ approach to store additional metadata

• Disadvantages

• Complex to use

• Need to build trees and commits to prevent being garbage collected

Levels of Embedding0. System.exec(“java -jar jgit.sh ...”)

1. Main.main([]“--git-dir”, “/path/.git”, “...”)

2. Git.open(new File(“.../.git”)).clean().call()

3. FileRepositoryBuilder.create(...).getRef()

4. new TreeWalk/RevWalk(repository)

5. repository.newObjectInserter/Reader

ThankyouAlex Blewitt

@alblue

Winners of Eclipse 4 Plug-in development

Vincenzo Caselli@vcaselli

Lorenzo Bettini@lorenzo_bettini