Java .gitignore template

*.class is the compiled bytecode for each source file. It is produced by the compiler from the .java file next to it, it changes whenever the compiler version changes, and a committed one will shadow the source in confusing ways when the two drift apart. The build and .gradle directories hold the same idea at a larger scale, plus the incremental compilation state that only makes sense on the machine that produced it.

The archive rules cover the packaged outputs: jar, war, ear, nar. These are the deliverables, produced by the build from the sources, and a release process that publishes them to an artifact repository has no reason to also keep them in git. The same rules also catch zip, tar.gz and rar, which is where the template starts being broader than it looks.

The three negation lines are the interesting part. Ignoring *.jar would also ignore gradle/wrapper/gradle-wrapper.jar, and that file is not a build output at all: it is the small bootstrapper that lets somebody who has never installed Gradle run ./gradlew and get the exact version the project expects. The negation puts it back. The other two negations protect source directories that happen to contain a folder named build.

hs_err_pid and replay_pid files are crash dumps written by the virtual machine when it dies hard. They contain the full command line, environment and loaded module list of the machine that crashed, which makes them both large and mildly revealing, and they are only ever useful to the person debugging that specific crash.

*.ctxt and .mtj.tmp come from teaching and profiling tools respectively, and are harmless leftovers to keep in the list. What the template does not cover is a local configuration file holding a database password, which in Java projects tends to live in an application properties file under src/main/resources.