Make a single file Java jar launcher

jar-launcher.md

This is a little trick to turn an executable Java jar.

It works on all unixy like systems including Linux, MacOS, Cygwin, and Windows Linux subsystem.

Step 1 Start with an all-in-one jar (with entry point and all deps within)

$ ls 
hello.jar

$ java -jar hello.jar
Hello from Java

Step 2: Prepend launcher to beginning of jar file

Add a shell script to the header of the jar, rename to anything, and make it executable

$ (echo '#!/bin/sh' && echo 'exec java -jar $0 "$@"' && cat hello.jar) > hello
$ chmod +x hello
$ rm hello.jar

$ ls
hello

Step 3: Deploy a single all-in-one executable launcher

$ ./hello
Hello from Java

This single file can be copied to a server. The only dependency is it expects to find a java JVM in the PATH.

How does it work???

This relies on a little known trick about the Zip format allowing arbitrary data to be prepended/appended around the main zip file.

So:

• The file looks like a shell script. Executing it will run it
• The shell script calls exec java -jar [myfilename] [args...] replacing the current process
• The Java VM loads, loads the same script as a Jar and happily ignores the initial shell script because Zip allows that

Bonus: It also works with Python!

Yep. This makes it much simpler to distribute Python tools without having to worry about pip, virtual-env, or Conda. Just ship a single self-contained executable.

In Python you use .pex files, which like .jar files are just Zips. See PEP-441.

Create a Zip containing all your Python files (and dependencies). Include __main__.py as the entry point. Prepend #!/usr/bin/env python to the Zip, make executable, remove the .zip suffix. This is easy to script, but if you want something out of the box there's an existing tool

Can it work if i use it to call .java ?