How to make a JAR file Linux executable

Every Java programmer knows - or should known - that it is possible to create a runnable Java package archive (JAR), so that in order to launch an application it is enough to specify the jar file name on the Java interpreter command line along with the -jar parameter. For example:

$ java -jar helloworld.jar

There are plenty of tutorials showing how to implement this feature using Ant, Maven, Eclipse, Netbens, etc.

Anyway in its basic form, it just requires to add a MANIFEST.MF file to the jar package. The manifest must contain an entry Main-Class that specifies which is the class defining the main method for your application. For example:

$ javac HelloWorld.java
$ echo Main-Class: HelloWorld > MANIFEST.MF
$ jar -cvmf MANIFEST.MF helloworld.jar HelloWorld.class 

But this still requires your users to invoke the Java interpreter with the -jar option. There are many reasons why it would be preferable to have your app runnable by simply invoking it on the terminal shell like any other command.

Here comes the protip!

This technique it is based on the ability to append a generic binary payload to a Linux shell script. Read more about this here: http://www.linuxjournal.com/content/add-binary-payload-your-shell-scripts

Taking advantage of this possibility the trick is just to embed a runnable jar file into a Bash script file. The script when executed will launch the Java interpreter specifying itself as the jar to run. Too complex? Much more easier to do in practice, than to explain!

Let's say that you have a runnable jar named helloworld.jar

Copy the Bash script below to a file named stub.sh

#!/bin/sh
MYSELF=`which "$0" 2>/dev/null`
[ $? -gt 0 -a -f "$0" ] && MYSELF="./$0"
java=java
if test -n "$JAVA_HOME"; then
    java="$JAVA_HOME/bin/java"
fi
exec "$java" $java_args -jar $MYSELF "$@"
exit 1 

Than append the jar file to the saved script and grant the execute permission to the file resulting with the following command:

cat stub.sh helloworld.jar > hello.run && chmod +x helloworld.run 

That's all!

Now you can execute the app just typing helloworld.run on your shell terminal.

The script is smart enough to pass any command line parameters to the Java application transparently. Cool! Isn't it ?!

In the case your are a Windows guy, obviously this will not work (except you will run a Linux compatibility layer like Cygwin).

Anyway exist tools that are able to wrap a Java application into a native Windows .exe binary file, producing a result similar to the one explained in this tutorial. See for example http://launch4j.sourceforge.net/