Multiple runnable classes inside JAR, how to run them?

Question

Hello,
I'm having problems with running multiple different classes from one JAR file. I know that I can set one of the classes inside JAR to by Main class that will be run after command java -jar myjar.jar, but what I want is something like:

java -jar myjar.jar MyClass

Is it possible to do this that way, or do I have to create multiple JAR (each for one runnable class), or is it better to create 'manager' class that will run my other classes passing to them command line arguments?
I was looking for documentation or reference, but I couldn't find any.

Answer 1

you will have to use:

java -cp myjar.jar MyClass

and

java -cp myjar.jar OtherMainClass

Answer 2

You do it like this:

java -cp myjar.jar MyClass

i.e. put the JAR into the classpath, then any class with a main method can be run by specifying its fully qualified name. The -jar option only exists as a shortcut to use the information in the JAR's manifest instead (which can also include other JARs in the classpath as well as specify the main class).

Answer 3

The executable Jar file format only allows you to specify one main class. In order for you to be able to execute different applications, you'll need to either create a "manager" as you suggest, or to use the classpath instead:

java -cp myjar.jar MyClass

However, this approach will ignore the classpath you have configured in the Jar's manifest file.

Answer 4

Jar files can contain only one Main-Class attribute in the manifest, which means java -jar myjar.jar can only start one class.

You can start other runnable classes with

java -cp myjar.jar OtherClass

but that won't support users double-clicking on the jar file.

Depending on how skilled your users are, maybe the command line is OK for them. If not, you can create a script for each runnable class or one script that takes arguments to choose the right class.

Answer 5

This doc will help you.