导出时从另一个Jar运行一个Jar不起作用

Question

I've been reading about the subject for a few hours:

What I'd like to do (and succeeded to do) is to run a runnable jar from my code, the thing is, when I export the code into a runnable jar, then it stops working.

The method that works from my compiler is :

ProcessBuilder pb = new ProcessBuilder("java", "-jar", "JAR I'M LOADING");
pb.start

This is what I've tried :

I've created a packet with the jar that I wanna run, and I created a static JarLoader class where I do :

public class JarLoader {

    static JarLoader jl = new JarLoader();

    public static ProcessBuilder loadJar(){
        return new  ProcessBuilder("java", "-jar", jl.getClass().getResource("JAR I'M TRYING TO LOAD").toString());

}

And of course I switched the method that is runs the jar to :

ProcessBuilder pb = JarLoader.loadJar();
pb.start();

This technique worked very well with other type of resources, such as images, but it doesn't work with the jar.

Am I missing something ? (and yes, I did add the folder containing the Jar and the JarLoader to the project's source)

Answer 1

The technique you're currently using allows you to get to a resource that's already packed inside your JAR file. But these resources are coherent only within the application, and not from outside it.

When you execute your JAR file using java -jar ... , that's an external application being executed, and it won't understand how to retrieve a JAR from inside another JAR. You will need to write it temporarily to disk, and then run the temporary JAR file.

You need to:

1. Open the resource as a stream.
2. Use Files.createTempFile() to create a temporary JAR file somewhere.
3. Use Files.copy() to copy from the resource stream to the temporary file.
4. Use File.deleteOnExit() to make sure that the temporary file will get cleaned up when the main (host) application terminates.
5. Execute the temporary file with java -jar tempfile.jar or whatever you called it.