Reloading the same jar at runtime

Question

Hi Friends,
I have loaded the jar at runtime using the urlclassloader, but now I want to reload the same jar of different version again.
Is it possible, if yes ..Could you please explain.
Thanks in advance,
Kk

Answer 1

If your application permits, you can do this:
1. Create an URLClassLoader with an URL referencing the original version of the JAR;
2. Create an instance of a class, most probably a Runnable, keeping a reference to it;
3. Use the object created in 2.
And when you want to reload the JAR, you set the reference to null, perform any cleanup needed, and repeat those three steps again.
Something like
public class MyApplication {
  public static void main(String[] args) throws Exception /* error handling removed for clarity */ {
    URLClassLoader loader = new URLClassLoader(new URL[] {new URL("name.jar"});
    Runnable run = (Runnable) loader.loadClass("com.mypackage.MyApplicationStage2").getConstructor().newInstance();
    run.run();
    System.out.print("Press Enter to reload name.jar, or Ctrl+C to exit: ");
    new BufferedReader(new InputStreamReader(System.in)).readLine();
    run = null;
    loader = new URLClassLoader(new URL[] {new URL("name.jar"});
    Runnable run = (Runnable) loader.loadClass("com.mypackage.MyApplicationStage2").getConstructor().newInstance();
    run.run();
}And then the class in name.jar could look like this (old, then new):
package com.mypackage;
public class MyApplicationStage2 implements Runnable {
  public void run() { System.out.println("Old code"); }
package com.mypackage;
public class MyApplicationStage2 implements Runnable {
  public void run() { System.out.println("New code"); }
}Edited by Looce on Nov 8, 2008 6:00 PM (three steps, not two)