Adding jars at runtime

Shared Code
#1

Here’s a class I came across while trying to solve a problem I had recently. Its a ‘hack’ so to speak, but it does work well. I have an app that uses the Comm API with java. The issue I had was the nusance of installing the app. You had to copy a few files to the jre and then set some classpath stuff. Not a big deal per se, but annoying, especially when I wanted a simple drag and drop solution.

This class helped provide that, by allowing me to add the comm.jar at runtime without having to make a batch file or modify the environment variables.


import java.io.*;
import java.net.*;
import java.lang.reflect.*;

public class ClassPathHacker {

      private static final Class[] parameters = new Class[] { URL.class };

      public static void addFile(String s) throws IOException {
            File f = new File(s);
            addFile(f);
      }
      
      public static void addFile(File f) throws IOException {
            addURL(f.toURL());
      }

      public static void addURL(URL u) throws IOException {

            URLClassLoader sysloader =
                  (URLClassLoader) ClassLoader.getSystemClassLoader();
            Class sysclass = URLClassLoader.class;

            try {
                  Method method = sysclass.getDeclaredMethod("addURL", parameters);
                  method.setAccessible(true);
                  method.invoke(sysloader, new Object[] { u });
            } catch (Throwable t) {
                  t.printStackTrace();
                  throw new IOException("Error, could not add URL to system classloader");
            }

      }
}

I don’t take credit for this, I came across it at java.net on the forums.

Regards,
Dr. A>

#2

very handy! Great for loading plugins and maps bundled as .jar files on the fly.

Will.

#3

Wheres the difference to creating a new URLClassLoader?

#4

It appears that this is acutally dropping the new URL into the system class loader which means I suppose you can pick up the new classes without having to use the new classloader anywhere else?

Kev

#5

Looking at it tho, a far prettier solution would probably be (just typed, no compiler handy):



public class MySystemClassLoader {
     private static MySystemClassLoader single;

     public static MySystemClassLoader get() {
          return single;
     }

     private URLClassLoader loader;

     public MySystemClassLoader(ClassLoader loader) {
           super(new URL[0],loader);
 
           single = this;
     }

     public void addURL(URL u) {
          loader.addURL(u);
     }
}

public class Hook {
  public static void main(String argv[]) {
        MySystemClassLoader.get().addURL(new URL("whatever"));
  }
}

EDIT: actually you’d need to set the property “java.system.class.loader” to “MySystemClassLoader” outside, i.e. java -Djava.system.class.loader=MySystemClassLoader

Reference for why I think this should work: http://java.sun.com/j2se/1.4.2/docs/api/java/lang/ClassLoader.html#getSystemClassLoader()

This way there’s no reflection involved or naughty assumptions about what the system class loader will or won’t be implemented as.

Anyone tell me if this strategy would work?

Kev

#6

Thank you for this post!

I’ve been fighting with getting JavaFX apps to run from a JAR and this hack works perfectly. I added a few modifications so it is easier to use for loading JAR’s at runtime. My problem was that I need JavaFX, MySQL, and Java EE support, but I can’t expect the end user to play with their configuration settings to get the application to run.

As for the other responses there is a HUGE difference between this method and creating a new class loader. My first attempt was to load everything via class loaders but JavaFX will not work properly when loaded from a class loader since classes built by different loaders aren’t compatible.

Using this method I can search the users computer for their installation of JavaFX in the standard install folders. For the MySQL driver I include it inside the jar (jar within a jar). I save the mysql driver to the users home directory. After that loading it with the above method is trivial!