Adding jars at runtime

[dranonymous]

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.

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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>

[William Denniss]

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

Will.

[Herkules]

Wheres the difference to creating a new URLClassLoader?

Games published by our own members! Check 'em out!

[kevglass]

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

[kevglass]

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

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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

[rritoch]

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!