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Combining the information from many posts on this site and many others, I got the following code to dynamically add (at run time) a directory containing classes to the classpath and load a class within that directory.

I'm using OSGi bundles and running from eclipse an "Eclipse Application" (a kind of Run Configuration).

This is the code I'm using:

CASE 1: (both cases are different things I've tried to do the same thing.)

File file = new File("/Users/alek/fastFIX/myJPass/"); URL url = file.toURI().toURL(); URL[] urls = new URL[]{url}; ClassLoader cl = new URLClassLoader(urls); Class cls = cl.loadClass("GuiLauncher"); //the file GuiLauncher.class is in the /Users/alek/fastFIX/myJPass/ directory Class[] argTypes = new Class[] { String[].class }; Method main = cls.getDeclaredMethod("main", argTypes); //trying to run the main class main.invoke(null, (Object) args);

I don't get any error, and nothing happens. I've also tryied the following, as I actually need the loaded class to interact with other (already loaded) classes.

CASE 2:

ClassLoader currentThreadClassLoader = Thread.currentThread().getContextClassLoader(); URLClassLoader urlClassLoader = new URLClassLoader(new URL[] { new File("/Users/alek/fastFIX/myJPass/").toURL() }, currentThreadClassLoader); Thread.currentThread().setContextClassLoader(urlClassLoader);

then i load like this:

 Class<?> c = Class.forName("GuiLauncher");

or like this:

Class<?> c = Thread.currentThread().getContextClassLoader().loadClass("GuiLauncher");

and try to invoke the main function like this:

Class[] argTypes = new Class[] { String[].class }; Method main = cls.getDeclaredMethod("main", argTypes); //trying to run the main class main.invoke(null, (Object) args);

here also nothing happens.

Any clue of what could be happening? I've read all related posts here and many places else with no luck.

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    For the record, a much easier way to call methods on dynamically loaded objects is to make them implement known interfaces. MyPlugin m = cls.newInstance(); m.runPlugin(magic, hax); where MyPlugin is an interface that your runtime class implements. Commented Oct 3, 2012 at 18:23
  • You wrote "Class.forName("GuiLauncher");". Is your class in the default package? Maybe the folder of your class does not match the package declaration? Commented Oct 3, 2012 at 18:53
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    If you have referenced the GuiLauncher class before the execution of the classloader, it would get loaded from the classpath by the default classloader. So in that case your loadClass will not have any effect as it will check with parent classloader to see if the class it already loaded and find that it is loaded. Commented Oct 3, 2012 at 18:55
  • @Wug: I have no access to the code of the class I'm loading. Commented Oct 4, 2012 at 11:04
  • @basiljames, I have not done that before. In any case, the main method should run if the class is already loaded. correct me if I'm wrong. Commented Oct 4, 2012 at 11:06

2 Answers 2

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In OSGI framework it is necessary to add the OSGI class loader as a parent, something like this: ... ClassLoader cl = new URLClassLoader(new URL[]{file.toURI().toURL()}, this.getClass().getClassLoader()); ...

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In case 1, I suspect that the GuiLauncher class is already on the classpath, so may get loaded by the default classloader. Try doing Class.forName() before setting up the dynamic classloader, to confirm that there's no class available. If you are in Eclipse, you need to be careful that the class is not included on the Eclipse classpath, which is what would normally happen. You might need to compile it once then move the .java and .class files elsewhere to hide them from Eclipse!

In case 2:

Class.forName("GuiLauncher");

will not work as you expect, because this will use the system classloader. This should fail, hence my suspicion above. You need use the other version of this method that specifies your dynamic classloader:

Class.forName("GuiLauncher", true, urlClassLoader)

The following code works for me. 

import java.net.*; import java.lang.reflect.*; import java.io.File; public class Main{ public static void main(String[] args) { try{ Class cls = Class.forName("Plugin"); } catch(Exception e){ System.out.println("Nothing there!"); } try{ File file = new File("plugin"); ClassLoader cl = new URLClassLoader(new URL[]{file.toURI().toURL()}); Class cls = Class.forName("Plugin", true, cl); Method main = cls.getDeclaredMethod("main", new Class[] { String[].class }); main.invoke(null, (Object) args); } catch(Exception e){ e.printStackTrace(); } } }

The Plugin class is compiled in the plugin subfolder, so it's not on the classpath used to run Main, as shown by the first Class.forName().

public class Plugin{ public static void main(String[] args) { System.out.println("Plugin was invoked!"); } }

and prints out:

Nothing there! Plugin was invoked!
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thanks for the reply. it is the same, nothing happens. what is the difference between this you say and case 1?
the main method should open an awt window. i've already tried putting the class in the classpath (by hand) and works fine, but I can not do this in my real scenario. when I do class.forname(class), i get classnotfoundException. I've tried the version of the class.forname you say also with no luck
if I print the result of Class.forName("GuiLauncher", true, urlClassLoader) i get "class GuiLauncher". I think this means the class was successfully loaded. maybe a problem when loading the main method? the args array is an array to send arguments to the main function, which in this case is empty.
Hi @DNA, thank you again. This gets me the same problem, no error message, but nothing happening. Something that maybe is important, is that I'm running OSGi bundles. I still don't know how this could affect or if it would affect at all, but will go on reading. thanks again.
OK, sounds like OSGi is the issue then - I believe that OSGi assigns one classloader per bundle. I'm not even sure why you are creating classloaders yourself if you are using OSGI though - see this blog post. You could try running the code in the debugger and see what shows up...

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