Question

I’m trying to use Class.forName to dynamically load a class from a .jar file on the filesystem at runtime. The class I am trying to load implements an interface in another .jar file, so I am using my own URLClassLoader to reference the two .jars.

The code works when it is called not in the context of the web app (I have tested this by copying and pasting the method into a separate program and calling it from main). However, when I run/debug the web app (I’m using NetBeans) the code fails. The newInstance method throws a ClassCastException when I try to cast the instance to the interface specified in my jar_file_dependencies.jar.

Here is the relevant code if this helps:

       File gameJar = new File("C:\\file_path\\jar_file.jar");

       File gameDependenciesJar = new File("C:\\file_path\\jar_file_dependencies.jar");

        URLClassLoader cl = new URLClassLoader(new URL[]
                {
                    gameJar.toURI().toURL(),
                    gameDependenciesJar.toURI().toURL()
                });

        Class clazz = Class.forName("MyClass", true, cl);

        IMyClass myClass = (IMyClass)clazz.newInstance();

        System.out.println(game);

    } catch (Exception e)
    {
        System.out.println(e.getMessage());
    }

Any suggestions as to why this code is working in one program and not another would be greatly appreciated.

Many thanks,
Dan

Answer 1

short answer without going into too many of the hairy details: one or both of the gameJar and gameDependenciesJar probably contain a definition of the IMyClass class/interface. the rule of thumb when using child classloaders is that the child classloader should not contain any of the “shared” classes–these should exist only in the parent classloader.

partial explanation: Web app classloaders usually have different delegation policies from normal classloaders. often they prefer the child’s class to the parent’s. normal classloaders generally prefer the parent’s class to the child’s. in your web app, you are ending up with 2 separate definitions of the IMyClass class (one def in the parent classloader, one in the child). in your normal app, the IMyClass definition in the child classloader is being ignored, so only one definition gets loaded (in the parent classloader), and everything is happy.