Classloader java.lang.NoClassDefFoundError?

Question

So I have a classloader loading a class like so:

ClassLoader cl = new URLClassLoader(urls);
Class cls = cl.loadClass("modules.Test");
Method method = cls.getDeclaredMethod("getModule", noparams);
Class<?> type = method.getReturnType();

if(type.newInstance() instanceof Module){
    System.out.println("Accessed field with type: Module");
}

The class Module is in another jar at runtime. And the Test.class was generated within that main jar then i unarchived it, so the dependency would be there.

How can I access other dependencies from the external .class file I have loaded?

The exception:

java.lang.NoClassDefFoundError: com/xxxxxxx/xxxx/objects/Module
Caused by: java.lang.ClassNotFoundException: com.xxxxxxx.xxxx.objects.Module

Answer 1

I think that the problem is happening because your modules.Test class depends the Modules class, but your custom class loader can't find that class.

I think that is because you have instantiated the custom classloader incorrectly. You wrote:

ClassLoader cl = new URLClassLoader(urls);

That creates a classloader whose parent classloader is the default system classloader. But the error implies that the default classloader is not the one that knows about Modules . Try this instead:

Classloader cl = new URLClassLoader(
        urls, this.getClass().getClassLoader());

This should at least give you a classloader that knows about Modules .

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Note: adding the URL for the JAR containing Modules to the urls array is a non-solution. You are liable to end up loading the Modules class twice, and that is liable to lead to other problems. (The instanceof won't work, for example.)