URLClassloader Dependencies

Question

In my previous question, I asked how to load remote jar files. My current code is this:

//f is the path to the jar
URLClassLoader loader = new URLClassLoader(new URL[]{f.toURI().toURL()});
Class<?> jarClass = Class.forName(main, true, loader);
Class<? extends Module> module = jarClass.asSubclass(Module.class);

Constructor<? extends Module> constructor = module.getConstructor();
System.out.println(constructor);

Module module = constructor.newInstance();

This works well, but the remotely-loaded modules extend a class that is in the jar that is loading them, which gives this error:

Caused by: java.lang.ClassNotFoundException: package.whatever.Module, which I presume is because it is using URLClassLoader instead of getClass().getClassLoader().. how can I make it use URLClassLoader and then fall back to the default one?

Thanks,
Bart

Answer 1

You can set your application class loader to be the parent of the url class loader:

URLClassLoader loader = new URLClassLoader(
       new URL[]{f.toURI().toURL()}, Module.class.getClassLoader());

From the Oracle Java tutorial (class loading mechanism):

The Java platform uses a delegation model for loading classes. The basic idea is that every class loader has a "parent" class loader. When loading a class, a class loader first "delegates" the search for the class to its parent class loader before attempting to find the class itself.

Answer 2

我遇到了同样的问题而我正在使用Java 9，将其降级为Java 8解决了我的问题。