Question

I seem to be having a problem with loading classes in a module loader for an application I’m developing. Basically, all classes I’m going to be loading with it extend another class, which is located in a package in the actual application. For our purposes, we’ll call it Module. Modules are located in a separate folder outside the actual application.

The loader iterates through a folder and executes the loadFile() method on any file with the extension .class. All classes have the package declaration as the Module class, as well as the extends Module declaration in the class header.

This is the loadFile() method, header and exception clauses excluded:

String fileName = file.getName();
String className = fileName.replace(".class", ""); //Strips extension
Class<?> aClass = Class.forName(className, true, new URLClassLoader(new URL[] { file.toURI().toURL() }));
Class<? extends Module> modClass = aClass.asSubclass(Module.class);
return modClass.getConstructor().newInstance();

I keep getting a ClassNotFoundException on the third line. And past that, if it ClassNotFoundException weren’t thown, would all dependencies be resolved?

Answer 1

From the documentation for URLCLassLoader:

This class loader is used to load classes and resources from a search path of URLs referring to
both JAR files and directories. Any URL that ends with a ‘/’ is assumed to refer to a directory.
Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed.

So, you must use URLs for either directories or .jars

Two solutions:

1. Force your users to give you .jar files, including a manifest of some sort inside with the classname that they wish to be loaded. This approach is used by the Bukkit developers. Having used this method in the past, the dependencies should all be packaged in the .jar file and thus in the URLClassLoader’s search path and able to be loaded.
2. Use the URL of the file’s directory, and search that directory for .class files. I’m not sure if dependencies will be loaded using this method.