Java Classloader unable to load modules?

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?

Maybe the URL is an absolute file path instead of a path that points to a class inside your application's classpath. — Luciano
– Luciano, Commented Jun 1, 2012 at 21:44
Argh, that might be it, since it's referring to the location of a .class file in a separate folder. — user1431637
– user1431637, Commented Jun 1, 2012 at 21:51

cabbagebot · Accepted Answer · 2012-06-01 21:54:24Z

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:

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.
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.

answered Jun 1, 2012 at 21:54

cabbagebot

3861 silver badge12 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user1431637 Over a year ago

Made a switch over to a system of .jar files, and it works just fine. Thanks for the advice.

Simone Gianni · Accepted Answer · 2012-06-01 22:16:02Z

In the URLClassLoader, do not pass the file, but the parent folder. However, this works correctly if classes are all in the "default package", so the .class files you are loading must not have a package declaration on top.

By default, a class loader will also trigger loading of all classes required to properly build the class: it will try to load the super class, the super super class etc... all the interfaces and super interfaces, the classes needed for static fields and methods, the classes needed for method signatures (return types and parameters). It will not usually try to load classes used internally by methods, not until you execute those methods.

However, usually a class loader does not "contain" all those classes, for example your class will end up inheriting from java.lang.Object, and your URLClassLoader will not contain the Object.class file. So, class loaders delegate to their parent class loaders.

You are currently creating a URLClassLoader without specifying a parent, in Java 7 at least the parent will default to the "system class loader", which is fine as long as you are in a plain java application, and not executing your code itself inside a specific hierarchy of class loaders. If however you are running that code in a web application, or in an OSGI container etc.. the you should give the URLClassLoader a proper parent to delegate to, for example Thread.currentThread().getContextClassLoader() or this.getClass().getClassLoader().

I suppose you need all of this because you need to load those class dynamically at runtime.

Collectives™ on Stack Overflow

Java Classloader unable to load modules?

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related