convert Class object to bytes

Question

If I have a Class instance at runtime, can I get its byte[] representation? The bytes I'm interested in would be in the Class file format, such that they'd be valid input to [ClassLoader.defineClass][3].

[3]: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/ClassLoader.html#defineClass(java.lang.String, byte[], int, int)

EDIT: I've accepted a getResourceAsStream answer, because it's very simple and will work most of the time. ClassFileTransformer seems like a more robust solution because it doesn't require that classes be loaded from .class files; it would handle network-loaded classes for example. There's a few hoops to jump through with that approach, but I'll keep in in mind. Thanks all!

Answer 1

You can usually just load the class as a resource from the Classloader.

Class c = ... String className = c.getName(); String classAsPath = className.replace('.', '/') + ".class"; InputStream stream = c.getClassLoader().getResourceAsStream(classAsPath); 

I would probably recommend using something from Apache commons-io to read the InputStream into a byte[], but IOUtils.toByteArray() should do the trick. Writing that code is really easy to get wrong and/or make slow.

Answer 2

In general you can't go back like that. However, for some class loaders you may be able to get the resource file be taking the fully qualified class name, replacing . with / and appending .class (so mypackage.MyClass becomes mypackage/MyClass.class (remember, may be case sensitive)).