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!

Spumescent

See also #7980633 – Doubleteam

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.

Promissory

and how to compile the class back on the other side when I receive this message as ByteArrayInputStream? – Uplift

Creating classes from bytes is the job of a ClassLoader. – Promissory

Hi, if the class is a proxy class and dynamic be generated at runtime, and stored in the memory, how can we save it to a file? – Feller

How to do the reverse? that is to convert byte[] to java.lang.Class? – Oeildeboeuf

If you want to recreate the class from the buffer, then look into this – Chaplain

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)).

Reimer

Answer 3

You can try java instrumentation! In particular ClassFileTransformer maybe of interest to you!!

You simply override the transform method (from ClassFileTransformer), and your transform method will be called before each class is loaded. So then you cna do whatever class bytes.

Basile

Answer 4

Make use of the ClassFileTransformer.

Tuckie

Answer 5

Here's a bit of help,

use

final Class<T> clazz = ...
final byte[] buffer = clazz.getClassLoader().getResourceAsStream(clazz.getName().replace('.', '/').concat(".class"));

to get the compiled byte code of the class.

---

And use

class SimpleClassLoader extends ClassLoader {

    public Clazz<?> loadFromBuffer(final byte[] buffer) {
        return defineClass(buffer, 0, buffer.length);
    }

    public Object newObjectFromBuffer(final byte[] buffer) {
        return loadFromBuffer(buffer).newInstance();
    }

}

to instantiate an Object or to load the Class back from the buffer.

Chaplain