Java: new instance from bytecode

Question

ClassWriter cw = new ClassWriter(...);
byte[] bytes = cw.toByteArray();

I would like to create new class instance from bytes array. How do I do this? Is it possible at all?

possible duplicate of [Java: How to load Class stored as byte\[\] into the JVM?](http://stackoverflow.com/questions/1781091/java-how-to-load-class-stored-as-byte-into-the-jvm) — bruno conde, Nov 23 '10 at 17:44
@bruno conde: It looks like. We just express things in a more concise way here — Denys Kniazhev-Support Ukraine, Nov 23 '10 at 17:54
You can use defineClass, however unless you use a custom class loader you may need to use reflections to call it as its not a public method. — Peter Lawrey, Nov 23 '10 at 22:39

score 8 · Accepted Answer · edited Nov 23 '10 at 17:48

8

ClassLoader.defineClass()

Reference:

ClassLoader.defineClass(String name, byte[] b, int off, int len)

edited Nov 23 '10 at 17:48

Sean Patrick Floyd

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answered Nov 23 '10 at 17:45

Vladimir Ivanov

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Nice answer but awful formatting and a java 1.4 link. Changed both. – Sean Patrick Floyd Nov 23 '10 at 17:49
thanks for editing, was struggling with that, but parser didn't want to show the link properly. – Vladimir Ivanov Nov 23 '10 at 17:50
3

+1. Anyway, the method is protected, so you'll need a way to access it. Extending the ClassLoader and providing a public method could be a solution. – Tomas Narros Nov 23 '10 at 17:51

score 2 · Answer 2 · answered Nov 23 '10 at 17:48

2

This is possible, and you need to use Reflection in order to achieve this. The psuedo code is:

final Class clazz = loadIntoCurrentClassLoader(bytes); //I'm assuming you wrote this already using defineClass

final YourClass foo ;
try {
    foo = (YourClass) clazz.newInstance();
}
catch (final Exception e) {
    throw new RuntimeException(e);
}

answered Nov 23 '10 at 17:48

Amir Afghani

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I'm pretty sure the `loadIntoCurrentClassLoader(bytes)` part is what the question is about :-) – Sean Patrick Floyd Nov 23 '10 at 17:49
Ok so I went a little overboard ;) – Amir Afghani Nov 23 '10 at 17:50
It's ambiguous from the way the question is phrased. – Amir Afghani Nov 23 '10 at 17:50
The point is not clazz.newInstance(). The point here was how to get clazz. Reflection does not help here, you should use classLoader.defineClass() – AlexR Nov 23 '10 at 19:03

score 0 · Answer 3 · answered Jan 08 '11 at 20:47

I can create the class by extending ClassLoader and using defineClass. But then the created class has my extended ClassLoader as its ClassLoader, which causes failures when the code of my class loads other classes. Presumably I can get around this by creating my ClassLoader to delegate everything in the right way, but it's not obvious how to get this right.

Java: new instance from bytecode

3 Answers3