CODEWITHSUNDEEP
javacode-generationbytecodebytecode-manipulation
leventov
leventov

Reputation: 15313

How to make a copy of a Java class in runtime?

I have a class

class Foo {
    int increment(int x) {
        return x + 1;
    }
}

I want to obtain a copy of this class in runtime, e. g. a class like

class Foo$Copy1 {
    int increment(int x) {
        return x + 1;
    }
}

Which has all the same methods, but a different name.

Proxy seem to help with delegating, but not copying the methods with all their bodies.

Upvotes: 7

Views: 1266

Answers (3)

sozal
sozal

Reputation: 41

@leventov what about this one? https://gist.github.com/serkan-ozal/8737583841b4b12e3a42d39d4af051ae

Upvotes: 2

Rafael Winterhalter
Rafael Winterhalter

Reputation: 44042

You can use Byte Buddy for this:

Class<?> type = new ByteBuddy()
  .redefine(Foo.class)
  .name("Foo$Copy1")
  .make()
  .load(Foo.class.getClassLoader())
  .getLoaded();

Method method = type.getDeclaredMethod("increment", int.class);
int result = (Integer) method.invoke(type.newInstance(), 1);

Note that this approach redefines any uses of the class within Foo, e.g. if a method returned Foo, it would now return Foo$Copy1. Same goes for all code-references.

Upvotes: 12

Oleg Šelajev
Oleg Šelajev

Reputation: 3800

I think using Unsafe could be enough if you have normal access to the bytecode.

Something like Foo.class.getClassLoader().getResourceAsStream() can give you the bytecode for the class.

Then use sun.misc.Unsafe.defineClass(String name, byte[] code, int off, int len, ClassLoader classLoader, ProtectionDomain protectionDomain) to define the class in the same classloader and protection domain as Foo but with a different name.

The details are to figure out, but it might be the simplest approach without any third party libraries.

Upvotes: 1

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