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javaclassclassloaderjava-bytecode-asmbytecode-manipulation
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Reputation: 43

How to create a transforming class loader

I am trying to make a class loader which transforms the class bytes of the class being loaded using some kind of class transformer.

Suppose I have the following code:

static class Test {
  void foo();
}

void test() {
  ClassLoader transformingLoader = transformingClassLoader(..., this.getClass().getClassLoader(), ...);
  Class<?> klass = Class.forName("test.package.Tests$TestClass", true, loader);

  // I want to use the class easily here, but using it like this
  // will cause it to load a new instance of the class, ignoring the already
  // defined class by the transforming class loader and so all the transformations.
  TestClass test1 = new TestClass();
  test2.foo();

  // Using something like this doesn't work either, it throws a ClassCastException
  // because both classes are different from different loaders
  TestClass test2 = (TestClass) klass.newInstance();
}

How could I make this work? I have tried using an interface like:

interface Test {
  void foo();
}

static class TestClass implements Test { 
  // ...
}

void test() {
  ClassLoader transformingLoader = ...
  Class<?> klass = Class.forName("test.package.Tests$TestClass", true, loader);

  // This leads to another ClassCastException though, because the transforming
  // class loader defines it's own version of the Test interface.
  Test test = (Test) klass.newInstance();
  test.foo();
}

This is my current code for the transforming class loader: pastes.dev

So how could I fix these issues? I think most of the issues would be fixed if the class loader properly used already loaded classes when available but it doesn't seem like it does.

Upvotes: 0

Views: 82

Answers (0)

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