如何在java中重新定义lambda匿名类

Question

JDK8 convert lambda expressions to anonymous class in

InnerClassLambdaMetafactory.spinInnerClass() {
    return UNSAFE.defineAnonymousClass(targetClass, classBytes, null);
}

I'm writing a javaagent, use asm to modify classBytes (add a method) and pass it to defineAnonymousClass , but the method end up with a ClassNotFoundException of this anonymous class. is there any way I can modify the content of annonymous classBytes?

Answer 1

Transforming anonymously loaded classes is tricky. Are you retranforming already loaded classes that represent such classes? If so, watch out that Class::getName does not return the actual binary name of the class but adds a random hash such as my.DefinedType/12345 where you would need to strip the latest numbers.

Also, you cannot reference such classes directly from another class but you need to reference them directly from either the reflection API or ideally a method handle. You cannot look up such classes from a class loader which is why they are called anonymous.

Finally, when installing a class file transformer, the loading of such anonymous classes is not registered with the transformer. The easiest way of dealing with such classes is to patch the lambda meta factory that is responsible for creating lambda types. You can do so easily by using for example Byte Buddy which allows you to create an agent:

new AgentBuilder.Default()
  .with(LambdaTransformationStrategy.ENABLED)
  .type(someMatcher)
  .transform(someTransformer)
  .installOn(instrumentation);

Calling this, under the covers, Byte Buddy rewrites the JVM's default lambda meta factory class and replaces it with its own code generation where such instrumentation is possible.