Inject code into method with Javassist

Question

I'm trying to add some code to a class that is inside a jar(maven dependecy) and i'm doing it in the following way:

ClassPool classPool = ClassPool.getDefault();

CtClass ctClass = classPool.get("xyz.abc.ClassInADependecy");

CtMethod method =  ctClass.getDeclaredMethod("getSomeValue");

method.insertBefore("{ System.out.println(\"modified\"); }");

I'm using Spring and the above code is being called using a @Configuration annotation.

When i call the method getSomeValue nothing is printed.

Can you help me find out what i'm doing wrong?

Thank you very much.

Answer 1

You are only changing the implementation as it is represented in Javassists type pool. You have to make sure that the class is also loaded by the respective class loader. Also, this must happen before the class is loaded for the first time, i.e. before your Spring application loads that class.

One way to do so is to manipulate the class from a Java agent: https://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html