How to instrument java system classes?
instrumentation, java
Solution
Some (not all) system classes are already loaded before the pre-main method is invoked and your ClassFileTransformer is added. If you want to transform these classes as well, you can invoke something like Instrumentation#retransformClasses(Instrumentation#getAllLoadedClasses()) after adding your ClassFileTransformer. Note, you have to use Instrumentation#addTransformer(ClassFileTransformer, true) to indicate that your transformer supports retransformation of classes.
Problem
I'm trying to add custom behaviour to system classes (FileInputStream/FileOutputStream). I wrote custom ClassFileTransformer with the following transform method: ``` public byte[] transform(ClassLoader arg0, String arg1, Class arg2, ProtectionDomain arg3, byte[] arg4) throws IllegalClassFormatException { System.out.println("class name: " + arg1); return arg4; } ``` When I run sample program: ``` public static void main(String[] args) throws Exception { new FileOutputStream("file"); } ``` I see that no system classes are not passed to transform. Is there any way to modify system classes? Thanks in advance!