Java agent : transform() not invoked for all classes - transform

Question

I have a very simple transformer:
import java.io.ByteArrayInputStream;
import java.lang.instrument.ClassFileTransformer;
import java.lang.instrument.IllegalClassFormatException;
import java.security.ProtectionDomain;
import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
//this class will be registered with instrumentation agent
public class PizzaTransformer implements ClassFileTransformer {
public byte[] transform(ClassLoader loader,
String className,
Class classBeingRedefined,
ProtectionDomain protectionDomain,
byte[] classfileBuffer) throws
IllegalClassFormatException {
byte[] byteCode = classfileBuffer;
System.out.println("This class is "+className);
return byteCode;
}
}
The agent code is as follows:
import java.lang.instrument.Instrumentation;
public class PizzaAgent {
public static void premain(String agentArgs, Instrumentation inst) {
System.out.println("Executing premain.........");
inst.addTransformer(new PizzaTransformer(),true);
}
}
The manifest is :
Boot-Class-Path: javassist.jar
Premain-Class: PizzaAgent
Can-Redefine-Classes: true
Can-Retransform-Classes: true
What is puzzling is only 4000+ classes were printed by the transform(), when 19000+ classes are reported as loaded by turning on -verbose:class with command line launching.
Why more than 10000 classes are not invoking transform()?
Thank you

Answer 1

There may be two scenarios,
Class yet not loaded by JVM
If any class yet not loaded by JVM, then there are chances that Java agent : transform() will not be invoked for that classes.
Whenever it will be loaded by JVM Java agent : transform() will be invoked for that class.
Class is loaded before initializing ClassFileTransformer
If you have used any class inside your ClassFileTransformer implementation or any other Agent class, then that class may be loaded before ClassFileTransformer initialization, so Java agent : transform() will not be invoked for that class

Answer 2

You are only seeing classes that are not yet loaded when the agent is attached. If you want to handle loaded classes, too, you have to explicitly retransform these classes. You can do so by:
instrumentation.retransformClasses(instrumentation.getLoadedClasses());
However, some classes are not retransformable (Instrumentation::isModifiable) and you most likely need to split up the array to avoid draining your memory.