A ClassFileTransformer registered through an Instrumentation object is offered a chance to transform every class that is subsequently loaded or redefined. The final argument to transform( ) is a byte array that contains the raw bytes of the class file (or bytes returned by a previously invoked ClassFileTransformer). If the TRansform( ) method wishes to transform the class, it should return the transformed bytes in a newly allocated array. The array passed to transform( ) should not be modified. If the transform( ) method does not wish to transform a given class, it should return null.

public interface ClassFileTransformer {
// Public Instance Methods
     byte[ ] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, 
java.security.ProtectionDomain protectionDomain, byte[ ] classfileBuffer) 
throws IllegalClassFormatException;  
}

Passed To

Instrumentation.{addTransformer( ), removeTransformer( )}