Bytecode manipulation to intercept setting the value of a field

Question

Using a library like ASM or cglib, is there a way to add bytecode instructions to a class to execute code whenever the value of a class field is set?

For example, let’s say I have this class:

   public class Person
   {  
       bool dirty;
       public String name;
       public Date birthDate;
       public double salary;
   }

Let’s say a section of code contains this line:

person.name = "Joe";

I want this instruction to be intercepted so the dirty flag is set to true. I know this is possible for setter methods -- person.setName (“Joe”) -- as class methods can be modified by bytecode manipulation, but I want to do the same thing for a field.

Is this possible, and if so, how?

EDIT

I want to avoid modifying the code section that accesses the class, I'm looking for a way to keep the interception code as part of the Person class. Are there a pseudo-methods for field access, similar to properties in Python classes?

Answer 1

There are two bytecodes for updating fields: putfield and putstatic (see http://java.sun.com/docs/books/jvms/second_edition/html/Instructions2.doc11.html). These will be found in the code for the using class, so there's no way to simply modify Person.

Answer 2

In short, you need to inject bytecode that does the following in the method of interest :

if (person.name.equals("Joe") { 
   dirty = true;
}

You cannot evaluate the field at instrumentation time - it has to be at runtime when the method is executing.

Regarding your question of how, try the following:

• Write the code in a test class and generate an ascii version of the bytecode to see what was generated. You can do this easily with javap.