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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?

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  • post EDIT. You can specifically check if the class being loaded is Person, and if so, choose to apply this particular instrumentation to it. Does that not meet your requirements? Commented Nov 16, 2010 at 20:00
  • Thanks, what I'm trying to find out is this: Where do I put the bytecode? Is there a method that is called on the class whenever one of its fields is accessed? Or will I have to hunt through all the other classes, find instances of field access and replace them with method calls? Commented Nov 17, 2010 at 6:55
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    thought i'd mention it, but when a field is private, but that class is an inner class of another class, that outer class can still access the field directly even tho it is private. It does this by generating a synthetic method to access the field. If you are limited to this case, then you can just modify the synthetic method and this then works like python does. (but in general the answer is no, you must modify normal methods.) Commented Jun 10, 2011 at 3:40

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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.

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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.
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you can modify the source class to be what you want, and then use asmify to generate the asm code to produce that transformation.

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