How to inspect the stack using an ASM visitor?

Question

I am attempting to use the Java byte code engineering library ASM to perform static analysis. I have the situation where I would like to inspect the variables being assigned to a field.

I have MethodVisitor which implements the visitFieldInsn() method. I am specifically looking for the putfield command. That is no problem. The problem is that when I encounter putfield, I want to be able to access the variable that's going to be assigned to the field. Specifically I want to access information about the type of the variable.

At the moment I really only need to look at what's at the top of the stack, but if there's a more general way to inspect it that's even better.

Is there a way using ASM to inspect the variables on the stack?

Answer 1

I am not familiar with ASM, but I have done something that sounds similar with the Eclipse Java AST framework. To know about variables, I had to keep track of variable declarations myself in the appropriate visitX() methods of the AST visitor. It wasn't very difficult once I knew which AST nodes corresponded to variable declarations.

Answer 2

First of all, if you can assume that bytecode is valid, the type of value assigned to a field should match the field type, which you can read in advance using ClassReader API.

However if you need to track where each individual value on a stack or variable slot for given instruction pointer came from, you can use the Analyzer API with SourceInterpreter. Basically it would allow to find instruction that produced given value and you can use information about that instruction to deduce a type (e.g. if it reads from a variable which corresponds to a method parameter or if value been returned from a method call, so in both cases you can get the type from method descriptor). Also see my old blog post that has an example of using SourceInterpreter.