Question

I was really looking at the differences between pass by value and how Java allocates objects and what java does to put objects on the stack.

Is there anyway to access objects allocated on the heap? What mechanisms does java enforce to guarantee that the right method can access the right data off the heap?

It seems like if you were crafty and maybe even manipulate the java bytecode during runtime, that you might be able to manipulate data off the heap when you aren’t supposed to?

Answer 1

There is no instruction in the JVM instruction set that gives arbitrary access to the heap. Hence, bytecode manipulation will not help you here.

The JVM also has a verifier. It checks the code of every method (as a class is being loaded) to verify that the method does not try to pop more values off the execution stack than what it had pushed onto it. This ensures that a method cannot ‘see’ the objects pointed by its calling method.

Finally, local variables are stored in a per-method array (known as the ‘local variables array’). Again, the verifier makes sure that every read/write instruction from-/to- that array specifies an index that is less than the size of the array. Note that these JVM instructions can only specify a constant index. They cannot take a computed value and use it as an index.

So to recap, the answer is No.