to ensure a java method is thread safe

Question

is it enough to use only local variables and no instance variables. Thus only using memory on the stack (per thread).

But what happens when you create a new MyObject that is local to the method. Doesn't the new object get created on the heap ? Is it thread safe becuase the reference to it is local (thread safe) ?

Answer 1

It is thread safe because if it is only referenced by variables in that particular method (it is, as you said, a local variable), then no other threads can possibly have a reference to the object, and therefore cannot change it.

Imagine you and I are pirates (threads). You go and bury your booty (the object) on an island (the heap), keeping a map to it (the reference). I happen to use the same island for burying my booty, but unless you give me your map, or I go digging all over the island (which isn't allowed on the island of Java), I can't mess with your stash.

Answer 2

Your new MyObject is thread-safe because each call to the method will create its own local instance on the heap. None of the calls refer to a common method; if there are N calls, that means N instances of MyObject on the heap. When the method exits, each instance is eligible for GC as long as you don't return it to the caller.

Answer 3

Well, let me ask you a question: does limiting your method to local variables mean your method can't share a resource with another thread? If not, then obviously this isn't sufficient for thread safety in general.

If you're worried about whether another thread can modify an object you created in another thread, then the only thing you need to worry about is never leaking a reference to that object out of the thread. If you achieve that, your object will be in the heap, but no other thread will be able to reference it so it doesn't matter.

Edit

Regarding my first statement, here's a method with no instance variables:

public void methodA() {
    File f = new File("/tmp/file");
    //...
}

This doesn't mean there can't be a shared resource between two threads :-).

Answer 4

Threre's no way to other threads to access such object reference. But if that object is not thread-safe, then the overall thread-safety is compromised.

Consider for example that MyObject is a HashMap.

The argument that if it's in the heap, it's not thread-safe, is not valid. The heap is not accessible via pointer arithmetic, so it doesn't affect where the object is actually stored (besides ThreadLocal's).