Local variables in static method and thread safety

Question

I have a question about variable scope.

For example:

class A {
    private static void test() {
        // do something with local variables
    }
}


        

Answer 1

Local variables are stored in each thread's own stack. That means that local variables are never shared between threads. That also means that all local primitive variables are thread safe.

Local references to objects are a bit different. The reference itself is not shared. The object referenced however, is not stored in each threads's local stack. All objects are stored in the shared heap. If an object created locally never escapes the method it was created in, it is thread safe. In fact you can also pass it on to other methods and objects as long as none of these methods or objects make the passed object available to other threads.

Object members are stored on the heap along with the object. Therefore, if two threads call a method on the same object instance and this method updates object members, the method is not thread safe.

Thread safety check: If a resource is created, used and disposed within the control of the same thread, and never escapes the control of this thread,the use of that resource is thread safe.

From: http://tutorials.jenkov.com/java-concurrency/thread-safety.html

Answer 2

For number 1, I don't know what test() does, so I cannot answer. If they modify some static variable of the class A, then it may not be thread safe. If both threads along the way are given reference to the same object, depending on how the object is defined, it might not be thread safe.

For number 2, local variables are in the stack of each thread (or at least conceptually like that), so there is no worry about the local variables being modified by the other threads.

Answer 3

When I call test() in each thread, can I guarantee that test() is thread safe?

Yes it would be thread safe if in test() method you are working on method local variables.

Where are the local varibles in test() stored? each threads' stack? heap space?

Method Local variable are stored each thread's own stack.