Thread Safety - Where to Worry About

Given the architecture of the JVM, you need only be concerned with instance and class variables when you worry about thread safety. Because all threads share the same heap, and the heap is where all instance variables are stored, multiple threads can attempt to use the same object's instance variables concurrently. Likewise, because all threads share the same method area, and the method area is where all class variables are stored, multiple threads can attempt to use the same class variables concurrently. When you do choose to make a class thread-safe, your goal is to guarantee the integrity -- in a multithreaded environment -- of instance and class variables declared in that class.

You needn't worry about multithreaded access to local variables, method parameters, and return values, because these variables reside on the Java stack. In the JVM, each thread is awarded its own Java stack. No thread can see or use any local variables, return values, or parameters belonging to another thread.