Are Java pointer writes atomic?

Question

Simple question: does the Java memory/synchronization model guarantee atomic pointer writes? That is, if we have competing threads:

String shared;

thread1()
{
    shared = "a";
}

thread2()
{
    shared = "hello world";
}

started at the same time, is shared always guaranteed to be null, "a", or "hello world"?

Answer 1

Reads and writes are atomic for reference variables.

Source: http://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html

Answer 2

It is atomic.

However, in the example you gave, shared's value isn't necessarly one of null, a or hello world. It is possible that, without proper synchronization, each thread will never see the value set by other threads. So thread 1 will see a and thread 2 will see hello world at the same time.

Edit: Added references for the last paragraph for further reading

The JLS explains the order of operation performed by different threads, in Chapter 17 - Threads and Locks. Specifically, in the 17.4.5 Happens-before Order section. Also, the well-written Java Concurrency in Practice explains all of this thoroughly.