Object creation (state initialisation) and thread safety

Question

I am look into the book \"Java Concurrency in Practice\" and found really hard to believe below quoted statement (But unfortunately it make sense).

http://www.informit.c

Answer 1

I guess theoretically it is possible. It is similar to double checked locking problem.

public class Test {
    static Holder holder;

    static void test() {
        if (holder == null) {
            holder = new Holder(1);
        }
        holder.assertSanity();
    }
...

If test() is called by 2 threads, thread-2 might see the holder in a state when initialization is still in progress so n != n may happen to be true. Here is bytecode for n != n:

ALOAD 0
GETFIELD x/Holder.n : I
ALOAD 0
GETFIELD x/Holder.n : I
IF_ICMPEQ L1

as you can see JVM loads field n to operand stack twice. So it may happen that the first var gets value before init and the seccond after init