Java – Is the != check thread safe

atomicjavamultithreadingrace conditionthread-safety

I know that compound operations such as i++ are not thread safe as they involve multiple operations.

But is checking the reference with itself a thread safe operation?

a != a //is this thread-safe

I tried to program this and use multiple threads but it didn't fail. I guess I could not simulate race on my machine.

EDIT:

public class TestThreadSafety {
    private Object a = new Object();

    public static void main(String[] args) {

        final TestThreadSafety instance = new TestThreadSafety();

        Thread testingReferenceThread = new Thread(new Runnable() {

            @Override
            public void run() {
                long countOfIterations = 0L;
                while(true){
                    boolean flag = instance.a != instance.a;
                    if(flag)
                        System.out.println(countOfIterations + ":" + flag);

                    countOfIterations++;
                }
            }
        });

        Thread updatingReferenceThread = new Thread(new Runnable() {

            @Override
            public void run() {
                while(true){
                    instance.a = new Object();
                }
            }
        });

        testingReferenceThread.start();
        updatingReferenceThread.start();
    }

}

This is the program that I am using to test the thread-safety.

Weird behavior

As my program starts between some iterations I get the output flag value, which means that the reference != check fails on the same reference. BUT after some iterations the output becomes constant value false and then executing the program for a long long time does not generate a single true output.

As the output suggests after some n (not fixed) iterations the output seems to be constant value and does not change.

Output:

For some iterations:

1494:true
1495:true
1496:true
19970:true
19972:true
19974:true
//after this there is not a single instance when the condition becomes true

Best Answer

In the absence of synchronization this code

Object a;

public boolean test() {
    return a != a;
}

may produce true. This is the bytecode for test()

    ALOAD 0
    GETFIELD test/Test1.a : Ljava/lang/Object;
    ALOAD 0
    GETFIELD test/Test1.a : Ljava/lang/Object;
    IF_ACMPEQ L1
...

as we can see it loads field a to local vars twice, it's a non-atomic operation, if a was changed in between by another thread comparison may produce false.

Also, memory visibility problem is relevant here, there is no guarantee that changes to a made by another thread will be visible to the current thread.

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