CODEWITHSUNDEEP
javamemorythread-safety
rom1v
rom1v

Reputation: 2979

Java Memory Model: is the assignment guaranteed to be visible from the other thread?

Consider this program:

public class Test {
    private int i = 1;

    public void f() {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                if (i != 2)
                    throw new AssertionError("i != 2");
            }
        };
        i = 2;
        new Thread(runnable).start();
    }
}

According to the Java Memory Model, in run(), i is guaranteed to be equal to 2, at least because:

A call to start() on a thread happens-before any actions in the started thread.

So i = 2 happens-before if (i != 2). So far, so good.

But what if we don't start any thread after the assignment:

public class Test {
    private static final ExecutorService EXECUTOR = Executors.newSingleThreadExecutor();
    private int i = 1;

    public void f() {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                if (i != 2)
                    throw new AssertionError("i != 2");
            }
        };
        i = 2;
        EXECUTOR.execute(runnable);
    }
}

The previous rule does not apply. What guarantees that i = 2 happens-before the execution of run() in that case?

Does assigning the variable before creating the Runnable have any affect on the happens-before order:

public class Test {
    private static final ExecutorService EXECUTOR = Executors.newSingleThreadExecutor();
    private int i = 1;

    public void f() {
        i = 2;
        EXECUTOR.execute(new Runnable() {
            @Override
            public void run() {
                if (i != 2)
                    throw new AssertionError("i != 2");
            }
        });
    }
}

?

Upvotes: 1

Views: 72

Answers (1)

Louis Wasserman
Louis Wasserman

Reputation: 198211

From the ExecutorService Javadoc:

Actions in a thread prior to the submission of a Runnable or Callable task to an ExecutorService happen-before any actions taken by that task.

Upvotes: 4

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