Question

Are the following two method definitions semantically equivalent? Why? Why not?

Version A:

private static synchronized void foo() {
    bar();
}

Version B:

private static Semaphore available = new Semaphore(1, true);
private static void foo() {
    available.acquire();
    try {
        bar();
    }
    finally {
       available.release();
    }
}

Answer 1

Yes, except

• that the first one synchronizes on the class, and that there might be other parts of the code that synchronize on the same class.
• that the semaphore could be fair, whereas synchronization is not
• that the thread blocked in semaphore.acquire could be interrupted, causing an InterruptedException to be thrown. Your code doesn’t compile as is: you must handle this exception somehow, whereas you don’t have to using synchronized.