Why does another thread get access to blocked object while synchronized section has not completed?

Question

The message "main Thread" is printed before message "new Thread" though method with message "new Thread" is in syncronized section by object which include print* methods.

    Test test = new Test();
    new Thread(() -> {
        try {
            synchronized (test) {
                Thread.sleep(5000);
                test.printMessage();
            }
        } catch (InterruptedException e) {}
    }).start();
    Thread.sleep(1000);
    test.printAnotherMessage();
}

public void printMessage() {
    System.out.println("new Thread");
}

public void printAnotherMessage() {
    System.out.println("main Thread");
}

}

Only, explicit synchonizations (synchonized method or synchonized statement) on an instance makes threads to require to get the monitor on that object to execute the synchronized code. — davidxxx, Sep 20 '19 at 10:29
The other thread is not going into any `synchronized` blocks. You need to have those on both sides. — Thilo, Sep 20 '19 at 10:39

score 3 · Answer 1 · answered Sep 20 '19 at 10:21

3

In this example there is no synchronization between printAnotherMessage and the synchronized block that sleeps for 5 seconds, that's why main thread sleeps for 1 second and then prints main Thread without any waiting.

You probably intended to make printAnotherMessage a synchronized method. In this case main thread will wait till the other thread finishes the execution of the block synchronized on test object.

answered Sep 20 '19 at 10:21

Roman-Stop RU aggression in UA

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I thought that "synchronized(object)" blocks access to any members of that object no matter synchronized or not – Jara G Sep 20 '19 at 10:40
@JaraG, You and many other beginners. But that's not what it does. It only prevents other threads from synchronizing on the same object at the same time. (IMO: This is just one of several Java features whose use or meaning is obvious to experts, but which were named or designed in a way that is confusing to beginners.) – Solomon Slow Sep 20 '19 at 13:30

Andrew Tobilko · Accepted Answer · 2019-09-20T10:58:45.997

There is no synchronisation on test.printAnotherMessage(); so it will be executed first assuming the timing is right. 4 seconds is a lot and should be enough.

synchronized (test) {
    test.printAnotherMessage();
}

Thread.sleep is rarely a good option, though. A more proper way would be

Test test = new Test();

new Thread(() -> {
    synchronized (test) {
        test.printMessage();
        test.notify();
    }
}).start();

synchronized (test) {
    test.wait();
    test.printAnotherMessage();
}

I am playing a dangerous game here since I am assuming the main thread will enter the synchronized block and execute wait() before another thread is created and it enters its synchronized block. It's reasonable since creating a thread will take some time.

Test test = new Test();
new Thread(() -> {
    try {
        // a lot of time to let the main thread execute wait()
        Thread.sleep(500); 

        synchronized (test) {
            test.printMessage();
            test.notify();
        }
    } catch (InterruptedException e) {}
}).start();

synchronized (test) {
    test.wait();
    test.printAnotherMessage();
}

Why does another thread get access to blocked object while synchronized section has not completed?

2 Answers2