Question

How can I prevent from concurrent access. I have code like this

public class MC implements Runnable {

    public void run() {
        sync();
    }

    public static void main(String p[]){
        MC mc = new MC();
        MC mc2 = new MC();
        MC mc3 = new MC();
        MC mc4 = new MC();
        Thread t = new Thread(mc);
            t.start();
            Thread t2 = new Thread(mc2);
            t2.start();
            Thread t3 = new Thread(mc3);
            t3.start();
            Thread t4 = new Thread(mc4);
            t4.start();
    }

    private synchronized void sync(){
        try {
            System.out.println(System.currentTimeMillis());
            Thread.sleep(10000);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
    }
}

and I am getting output like this

1307082622317
1307082622317
1307082622317
1307082622317
BUILD SUCCESSFUL (total time: 11 seconds)

any advice?

Answer 1

make your method static:

private static synchronized void sync();

your method as coded is synchronized on the instance, but each thread has its own instance, so there’s no synchronization.

static methods are synchronized on the Class object, of which there is only one per class, so all instances will synchronize on static methods.