同步两个线程不同步

Question

consider the following code:

public class Main {

    public static void main(String[] args){
        MyObject obj = new MyObject();
        Thread1 t1 = new Thread1(100,'#',obj);
        Thread1 t2 = new Thread1(100,'*',obj);

        t1.start();
        t2.start();
    }
}

---

public class Thread1 extends Thread {

    int myNum;
    char myChar;
    MyObject myObj;

    public Thread1(int num, char c, MyObject obj){
        myNum = num;
        myChar = c;
        myObj = obj;
    }

    public synchronized void run(){
        for(int i = 1; i<myNum; i++){
            if((i%10)==0)
                System.out.println("");
            System.out.print(myChar);
        }
    }

}

The class MyObject is empty class without anything spacial. My question is, why the synchronized is not working and I get printed the '#' and ' ' simultaneously and in random order and not one after the other?

Answer 1

Synchronization locks the monitor of an object. In your case, you are synchronizing against each of the threads objects, that is Thread A locks against Threads A's monitor and Thread B locks against Thread B's monitor. Thus they are not interacting.

I think that you meant the following for the run() method:

public void run(){
  synchronized (myObj) {
    for(int i = 1; i<myNum; i++){
        if((i%10)==0)
            System.out.println("");
        System.out.print(myChar);
    }
  }
}

If that was indeed your intended use for myObj, then I would suggest the following change too; as it will help make the code more readable.

Change:

MyObject myObj;

to:

private final MyObject lock;

EDIT an alternative approach, that does not lock against myObj but instead the Thread1's instance of Class.

public void run(){
    doJob();
}

private static synchronized void doJob() {
    for(int i = 1; i<myNum; i++){
        if((i%10)==0)
            System.out.println("");
        System.out.print(myChar);
    }
}

Answer 2

You can try to put thread.join() , wait for first thread in execution finished to start next one

public class hilos {
    public static void main(String[] args) throws InterruptedException{

        MyObject obj = new MyObject();
        Thread1 t1 = new Thread1(50,'#',obj);
        Thread1 t2 = new Thread1(50,'*',obj);

        Thread[] threads = {t1, t2}; 


        start(threads);


    }


    public synchronized static void start(Thread[] threads) throws InterruptedException{

        synchronized(threads){
            for(int i=0; i < threads.length; i++)   {
                threads[i].start();
                threads[i].join();
            }
        }

    }
}

OUTPUT:

#########
##########
##########
##########
##########*********
**********
**********
**********
**********