Java中的同步

Question

-I want to print the output below by using synchronization technique in Java

1
1
2
2
3
3
4
4
5
5

-I wrote the code below but I am getting a compile time error saying obj variable is not referenced

-obj variable is in the synchronization block in the run method

-I created 2 threads Child1 and Child2 and shared obj between them

-How to make obj visible to synchronization block so that I can get desired output?

package multi_threading;

public class inter_thread {
    boolean val=false;
    Thread t;
    public inter_thread(test_value obj,String msg){
        t=new Thread(obj,msg);
        t.start();
    }
}
class test_value implements Runnable{
    boolean val=false;
    public static void main(String args[]){
        test_value obj=new test_value();
        inter_thread obj1=new inter_thread(obj,"Child1"); 
        inter_thread obj2=new inter_thread(obj,"Child2");
        try{
            obj1.t.join();
            obj2.t.join();
        }catch(InterruptedException e){
            System.out.println("Interrupted");
        }
    }
public void run(){
    int i;
    synchronized(obj){
       for(i=1;i<=5;i++){
       System.out.println(i);
       obj.val=!obj.val;
           while(obj.val)
                try{
                    wait();
                }catch(InterruptedException e){
                    System.out.println("Interrupted");
                }
               notify();
        }
      }
    }
 }

Answer 1

I think the easiest way is to use a CyclicBarrier :

public class SyncTask implements Runnable {
    private final CyclicBarrier barrier;

    public SyncTask(CyclicBarrier barrier) { this.barrier = barrier; }

    @Override
    public void run() {
        try {
            for (int i = 1; i <= 5; i++) {
                System.out.println(i);
                barrier.await();
            }
        } catch (InterruptedException | BrokenBarrierException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        CyclicBarrier barrier = new CyclicBarrier(2);
        new Thread(new SyncTask(barrier)).start();
        new Thread(new SyncTask(barrier)).start();
    }
}

(Note that here the two tasks are not really alternating but run in parallel. But the CyclicBarrier forces them to wait for each other after each System.out.println )

Answer 2

Try this out...it worked for me! Please add parentheses for while loop and restrict synchronized.

And to obtain obj, make it an instance variable so that run method can access it

     class inter_thread {
    boolean val=false;
    Thread t;
    public inter_thread(test_value obj,String msg){
        t=new Thread(obj,msg);
        t.start();
    }
}
 public class test_value implements Runnable{
    boolean val=false;
    static test_value obj=new test_value();
    public static void main(String args[]){

        inter_thread obj1=new inter_thread(obj,"Child1"); 
        inter_thread obj2=new inter_thread(obj,"Child2");
    }
public void run(){
    int i;
       for(i=1;i<=5;i++){
       System.out.println(i);

       synchronized(obj){
        obj.val=!obj.val;
           while(obj.val){
                try{
                    wait();
                }catch(InterruptedException e){
                    System.out.println("Interrupted");
                }
           }
        notify();
        }
      }
    }
 }