Break the loop using threads

[Sandeep Kumar B]

I have a class with method calling getValues

getValues would print the values from 1 to 10

I make two objects and what gets printed is
1
2
3
...
\
..
.

1
2
3
.
..
10

What I want is

1
1
2
2
3
3
...
...
10
10



I used Threaad.sleep and also used a static variable in the class to merge the results
Now I want to know if its possible to get separately the similar values returned by each thread

1 1

2 2

[John Vorwald]

Try a semaphore with one permit, look for producer / consumer examples...

[Sandeep Kumar B]

thanks, that helps

[Rohan Dhapodkar]

John Vorwald wrote:Try a semaphore with one permit, look for producer / consumer examples...

Semaphore is not perfect solution to this problem. Semaphore is to guarantee that specified number of threads can only enter into restricted area but not their ordering.
Below code will work with high sleep time 100 but if you will reduce sleep from 100ms to 1ms, you can notice a difference.

import java.util.concurrent.Semaphore;


public class PrintUsingSemaphore {

	public static void main(String[] args)  throws Exception{
		final Semaphore semaphore = new Semaphore(1);
		class MyPrinter implements Runnable {
			public void run(){
				for (int i = 0; i < 20; i++) {
					try {
						semaphore.acquire();
						System.out.println(i+" "+Thread.currentThread().getName());
						semaphore.release();
						Thread.sleep(100);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
				}
			}
		};
		
		for (int i = 0; i < 2; i++) {
			Thread t = new Thread(new MyPrinter());
			t.setPriority(Thread.MAX_PRIORITY-i*2);
			t.start();
		}
		Thread.sleep(100000);
	}

}

output with 100 ms.
0 Thread-0
0 Thread-1
1 Thread-0
1 Thread-1
2 Thread-0
2 Thread-1
3 Thread-0
3 Thread-1
4 Thread-0
4 Thread-1
5 Thread-0
5 Thread-1
6 Thread-0
6 Thread-1
7 Thread-0
7 Thread-1
8 Thread-0
8 Thread-1
9 Thread-0
9 Thread-1
10 Thread-0
10 Thread-1
11 Thread-0
11 Thread-1
12 Thread-0
12 Thread-1
13 Thread-0
13 Thread-1
14 Thread-0
14 Thread-1
15 Thread-0
15 Thread-1
16 Thread-0
16 Thread-1
17 Thread-0
17 Thread-1
18 Thread-0
18 Thread-1
19 Thread-0
19 Thread-1

Output with 1 ms.
0 Thread-0
1 Thread-0
0 Thread-1
2 Thread-0
1 Thread-1
3 Thread-0
2 Thread-1
4 Thread-0
3 Thread-1
5 Thread-0
4 Thread-1
6 Thread-0
5 Thread-1
7 Thread-0
6 Thread-1
8 Thread-0
7 Thread-1
9 Thread-0
8 Thread-1
10 Thread-0
9 Thread-1
11 Thread-0
10 Thread-1
12 Thread-0
11 Thread-1
13 Thread-0
12 Thread-1
14 Thread-0
13 Thread-1
15 Thread-0
14 Thread-1
16 Thread-0
15 Thread-1
17 Thread-0
16 Thread-1
18 Thread-0
17 Thread-1
19 Thread-0
18 Thread-1
19 Thread-1

Cyclic Barrier is perfect solution for such problems. This is gauranteed to work with or without Thread.sleep().

import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;


public class PrintUsingCyclicBarrier {

	public static void main(String[] args)  throws Exception{
		final CyclicBarrier semaphore = new CyclicBarrier(2,new Runnable() {
			public void run() {
				System.out.println();
			}
		});
		class MyPrinter implements Runnable {
			public void run(){
				for (int i = 0; i < 20; i++) {
					try {
						semaphore.await();
						System.out.print(i+" ");
						Thread.sleep(100);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}catch (BrokenBarrierException e) {
						e.printStackTrace();
					}
				}
			}
		};
		
		for (int i = 0; i < 2; i++) {
			Thread t = new Thread(new MyPrinter());
			t.setPriority(Thread.MAX_PRIORITY-i*2);
			t.start();
		}
		Thread.sleep(100000);
	}

}

output:

0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 10
11 11
12 12
13 13
14 14
15 15
16 16
17 17
18 18
19 19

[Nomaan Butt]

quite informative, Rohan

[Harsha Smith]

without semaphore, cyclic barrier, the following code produced desired result for several runs.

public static void main(String[] args)  throws Exception{  
      
        class MyPrinter implements Runnable {  
            public void run(){  
                for (int i = 0; i < 20; i++) {  
                    try {  
                        System.out.println(i +" "+Thread.currentThread().getName());  
                        
                        TimeUnit.MILLISECONDS.sleep(1);  
                    } catch (InterruptedException e) {  
                        e.printStackTrace();  
                    }  
                }  
            }  
        };  
          
        for (int i = 0; i < 2; i++) {  
            Thread t = new Thread(new MyPrinter());  
            t.start();  
        }  
        
    } 

[Rohan Dhapodkar]

Harsha Smith wrote:without semaphore, cyclic barrier, the following code produced desired result for several runs.

This code is not perfect. Check results given below. Also what if i don;t want to put any sleep ? Above mentioned code execution is entirely based on Thread scheduler and it's not guaranteed to work always.
Try your code with just 3 threads and you will see difference.

0 Thread-0
0 Thread-1
0 Thread-2
1 Thread-0
1 Thread-1
1 Thread-2
2 Thread-0
2 Thread-1
2 Thread-2
3 Thread-0
3 Thread-1
3 Thread-2
4 Thread-0
4 Thread-1
4 Thread-2
5 Thread-0
5 Thread-1
5 Thread-2
6 Thread-0
6 Thread-1
6 Thread-2
7 Thread-0
7 Thread-1
7 Thread-2
8 Thread-0
8 Thread-1
8 Thread-2
9 Thread-0
9 Thread-1
9 Thread-2
10 Thread-0
10 Thread-1
10 Thread-2
11 Thread-0
11 Thread-1
11 Thread-2
12 Thread-0
12 Thread-1
12 Thread-2
13 Thread-0
13 Thread-1
13 Thread-2
14 Thread-0
14 Thread-1
15 Thread-0
15 Thread-1
14 Thread-2
16 Thread-0
16 Thread-1
15 Thread-2
17 Thread-0
17 Thread-1
16 Thread-2
18 Thread-0
18 Thread-1
17 Thread-2
19 Thread-0
19 Thread-1
18 Thread-2
19 Thread-2


Even with just 2 threads out put is not perfect.
Check this
0 Thread-0
0 Thread-1
1 Thread-0
1 Thread-1
2 Thread-0
2 Thread-1
3 Thread-0
3 Thread-1
4 Thread-0
4 Thread-1
5 Thread-0
5 Thread-1
6 Thread-0
6 Thread-1
7 Thread-0
7 Thread-1
8 Thread-0
8 Thread-1
9 Thread-0
9 Thread-1
10 Thread-0
10 Thread-1
11 Thread-0
11 Thread-1
12 Thread-0
12 Thread-1
13 Thread-0
14 Thread-0
13 Thread-1
15 Thread-0
14 Thread-1
16 Thread-0
15 Thread-1
17 Thread-0
16 Thread-1
18 Thread-0
17 Thread-1
19 Thread-0
18 Thread-1
19 Thread-1