CODEWITHSUNDEEP
javamultithreadingconcurrency
Jim
Jim

Reputation: 19552

Can not suspend my thread! Why?

I can't figure out what is the problem in the following code:
I have a thread that can be suspended and resumed
Code bellow: 

public class CustomThread implements Runnable {   

    private volatile boolean stop;  
    private volatile boolean suspend;  

    String[] names = new String[]{  
            "A", "B","C","D","E", "F", "G","H","I","J","K", "L"  
    };  

    public CustomThread(){  
        Collections.shuffle(Arrays.asList(names));  
        System.out.println("Available names:");  
        System.out.println(Arrays.asList(names));  

    }  

    @Override  
    public void run() {  

        while(!stop){             
            synchronized (this) {  
                if(suspend){  
                    try {  
                        System.out.println("Got suspended");  
                        wait();  
                        System.out.println("Resumed");  
                    } catch (InterruptedException e) {  
                        System.out.println("Got interupted");  
                    }  
                }     
                else System.out.println("Suspend false");  
            }  
            int randomIdx = new Random().nextInt(names.length);  
            System.out.println(names[randomIdx]);             
        }  
    }  

    public synchronized void suspend(){  
        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>Suspend true");  
        suspend = true;  
    }  

    public synchronized void resume(){  
        suspend = false;  
        notify();  
    }    
}

I run the following simple code: 

public class CustomTest {  

    /**
     * @param args
     * @throws InterruptedException 
     */
    public static void main(String[] args) throws InterruptedException {  
        CustomThread c = new CustomThread();  
        Thread t = new Thread(c);  
        t.start();  
        Thread.sleep(5000);  
        System.out.println("++++++++++++++++++++++++++++++++");         
        c.suspend();  
    }  
}

What I am expecting to see is:
Thread custom runs, main sleeps, main suspends the custom thread by c.suspend() and since main terminates and noone resumes the thread, the thread remains in wait state.
But what I see instead is that the CustomThread prints continually Suspend false and an element from names.

What is the problem here? It is like the Thread.sleep(5000) and c.suspend() in main don't do anything.

Upvotes: 1

Views: 603

Answers (2)

Marko Topolnik
Marko Topolnik

Reputation: 200148

The code is fine as written, but your problem is probably that you are running this through Eclipse and you are overwhelming the console. Put a shorter delay in main and you'll see good results.

Note: your suspend method doesn't need to be synchronized as it only writes to a volatile variable.

Upvotes: 1

Hakan Serce
Hakan Serce

Reputation: 11256

Instead of if(suspend) you should have while(suspend), see the explanation in javadoc here: http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#wait%28%29

From the javadoc of Object.wait():

...interrupts and spurious wakeups are possible, and this method should always be used in a loop

Upvotes: 0

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