Java Thread suspend does not work

Question

This is a sample code using java thread. The problem is that I cannot suspend the thread.

public class RunnableDemo implements Runnable {
   public Thread t;
   private String threadName;
   boolean suspended = false;
   RunnableDemo( String name){
       threadName = name;
       System.out.println("Creating " +  threadName );
   }
   public void run() {
      System.out.println("Running " +  threadName );
      try {
         while (true) {
            System.out.println("Thread: " + threadName + ", " + i);
            // Let the thread sleep for a while.
            Thread.sleep(300);
            synchronized(this) {
               while(suspended) {
                  wait();
               }
            }
        }
     } catch (InterruptedException e) {
         System.out.println("Thread " +  threadName + " interrupted.");
     }
     System.out.println("Thread " +  threadName + " exiting.");
   }
   public void start ()
   {
      System.out.println("Starting " +  threadName );
      if (t == null)
      {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
   public void suspend() {
      suspended = true;
   }

The suspend function below does not work:

RunnableDemo R1 = new RunnableDemo( "Thread-1");
R1.start();
R1.suspend();  // This line does not work and thread continues ...

In fact the thread does not see the new value for suspended when I call suspend() . Any idea ?

The `suspend` function isn't synchronized. So your `synchronized(this)` block doesn't actually do anything. Only one thread calls `run` anyway, so you aren't synchronizing it to anything else. You also need to call `notify` in `suspend`, otherwise the thread will not wake up. — David Schwartz, Feb 24 '16 at 01:30

score 3 · Answer 1 · edited May 23 '17 at 12:31

3

Declare suspended as volatile:

volatile boolean suspended = false;

edited May 23 '17 at 12:31

Community

1
1

answered Feb 23 '16 at 22:19

Jean Logeart

52,687
11
83
118

That won't allow the `wait()` call to return. – VGR Feb 23 '16 at 22:23
Thanks, but it did not work for me. Still in the thread 'suspended' is equal to false – Morteza Mashayekhi Feb 23 '16 at 22:32

Simon Fischer · Answer 2 · 2016-02-23T22:48:54.107

It is not absolutely clear to me what you are trying to achieve, but here are a few observations and tips that may help:

By the time the main thread has completed the suspend() method, the second thread, "Thread-1", has probably not even entered the loop. You will see this if you add more debug output in suspend() and before and after wait(). The opposite could also be true, and you would reach the call to wait() before suspend is set to true. Then, once wait() is called, you won't see any more updates in "Thread-1" since it is waiting.
The wait() method will never return. To achieve this, synchronize on the same object that you synchronize on when calling wait(), i.e. R1 in the main method and call notifyAll() on this object. You can also do that inside your suspend() method.
This approach makes methods like Thread.sleep() pretty much unnecessary. Calling wait() is no active waiting and won't consume significant resources.

In short, change your suspend-method to

void suspend() {
   suspended = true;
   synchronized (this) {
       this.notifyAll();
   }
}

This will change how your program behaves and make it more clear what happens. But either way, it does not seem like "suspend" is a good name for what the method does, before or after, but if you experiment with this you may get a better understanding of how you can achieve what you want to achieve.

The problem is that the 'suspended' value in the thread is always false although suspend() turns it to true. — Morteza Mashayekhi, Feb 23 '16 at 22:40
There is no guarantee. You could just as well reach your call to wait() before suspend becomes true. Then you wait() and will not see any more updates. Do the notifyAll() and you will revive Thread-1 and see the update. — Simon Fischer, Feb 23 '16 at 22:42
I did notifyAll() at the end of suspend(). I got an exception: java.lang.IllegalMonitorStateException — Morteza Mashayekhi, Feb 23 '16 at 22:45

score 1 · Accepted Answer · edited May 23 '17 at 12:31

Using wait() might not work as you expect. The wait/notify is built as a simple implementation of the sempaphore (as an object instance wide monitor) context of concurrent programming. Calling notify() from any other thread not locking the monitor of the thread object instance won't work, see Java Wait and Notify: IllegalMonitorStateException

You can easily achieve your goal by checking the suspended flag regularly, and just wait a bit if your thread shall not do anything:

public class SuspendDemo implements Runnable {
    public Thread t;
    private final String threadName;
    boolean suspended = false;
    SuspendDemo(final String name){
        threadName = name;
        System.out.println("Creating " +  threadName );
    }

    @Override
    public void run() {
        System.out.println("Running " +  threadName );
        try {
            while (true) {
                System.out.println("Thread: " + threadName);
                // Let the thread sleep for a while.
                Thread.sleep(300);
                while(suspended) {
                    System.out.println("suspended");
                    Thread.sleep(50);
                }
            }
        } catch (final InterruptedException e) {
            System.out.println("Thread " +  threadName + " interrupted.");
        }
        System.out.println("Thread " +  threadName + " exiting.");
    }

    public void start () {
        System.out.println("Starting " +  threadName );
        if (t == null) {
            t = new Thread (this, threadName);
            t.start ();
        }
    }

    void suspend() {
        suspended = true;
    }

    void resume() {
        suspended = false;
        notify();
    }

    public static void main(final String[] args) throws InterruptedException  {
        final SuspendDemo R1 = new SuspendDemo( "Thread-1");
        R1.start();
        R1.suspend();
        Thread.sleep(500);
        R1.resume();
    }
}

Resulting:

> Creating Thread-1 
> Starting Thread-1 
> Running Thread-1 
> Thread: Thread-1
> suspended
> suspended
> suspended
> suspended
> Thread: Thread-1
> Thread: Thread-1
> Thread: Thread-1

The suspend flag is a native boolean, so writing it is an atomic operation, therefore you can go without synchronization on anything. I suggest to fully understand concurrent programming and the monitor concurrency model what Java uses, before using synchronized. I found using synchronize(this) blocks hard to maintain and prone to mistakes.

The next example solves the problem with wait() and notifyAll(). Note the use of synchronized methods for locking the monitor of the thread object instead of the synchronize(this) blocks.

public class SuspendDemo implements Runnable {
    public Thread t;
    private final String threadName;
    boolean suspended = false;
    SuspendDemo(final String name){
        threadName = name;
        System.out.println("Creating " +  threadName );
    }

    @Override
    public void run() {
        System.out.println("Running " +  threadName );
        try {
            while (true) {
                System.out.println("Thread: " + threadName);
                // Let the thread sleep for a while.
                Thread.sleep(300);
                work();
            }
        } catch (final InterruptedException e) {
            System.out.println("Thread " +  threadName + " interrupted.");
        }
        System.out.println("Thread " +  threadName + " exiting.");
    }

    synchronized protected void work() throws InterruptedException {
        while(suspended) {
            System.out.println("suspended");
            wait();
            System.out.println("resumed");
        }
    }

    public void start () {
        System.out.println("Starting " +  threadName );
        if (t == null) {
            t = new Thread (this, threadName);
            t.start ();
        }
    }

    synchronized void suspend() {
        suspended = true;
        notifyAll();
    }

    synchronized void resume() {
        suspended = false;
        notifyAll();
    }

    public static void main(final String[] args) throws InterruptedException  {
        final SuspendDemo R1 = new SuspendDemo( "Thread-1");
        R1.start();
        R1.suspend();
        Thread.sleep(500);
        R1.resume();
    }
}

Thanks. I got java.lang.IllegalMonitorStateException when it reached to notify(). Moreover, when I make an object of this class in other class, it does not work. — Morteza Mashayekhi, Feb 24 '16 at 03:17
Please check the code again. If you omit the synchronized keyword from suspend or resume, then you get an IllegalMonitorStateException. It is important to first obtain the lock on the monitor (synchronized takes care of that). Can you elaborate what do you mean "object of this class in other class" and what does not work? — Gee Bee, Feb 24 '16 at 09:46
You are creating R1 inside the same class in the main function. I said that when you create R1 in another class then suspend() does not work — Morteza Mashayekhi, Feb 24 '16 at 15:36
I've just checked and it worked. I've moved the main() to another class, and it works flawlessly. The only way to get an IllegalMonitorStateException is to *not* use synchronized. Can you post your code so I can verify your observations? — Gee Bee, Feb 24 '16 at 15:50
Here is my function that uses your second code: `public void startStopRE(int start) { SuspendDemo sd = new SuspendDemo ("RE"); if (start == 1) sd.start(); else sd.suspend(); }` — Morteza Mashayekhi, Feb 24 '16 at 16:01
I am afraid your code is not doing what you expect. If you pass start=0, then literally nothing happens. You call suspend but the thread is not even started. If you pass=1, a new thread is started and nothing is suspended. Since you always start a new thread, you can not actually suspend any thread. What is your goal? If you want that the tread to have a start/stop method, then implement that right in the thread cass. If you need some sort of external code which can create a new thread if needed, and can suspend that thread and resume that thread, you need this startStopRE method. Letmeknow — Gee Bee, Feb 24 '16 at 16:17
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/104447/discussion-between-gee-bee-and-mashaye). — Gee Bee, Feb 24 '16 at 16:19
Fixed via discussion. The code was expected to work in a REST controller, which is getting re-created by each page hit causing creating a new thread, and it was impossible to stop any thread then. — Gee Bee, Feb 24 '16 at 17:59

Java Thread suspend does not work

3 Answers3