具有Java多线程应用程序的Singleton

Question

Briefly, I want to click on a button to run a background task (separated thread). I faced two problems:

• What if user click many times on that button ==>Many thread will be created.
• Even if I use Singleton mechanism, I face another problem which is the fact that only one time that instance will be created even though after task accomplishes, user can't anymore re-run the process (second click on the button ).

My class:

package mypack.check;

public class RunnableCheck implements Runnable {

    private Thread t;
    private static RunnableCheckFeeders instance;

    public RunnableCheckFeeders getDefault() {
        if (instance == null) {
            instance = new RunnableCheckFeeders();
        }
        return instance;
    }

    @Override
    public void run() {
        //What the thread is supposed to do...
    }

    public void start() {
        if (t == null) {
            t = new Thread(this, "My task");
            t.start();
        }
    }
}

In the caller class:

RunnableCheckFeeders.getDefault().start();

I tried with Synchronized methods but in vain, any proposition is welcome.

Answer 1

Your start method doesn't guarantee that only one "my task" thread will be created, even if there is only one instance of RunnableCheck : because the checking of the thread reference and subsequent assignment is not atomic, it is possible for two threads to be created and started if both happen to evaluate t == null to true at the same time (or, at least, a second thread can evaluate it to true before the first thread was able to assign a non-null value to t ).

You can guard for this by:

• Making the start method synchronized, so multiple threads cannot run the method at the same time;

• Add an AtomicBoolean to record if the thread has been created/started. By updating the value of this flag atomically, it is not possible for two threads to set it to true , and thus impossible for two new Thread s to be created and started:

   private final AtomicBoolean started = new AtomicBoolean(); private Thread t; public void start() { if (!started.compareAndSet(false, true)) { return; } t = new Thread(this, "My task"); t.start(); } 

Answer 2

I suggest you use an ExecutorService.

 enum RunOne {; // no instances
      static final ExecutorService service = Executors.newSingleThreadedExecutor();
      static Future last = null;

      static synchronized void run(Runnable run) {
          if (last != null && !last.isDone()) return;
          last = service.submit(run);
      }
 }

This will submit a new task only if there is not already one running. It won't create more than one thread but you can submit tasks after a previous one finishes. You can call service.shutdown() to stop the service.

Answer 3

When a user clicks multiple times then do you want the event to happen multiple times, or just the once? If multiple times, you don't want a singleton, but rather to create a queue of work.

You can do this in one of two ways, the first is to use aa want a thread pool, likely using one from the ExecutorService .

Your second option is to have a single queue reading from a queue and a single thread reading from the queue.

If you want the event to only happen the once then you need to disable the button until it is completed, make start synchronized so only one thread can call it at a time and then set t to null once the thread finishes (ie the last item in run() ).