How can I make sure a method is only called once by multiple threads?

Question

I have the following structure:

public void someMethod(){  
   //DO SOME STUFF
   try{  
    doSomeProcessing();  
   }  
   catch (Exception e){  
                


        

Answer 1

Your loadSomeHeavyData method could use a blocking mechanism to make all threads wait until it has finished its update, but only let one of them actually do the update:

private final AtomicBoolean updateStarted = new AtomicBoolean();
private final CountDownLatch updateFinished = new CountDownLatch(1);

public void loadSomeHeavyData() {
    if (updateStarted.compareAndSet(false, true)) {
        //do the loading
        updateFinished.countDown();
    } else {
        //update already running, wait
        updateFinished.await();
    }
}

Note my assumptions:

• you want all the threads to wait until the loading completes so they can call doSomeProcessing a second time with updated data
• you only call loadSomeHeavyData once, ever - if not you will need to reset the flag and the CountdownLatch (which would then probably not be the most appropriate mechanism).

EDIT

Your latest comment indicates that you actually want to call loadSomeHeavyData more than once, just not more than once at a time.

private final Semaphore updatePermit = new Semaphore(1);

public void loadSomeHeavyData() {
    if (updatePermit.tryAcquire()) {
        //do the loading and release updatePermit when done
        updatePermit.release();
    } else {
        //update already running, wait
        updatePermit.acquire();
        //release the permit immediately
        updatePermit.release();
    }
}