27.9. Review Questions

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24.14. (Optional) Semaphores

Semaphores can be used to restrict the number of threads that access a shared resource. Before accessing the resource, a thread must acquire a permit from the semaphore. After finishing with the resource, the thread must return the permit back to the semaphore, as shown in Figure 24.23.

Figure 24.23. A limited number of threads can access a shared resource controlled by a semaphore.

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To create a semaphore, you have to specify the number of permits with an optional fairness policy, as shown in Figure 24.24. A task acquires a permit by invoking the semaphore's acquire() method and releases the permit by invoking the semaphore's release() method. Once a permit is acquired , the total number of available permits in a semaphore is reduced by 1. Once a permit is released, the total number of available permits in a semaphore is increased by 1.

Figure 24.24. The Semaphore class contains the methods for accessing a semaphore.

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A semaphore with just one permit can be used to simulate a mutually exclusive lock. Listing 24.12 revises the Account inner class in Listing 24.10 using a semaphore to ensure that only one thread can access the deposit method at a time.

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Listing 24.12. New Account Inner Class

 1  // An inner class for account  2   private static class   Account {  3  // Create a semaphore  4    private static   Semaphore semaphore =   new   Semaphore(   1   );  5   private int   balance =     ;  6  7   public int   getBalance() {  8   return   balance;  9    } 10 11   public void   deposit(   int   amount) { 12   try   { 13  semaphore.acquire();  // Acquire a permit   14   int   newBalance = balance + amount; 15 16  // This delay is deliberately added to magnify the  17  // data-corruption problem and make it easy to see.  18        Thread.sleep(   5   ); 19 20        balance = newBalance; 21      } 22   catch   (InterruptedException ex) { 23      } 24   finally   { 25  semaphore.release();  // Release a permit   26      } 27    } 28  } 

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A semaphore with one permit is created in line 4. A thread first acquires a permit when executing the deposit method in line 13. After the balance is updated, the thread releases the permit in line 25. It is a good practice to always place the release() method in the finally clause to ensure that the permit is finally released even in the case of exceptions.