MULTITHREADING AND PARALLEL PROGRAMMING - Introduction to Java Programming: Comprehensive Version - page 1125

To create a semaphore, you have to specify the number of permits with an optional fair-
ness policy, as shown in Figure 30.23. 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 .

java.util.concurrent.Semaphore
+Semaphore(numberOfPermits: int)

Creates a semaphore with the specified number of permits. The
fairness policy is false.
Creates a semaphore with the specified number of permits and
the fairness policy.
Acquires a permit from this semaphore. If no permit is
available, the thread is blocked until one is available.
Releases a permit back to the semaphore.

+Semaphore(numberOfPermits: int, fair:
boolean)
+acquire(): void

+release(): void

F IGURE 30.23

The Semaphore class contains the methods for accessing a semaphore.

A semaphore with just one permit can be used to simulate a mutually exclusive lock.
Listing 30.9 revises the Account inner class in Listing 30.9 using a semaphore to ensure that
only one thread at a time can access the deposit method.

L ISTING 30.9

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 ); create a semaphore

5 private int balance = 0 ;

6
7 public int getBalance() {

8 return balance;

9 }
10
11

public void deposit( int amount) {

12 try {

13 semaphore.acquire(); // Acquire a permit 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 release a permit

26 }
27 }
28 }

A semaphore with one permit is created in line 4. A thread first acquires a permit when execut-
ing 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.