MULTITHREADING AND PARALLEL PROGRAMMING - Introduction to Java Programming: Comprehensive Version - page 1113

«interface»
java.util.concurrent.locks.Lock

+ lock(): void
+ unlock(): void
+ newCondition(): Condition

Acquires the lock.
Releases the lock.
Returns a new

Condition

instance that is bound to this

Lock

instance.

java.util.concurrent.locks.ReentrantLock
+ReentrantLock()
+ReentrantLock(fair: boolean)

ReentrantLock(false)

Same as .

Creates a lock with the given fairness policy. When the
fairness is true, the longest-waiting thread will get the
lock. Otherwise, there is no particular access order.

F IGURE 30.13

The ReentrantLock class implements the Lock interface to represent a lock.

ReentrantLock is a concrete implementation of Lock for creating mutually exclusive
locks. You can create a lock with the specified fairness policy . True fairness policies guar-
antee that the longest-waiting thread will obtain the lock first. False fairness policies grant a
lock to a waiting thread arbitrarily. Programs using fair locks accessed by many threads may
have poorer overall performance than those using the default setting, but they have smaller
variances in times to obtain locks and prevent starvation.
Listing 30.5 revises the program in Listing 30.7 to synchronize the account modification
using explicit locks.

fairness policy

L ISTING 30.5

AccountWithSyncUsingLock.java
1 import java.util.concurrent.*;
2 import java.util.concurrent.locks.*;
3
4 public class AccountWithSyncUsingLock {
5

package for locks

private static Account account = new Account();

6
7 public static void main(String[] args) {
8 ExecutorService executor = Executors.newCachedThreadPool();
9
10 // Create and launch 100 threads
11 for ( int i = 0 ; i < 100 ; i++) {
12 executor.execute( new AddAPennyTask());
13 }
14
15 executor.shutdown();
16
17 // Wait until all tasks are finished
18 while (!executor.isTerminated()) {
19 }
20
21 System.out.println( "What is balance? " + account.getBalance());
22 }
23
24 // A thread for adding a penny to the account
25 public static class AddAPennyTask implements Runnable {
26 public void run() {
27 account.deposit( 1 );
28 }
29 }
30