ReentrantLock和ReentrantReadWriteLock類的構造器都含有一個布爾參數fair，它允許你控制這兩個類的行為。默認fair值為false，它稱為非公平模式（Non-Fair Mode）。在非公平模式下，當有很多線程在等待鎖（ReentrantLock和ReentrantReadWriteLock）時，鎖將選擇它們當中的一個來訪問臨界區，這個選擇是沒有任何約束的。如果fair值是true，則稱為公平模式（Fair Mode）。在公平模式下，當有很多線程在等待鎖（ReentrantLock和ReentrantReadWriteLock）時，鎖將選擇它們中的一個來訪問臨界區，而且選擇的是等待時間最長的。這兩種模式只適用于lock()和unlock()方法。而Lock接口的tryLock()方法沒有將線程置于休眠，fair屬性并不影響這個方法。

　　下面我們將修改“使用鎖實現同步”當中的范例來使用這個屬性，并觀察公平模式和非公平模式之間的區別。

1. 創建一個打印隊列類PrintQueue。

import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;public class PrintQueue {//聲明一個鎖對象，并且用ReentrantLock類初始化private final Lock queueLock = new ReentrantLock(true);//實現打印方法public void printJob(Object doucument){queueLock.lock();Long duration = (long) (Math.random()*10000);System.out.println(Thread.currentThread().getName()+": PrintQueue: Printing a Job during "+(duration/1000)+" seconds");try {Thread.sleep(duration);} catch (InterruptedException e) {// TODO Auto-generated catch block e.printStackTrace();} finally {queueLock.unlock();}queueLock.lock();duration = (long) (Math.random()*10000);System.out.println(Thread.currentThread().getName()+": PrintQueue: Printing a Job during "+(duration/1000)+" seconds");try {Thread.sleep(duration);} catch (InterruptedException e) {// TODO Auto-generated catch block e.printStackTrace();} finally {queueLock.unlock();}}}

2. 創建打印工作類Job并且實現Runnable接口。

public class Job implements Runnable {private PrintQueue printQueue;public Job(PrintQueue printQueue){this.printQueue = printQueue;}@Overridepublic void run() {System.out.printf("%s: Going to print a document\n", Thread.currentThread().getName());printQueue.printJob(new Object());System.out.printf("%s: The Document has been printed\n", Thread.currentThread().getName());}}

3. 創建范例的主類Main

public class Main {public static void main(String[] args) {//創建一個共享的打印隊列對象PrintQueue printQueue = new PrintQueue();//創建10個打印工作Job對象Thread threads[] = new Thread[10];for(int i=0;i<10;i++){threads[i] = new Thread(new Job(printQueue), "Thread"+i);}//啟動10個線程try {for(int i=0;i<10;i++){threads[i].start();Thread.sleep(100);}} catch (InterruptedException e) {// TODO Auto-generated catch block e.printStackTrace();}} }

4. 程序運行結果如下

Thread0: Going to print a document Thread0: PrintQueue: Printing a Job during 4 seconds Thread3: Going to print a document Thread2: Going to print a document Thread1: Going to print a document Thread4: Going to print a document Thread5: Going to print a document Thread6: Going to print a document Thread7: Going to print a document Thread8: Going to print a document Thread9: Going to print a document Thread3: PrintQueue: Printing a Job during 9 seconds Thread2: PrintQueue: Printing a Job during 3 seconds Thread1: PrintQueue: Printing a Job during 7 seconds Thread4: PrintQueue: Printing a Job during 7 seconds Thread5: PrintQueue: Printing a Job during 5 seconds Thread6: PrintQueue: Printing a Job during 6 seconds Thread7: PrintQueue: Printing a Job during 0 seconds Thread8: PrintQueue: Printing a Job during 7 seconds Thread9: PrintQueue: Printing a Job during 5 seconds Thread0: PrintQueue: Printing a Job during 4 seconds Thread0: The Document has been printed Thread3: PrintQueue: Printing a Job during 5 seconds Thread3: The Document has been printed Thread2: PrintQueue: Printing a Job during 1 seconds Thread2: The Document has been printed Thread1: PrintQueue: Printing a Job during 9 seconds Thread1: The Document has been printed Thread4: PrintQueue: Printing a Job during 4 seconds Thread4: The Document has been printed Thread5: PrintQueue: Printing a Job during 2 seconds Thread5: The Document has been printed Thread6: PrintQueue: Printing a Job during 5 seconds Thread6: The Document has been printed Thread7: PrintQueue: Printing a Job during 9 seconds Thread7: The Document has been printed Thread8: PrintQueue: Printing a Job during 4 seconds Thread8: The Document has been printed Thread9: PrintQueue: Printing a Job during 6 seconds Thread9: The Document has been printed

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