ReentrantLock和ReentrantReadWriteLock類的構(gòu)造器都含有一個(gè)布爾參數(shù)fair，它允許你控制這兩個(gè)類的行為。默認(rèn)fair值為false，它稱為非公平模式（Non-Fair Mode）。在非公平模式下，當(dāng)有很多線程在等待鎖（ReentrantLock和ReentrantReadWriteLock）時(shí)，鎖將選擇它們當(dāng)中的一個(gè)來訪問臨界區(qū)，這個(gè)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(dāng)有很多線程在等待鎖（ReentrantLock和ReentrantReadWriteLock）時(shí)，鎖將選擇它們中的一個(gè)來訪問臨界區(qū)，而且選擇的是等待時(shí)間最長(zhǎng)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(gè)方法。

　　下面我們將修改“使用鎖實(shí)現(xiàn)同步”當(dāng)中的范例來使用這個(gè)屬性，并觀察公平模式和非公平模式之間的區(qū)別。

1. 創(chuàng)建一個(gè)打印隊(duì)列類PrintQueue。

import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;public class PrintQueue {//聲明一個(gè)鎖對(duì)象，并且用ReentrantLock類初始化private final Lock queueLock = new ReentrantLock(true);//實(shí)現(xiàn)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(chuàng)建打印工作類Job并且實(shí)現(xiàn)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(chuàng)建范例的主類Main

public class Main {public static void main(String[] args) {//創(chuàng)建一個(gè)共享的打印隊(duì)列對(duì)象PrintQueue printQueue = new PrintQueue();//創(chuàng)建10個(gè)打印工作Job對(duì)象Thread threads[] = new Thread[10];for(int i=0;i<10;i++){threads[i] = new Thread(new Job(printQueue), "Thread"+i);}//啟動(dòng)10個(gè)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(yùn)行結(jié)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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