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修改锁的竞争原则

ReentrantLock和ReentrantReadWriteLock的构造函数都有一个Boolean型参数fairness，默认为false，该参数用来控制改变锁的竞争原则，即两种竞争模式

• non-fair mode:当多个线程等待获取锁时，jvm随机选择其中一个线程获取锁对象，访问临界区；
• fair-mode:在该模式下，jvm会选择等待时间最长的线程来获取锁；

然而tryLock()方法可以不时线程睡眠，所以如果是使用tryLock()那么fairness参数将无效，不会有任何影响；

下面例子中使用一个属性作为实验，看在两种模式下的不同表现；

动手实验

（1）创建PrintQueue

public class PrintQueue {private final Lock queueLock=new ReentrantLock(true);public void printJob(Object document) {queueLock.lock();try {Long duration=(long)(Math.random()*10000);System.out.printf("%s:PrintQueue: Printing a Job during %d seconds\n",Thread.currentThread().getName(),(duration/1000));Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();}finally {queueLock.unlock();}//-------------queueLock.lock();try {Long duration=(long)(Math.random()*10000);System.out.printf("%s:PrintQueue: Printing a Job during %d seconds\n",Thread.currentThread().getName(),(duration/1000));Thread.sleep(duration);} catch (InterruptedException e) {e.printStackTrace();}finally {queueLock.unlock();}}
}

(2)创建Job

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());}public static void main(String[] args) {PrintQueue printQueue=new PrintQueue();Thread thread[]=new Thread[10];for (int i=0; i<10; i++){thread[i]=new Thread(new Job(printQueue),"Thread "+ i);}for (int i=0; i<10; i++){thread[i].start();try {Thread.sleep(100);} catch (InterruptedException e) {e.printStackTrace();}}}
}

一次运行结果：

Thread 0: Going to print a document
Thread 0:PrintQueue: Printing a Job during 0 seconds
Thread 1: Going to print a document
Thread 2: Going to print a document
Thread 3: Going to print a document
Thread 4: Going to print a document
Thread 5: Going to print a document
Thread 6: Going to print a document
Thread 7: Going to print a document
Thread 1:PrintQueue: Printing a Job during 9 seconds
Thread 8: Going to print a document
Thread 9: Going to print a document
Thread 2:PrintQueue: Printing a Job during 8 seconds
Thread 3:PrintQueue: Printing a Job during 1 seconds
Thread 4:PrintQueue: Printing a Job during 7 seconds
Thread 5:PrintQueue: Printing a Job during 4 seconds
Thread 6:PrintQueue: Printing a Job during 9 seconds
Thread 7:PrintQueue: Printing a Job during 8 seconds
Thread 0:PrintQueue: Printing a Job during 0 seconds
Thread 8:PrintQueue: Printing a Job during 0 seconds
Thread 0: The document has been printed
当fairness设置为true，当printJob()方法执行再次获取Lock时，此时应该是Thread-1等待的时间最长，也是应该最先获取锁的线程，如上输出结果；如果将fairness设置为false，在此观察输出结果，多运行几次发现结果都不一样；

要点

了解fairness参数的作用