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;    }    @Override    public 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