锁机制最大的改进之一就是ReadWriteLock接口和它的唯一实现类ReentrantReadWriteLock。这个类有两个锁，一个是读操作锁，另一个是写操作锁。使用读操作锁时可以允许多个线程同时访问，但是使用写操作锁时只允许一个线程进行。在一个线程执行写操作时，其他线程不能够执行读操作。

　　下面我们将通过范例学习如何使用ReadWriteLock接口编写程序。这个范例将使用ReadWriteLock接口控制对价格对象的访问，价格对象存储了两个产品的价格。

1. 创建一个价格信息类PricesInfo，并且存放两个产品的价格。

import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;public class PricesInfo {    //两个价格    private double price1;    private double price2;    //声明读写锁ReadWriteLock对象lock    private ReadWriteLock lock;    public PricesInfo(){        price1 = 1.0;        price2 = 2.0;        lock = new ReentrantReadWriteLock();    }        public double getPrice1(){        lock.readLock().lock();        double value = price1;        lock.readLock().unlock();        return value;    }        public double getPrice2()    {        lock.readLock().lock();        double value = price2;        lock.readLock().unlock();        return value;    }        public void setPrices(double price1, double price2){        lock.writeLock().lock();        this.price1 = price1;        this.price2 = price2;        lock.writeLock().unlock();    }    }

2. 创建读取类Reader，它实现了Runnable接口。

public class Reader implements Runnable {    private PricesInfo pricesInfo;    public Reader(PricesInfo pricesInfo){        this.pricesInfo = pricesInfo;    }    @Override    public void run() {        // 循环读取连个价格10次        for(int i=0;i<10;i++){            System.out.printf("%s: Price1: %f\n", Thread.currentThread().getName(), pricesInfo.getPrice1());            System.out.printf("%s: Price2: %f\n", Thread.currentThread().getName(), pricesInfo.getPrice2());        }    }}

3. 创建写入类Writer，它实现了Runnable接口。

public class Writer implements Runnable {    private PricesInfo pricesInfo;    public Writer(PricesInfo pricesInfo){        this.pricesInfo = pricesInfo;    }    @Override    public void run() {        // 循环修改两个价格3次        try {            for(int i=0;i<3;i++){                System.out.printf("Writer: Attempt to modify the prices.\n");                pricesInfo.setPrices(Math.random()*10, Math.random()*8);                System.out.println("Writer: Prices have been modified.");                Thread.sleep(2);            }        } catch (InterruptedException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}

4. 创建范例的主类Main

public class Main {    public static void main(String[] args) {        PricesInfo pricesInfo = new PricesInfo();        Reader[] readers = new Reader[5];        Thread[] threadsReader = new Thread[5];        for(int i=0;i<5;i++){            readers[i] = new Reader(pricesInfo);            threadsReader[i] = new Thread(readers[i]);        }        Writer writer = new Writer(pricesInfo);        Thread threadWriter = new Thread(writer);        for(int i=0;i<5;i++){            threadsReader[i].start();        }        threadWriter.start();    }}

5. 程序运行结果如下

Thread-1: Price1: 1.000000Thread-4: Price1: 1.000000Thread-2: Price1: 1.000000Thread-2: Price2: 2.000000Thread-2: Price1: 1.000000Thread-2: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-0: Price1: 1.000000Thread-0: Price2: 2.000000Thread-2: Price1: 1.000000Thread-2: Price2: 2.000000Writer: Attempt to modify the prices.Writer: Prices have been modified.Thread-3: Price1: 1.000000Thread-3: Price2: 4.840562Thread-1: Price2: 2.000000Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Thread-1: Price1: 6.220535Thread-1: Price2: 4.840562Writer: Attempt to modify the prices.Writer: Prices have been modified.Thread-4: Price2: 2.000000Thread-4: Price1: 5.640719Thread-4: Price2: 1.872038Thread-4: Price1: 5.640719Thread-4: Price2: 1.872038Thread-2: Price1: 6.220535Thread-2: Price2: 1.872038Thread-2: Price1: 5.640719Thread-2: Price2: 1.872038Thread-2: Price1: 5.640719Thread-2: Price2: 1.872038Thread-2: Price1: 5.640719Thread-2: Price2: 1.872038Thread-2: Price1: 5.640719Thread-2: Price2: 1.872038Thread-2: Price1: 5.640719Thread-2: Price2: 1.872038Thread-2: Price1: 5.640719Thread-2: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Thread-3: Price1: 5.640719Thread-3: Price2: 1.872038Writer: Attempt to modify the prices.Writer: Prices have been modified.Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420Thread-4: Price1: 5.491746Thread-4: Price2: 2.729420