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
", Thread.currentThread().getName(), pricesInfo.getPrice1());
System.out.printf("%s: Price2: %f
", 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.
");
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: 1.000000 Thread-4: Price1: 1.000000 Thread-2: Price1: 1.000000 Thread-2: Price2: 2.000000 Thread-2: Price1: 1.000000 Thread-2: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-0: Price1: 1.000000 Thread-0: Price2: 2.000000 Thread-2: Price1: 1.000000 Thread-2: Price2: 2.000000 Writer: Attempt to modify the prices. Writer: Prices have been modified. Thread-3: Price1: 1.000000 Thread-3: Price2: 4.840562 Thread-1: Price2: 2.000000 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Thread-1: Price1: 6.220535 Thread-1: Price2: 4.840562 Writer: Attempt to modify the prices. Writer: Prices have been modified. Thread-4: Price2: 2.000000 Thread-4: Price1: 5.640719 Thread-4: Price2: 1.872038 Thread-4: Price1: 5.640719 Thread-4: Price2: 1.872038 Thread-2: Price1: 6.220535 Thread-2: Price2: 1.872038 Thread-2: Price1: 5.640719 Thread-2: Price2: 1.872038 Thread-2: Price1: 5.640719 Thread-2: Price2: 1.872038 Thread-2: Price1: 5.640719 Thread-2: Price2: 1.872038 Thread-2: Price1: 5.640719 Thread-2: Price2: 1.872038 Thread-2: Price1: 5.640719 Thread-2: Price2: 1.872038 Thread-2: Price1: 5.640719 Thread-2: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Thread-3: Price1: 5.640719 Thread-3: Price2: 1.872038 Writer: Attempt to modify the prices. Writer: Prices have been modified. Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420 Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420 Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420 Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420 Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420 Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420 Thread-4: Price1: 5.491746 Thread-4: Price2: 2.729420