ReadWriteLock ReentrantReadWriteLock

ReadWriteLock管理一组锁，一个是只读的锁，一个是写锁。读锁可以在没有写锁的时候被多个线程同时持有，写锁是独占的。

所有读写锁的实现必须确保写操作对读操作的内存影响。换句话说，一个获得了读锁的线程必须能看到前一个释放的写锁所更新的内容。

读写锁比互斥锁允许对于共享数据更大程度的并发。每次只能有一个写线程，但是同时可以有多个线程并发地读数据。ReadWriteLock适用于读多写少的并发情况。

Java并发包中ReadWriteLock是一个接口，主要有两个方法，如下：

public interface ReadWriteLock {
    /**
     * 返回读锁
     */
    Lock readLock();

    /**
     * 返回写锁
     */
    Lock writeLock();
}

Java并发库中ReetrantReadWriteLock实现了ReadWriteLock接口并添加了可重入的特性。

实例的代码

package cn.itcast.heima2;

import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockTest {
    public static void main(String[] args) {
        final Queue3 q3 = new Queue3();
        for (int i = 0; i < 3; i++) {

            new Thread() {
                public void run() {
                    while (true) {
                        q3.get();
                    }
                }

            }.start();

            new Thread() {
                public void run() {
                    while (true) {
                        q3.put(new Random().nextInt(10000));
                    }
                }
            }.start();
        }

    }
}

class Queue3 {
    private Object data = null;//共享数据，只能有一个线程能写该数据，但可以有多个线程同时读该数据。
    ReadWriteLock rwl = new ReentrantReadWriteLock(true);  //true为公平锁，false则为打乱

    public void get() {
        rwl.readLock().lock();  //读锁上锁
        try {
            System.out.println(Thread.currentThread().getName() + " be ready to read data!");
            Thread.sleep((long) (Math.random() * 1000));
            System.out.println(Thread.currentThread().getName() + " have read data :" + data);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            rwl.readLock().unlock();  //读锁解锁
        }
    }

    public void put(Object data) {

        rwl.writeLock().lock();  //写锁上锁
        try {
            System.out.println(Thread.currentThread().getName() + " be ready to write data!");
            Thread.sleep((long) (Math.random() * 1000));
            this.data = data;
            System.out.println(Thread.currentThread().getName() + " have write data: " + data);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            rwl.writeLock().unlock(); //写锁解锁
        }

    }
}

结果：

Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-4 be ready to read data!
Thread-4 have read data :null
Thread-2 have read data :null
Thread-0 have read data :null
Thread-3 be ready to write data!
Thread-3 have write data: 8786
Thread-5 be ready to write data!
Thread-5 have write data: 4237
Thread-1 be ready to write data!
Thread-1 have write data: 2771
Thread-4 be ready to read data!
Thread-2 be ready to read data!
Thread-0 be ready to read data!
Thread-2 have read data :2771
Thread-0 have read data :2771
Thread-4 have read data :2771
Thread-3 be ready to write data!
Thread-3 have write data: 4449
Thread-5 be ready to write data!