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!
