线程-读写锁。
读写锁。我们上次温习了synchronized与Lock.我们遗留了一个问题，就是想让读读不互斥，因为多线程同时读取数据并不会破坏数据。如果能实现读读不用互斥，那将大大提升了多线程读性能。于是我们引入了读写锁ReentrantReadWriteLock。ReentrantReadWriteLock可以专门创建一个读锁和写锁。读锁：读读不互斥，但是读写互斥。 写锁：不但读写互斥，而且写写也互斥。
创建一个读写锁
ReadWriteLock rwl = new ReentrantReadWriteLock();
读锁： rwl.readLock().lock() 与 rwl.readLock().unlock(); 成对出现； 读完了公共变量，其它线程才能写这个公共变量（赋值），但是多个线程可以同时读。
写锁： rwl.writeLock().lock() 与 rwl.writeLock().unlock(); 成对出现；写完了公共变量，其它线程才能写或读这个公共变量。
以下示例代码展示了读锁与写锁。创建两个一个Queue自定义类，有一个get()和put()方法，在get()方法里使用读锁，在put()里使用写锁，它们都操作一个公共数据Object data。
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package testFuture;
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import java.util.Random;
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import java.util.concurrent.locks.ReadWriteLock;
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import java.util.concurrent.locks.ReentrantReadWriteLock;
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public class LockTest {
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public static void main(String[] args) {
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final Queue3 q3 = new Queue3();
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for(int i=0;i<3;i++)
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{
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new Thread(){
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public void run(){
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while(true){
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q3.get();
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}
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}
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}.start();
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new Thread(){
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public void run(){
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while(true){
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q3.put(new Random().nextInt(10000));
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}
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}
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}.start();
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}
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}
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}
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synchronized
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class Queue3{
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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。
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ReadWriteLock rwl = new ReentrantReadWriteLock();
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public void get(){
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rwl.readLock().lock();
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try {
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System.out.println(Thread.currentThread().getName() + " be ready to read data!");
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Thread.sleep((long)(Math.random()*1000));
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System.out.println(Thread.currentThread().getName() + "have read data :" + data);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally{
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rwl.readLock().unlock();
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}
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}
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public void put(Object data){
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rwl.writeLock().lock();
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try {
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System.out.println(Thread.currentThread().getName() + " be ready to write data!");
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Thread.sleep((long)(Math.random()*1000));
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this.data = data;
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System.out.println(Thread.currentThread().getName() + " have write data: " + data);
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} catch (InterruptedException e) {
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e.printStackTrace();
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}finally{
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rwl.writeLock().unlock();
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}
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}
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}
打印日志如下：
Thread-2 be ready to read data!
Thread-0 be ready to read data!
Thread-4 be ready to read data!
Thread-0have read data :null
Thread-2have read data :null
Thread-4have read data :null
Thread-5 be ready to write data!
Thread-5 have write data: 8687
Thread-5 be ready to write data!
Thread-5 have write data: 8653
Thread-1 be ready to write data!
Thread-1 have write data: 7614
Thread-1 be ready to write data!
Thread-1 have write data: 4680
Thread-3 be ready to write data!
Thread-3 have write data: 473
Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-4 be ready to read data!
Thread-0have read data :473
Thread-4have read data :473
Thread-2have read data :473
Thread-5 be ready to write data!
Thread-5 have write data: 7706
Thread-1 be ready to write data!
Thread-1 have write data: 7828
Thread-1 be ready to write data!
Thread-1 have write data: 6492
Thread-3 be ready to write data!
Thread-3 have write data: 6439
Thread-3 be ready to write data!
Thread-3 have write data: 4558
Thread-0 be ready to read data!
Thread-4 be ready to read data!
Thread-2 be ready to read data!
Thread-0have read data :4558
Thread-4have read data :4558
Thread-2have read data :4558
我们会发现总共有6个线程：0,2,4线程是读，1,3,5线程是写数据，当5在写数据的时候，put方法输出以下2条日志：
Thread-5 be ready to write data!
Thread-5 have write data: 8687
这2条日志之间，并没有其它线程打扰。5写完了，接着才是线程1写数据，输出日志：
Thread-1 be ready to write data!
Thread-1 have write data: 7614
同样，这2条日志之间，没有其它线程打扰。
假设线程5在执行put写数据的时候，输出日志如下，则是被线程1打扰了：
Thread-5 be ready to write data!
Thread-1 be ready to write data!
Thread-5 have write data: 8687
Thread-1 have write data: 7614
线程5的本意是让data变为8676，结果当线程5的put返回的时候，实际上data已被线程1修改成了7614，线程5还摸不着头脑，自己怎么就做错了，很简单的事情都能出错。
文章来源: blog.csdn.net，作者：冉航--小虾米，版权归原作者所有，如需转载，请联系作者。
原文链接：blog.csdn.net/gaoxiaoweiandy/article/details/116099251
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