Java语言ReadWriteLock特性实例测试

本文研究的主要是ReadWriteLock特性，具体如下。

ReadWriteLock几点特性

readLock 与 readLock 不互斥
readLock 与 writeLock 互斥
writeLock 与 readLock 互斥
writeLock 与 writeLock 互斥

举例来说:

线程1, 先拿到readLock, 线程2试图拿readLock, 可以拿到
线程1, 先拿到readLock, 线程2试图拿writeLock, 阻塞等待,直到线程1释放锁之后才可以拿到
线程1, 先拿到writeLock,线程2试图拿readLock, 阻塞等待,直到线程1释放锁之后才可以拿到
线程1, 先拿到writeLock,线程2试图拿writeLock, 阻塞等待,直到线程1释放锁之后才可以拿到

测试代码

package com.alioo.lock;import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;/** * */public class ReadWriteLockDemo {static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");public static void main(String[] args) {Data data = new Data();Worker t1 = new Worker(data, false);//写Worker t2 = new Worker(data, true);//读Worker t3 = new Worker(data, true);//读t1.start();t2.start();t3.start();}static class Worker extends Thread {Data data;Boolean read;public Worker(Data data, Boolean read) {this.data = data;this.read = read;}public void run() {if (read)    data.read(); else    data.write();}}static class Data {ReadWriteLock lock = new ReentrantReadWriteLock();Lock read = lock.readLock();Lock write = lock.writeLock();public void write() {try {Thread.sleep(2000);//}catch (Exception e) {}write.lock();System.out.println(Thread.currentThread() + " write:begin "     + sdf.format(new Date()));try {Thread.sleep(5000);//}catch (Exception e) {}finally {System.out.println(Thread.currentThread() + " write:end "      + sdf.format(new Date()));write.unlock();}}public int read() {read.lock();System.out.println(Thread.currentThread()+ " read :begin "     + sdf.format(new Date()));try {Thread.sleep(5000);//}catch (Exception e) {}finally {System.out.println(Thread.currentThread() + " read :end "      + sdf.format(new Date()));read.unlock();}return 1;}}}

测试结果:

Thread[Thread-2,5,main] read :begin 2018-01-22 13:54:16.794
Thread[Thread-1,5,main] read :begin 2018-01-22 13:54:16.794
Thread[Thread-2,5,main] read :end 2018-01-22 13:54:21.795
Thread[Thread-1,5,main] read :end 2018-01-22 13:54:21.795
Thread[Thread-0,5,main] write:begin 2018-01-22 13:54:21.795
Thread[Thread-0,5,main] write:end 2018-01-22 13:54:26.795

测试结果解读:

同时启动了3个线程,其中1号线程Thread[Thread-0,5,main],执行write时先休眠了2秒.那么 2,3号线程Thread[Thread-1,5,main],Thread[Thread-2,5,main]将会优先执行代码

read.lock();

由于read.lock();是不互斥的(即可重入的),所以他们同时拿到了锁,通过日志可以看出来

Thread[Thread-2,5,main] read :begin 2018-01-22 13:54:16.794Thread[Thread-1,5,main] read :begin 2018-01-22 13:54:16.794

而且他们执行的时间开销是相同的(测试代码中都是休眠5秒),所以也将同时执行结束

Thread[Thread-2,5,main] read :end 2018-01-22 13:54:21.795Thread[Thread-1,5,main] read :end 2018-01-22 13:54:21.795

只有当所有的readLock被释放掉之后,writeLock才能拿到锁,而这个时候Thread[Thread-1,5,main],Thread[Thread-2,5,main]执行结束后就会释放锁readLock

所以Thread[Thread-0,5,main]这个时候拿到了writeLock并执行自己的业务代码

Thread[Thread-0,5,main] write:begin 2018-01-22 13:54:21.795Thread[Thread-0,5,main] write:end 2018-01-22 13:54:26.795

总结

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