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
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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
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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
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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
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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
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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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原文链接：http://blog.csdn.net/hl_java/article/details/79128667
