Java語言ReadWriteLock特性實例測試

本文研究的主要是ReadWriteLock特性，具體如下。

ReadWriteLock幾點特性

readLock 與readLock 不互斥
readLock 與writeLock 互斥
writeLock 與readLock 互斥
writeLock 與writeLock 互斥

舉例來說:

線程1, 先拿到readLock, 線程2試圖拿readLock, 可以拿到線程1, 先拿到readLock, 線程2試圖拿writeLock, 阻塞等待,直到線程1釋放鎖之後才可以拿到線程1, 先拿到writeLock,線程2試圖拿readLock, 阻塞等待,直到線程1釋放鎖之後才可以拿到線程1, 先拿到writeLock,線程2試圖拿writeLock, 阻塞等待,直到線程1釋放鎖之後才可以拿到

測試代碼

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);//寫Worker t2 = new Worker(data, true);//讀Worker t3 = new Worker(data, true);//讀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;}}}

測試結果:

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

測試結果解讀:

同時啟動了3個線程,其中1號線程Thread[Thread-0,5,main],執行write時先休眠了2秒.那麼2,3號線程Thread[Thread-1,5,main],Thread[Thread-2,5,main]將會優先執行代碼

read.lock();

由於read.lock();是不互斥的(即可重入的),所以他們同時拿到了鎖,通過日誌可以看出來

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

而且他們執行的時間開銷是相同的(測試代碼中都是休眠5秒),所以也將同時執行結束

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

只有當所有的readLock被釋放掉之後,writeLock才能拿到鎖,而這個時候Thread[Thread-1,5,main],Thread[Thread-2,5,main]執行結束後就會釋放鎖readLock

所以Thread[Thread-0,5,main]這個時候拿到了writeLock並執行自己的業務代碼

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

總結

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