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釋放鎖之后才可以拿到
線程1, 先拿到writeLock,線程2試圖拿readLock, 阻塞等待,直到線程1釋放鎖之后才可以拿到
線程1, 先拿到writeLock,線程2試圖拿writeLock, 阻塞等待,直到線程1釋放鎖之后才可以拿到
測試代碼
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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); //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.那么 2,3號線程Thread[Thread-1,5,main],Thread[Thread-2,5,main]將會優先執行代碼
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read.lock(); |
由于read.lock();是不互斥的(即可重入的),所以他們同時拿到了鎖,通過日志可以看出來
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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
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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 |
只有當所有的readLock被釋放掉之后,writeLock才能拿到鎖,而這個時候Thread[Thread-1,5,main],Thread[Thread-2,5,main]執行結束后就會釋放鎖readLock
所以Thread[Thread-0,5,main]這個時候拿到了writeLock并執行自己的業務代碼
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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 |
總結
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原文鏈接：http://blog.csdn.net/hl_java/article/details/79128667
