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本文研究的主要是ReadWriteLock特性,具體如下。
readLock 與 readLock 不互斥
readLock 與 writeLock 互斥
writeLock 與 readLock 互斥
writeLock 與 writeLock 互斥
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
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; } } }
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
同時啟動了3個線程,其中1號線程Thread[Thread-0,5,main],
執(zhí)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(yōu)先執(zhí)行代碼
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
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
而且他們執(zhí)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(zhí)行結(jié)束
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
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(zhí)行結(jié)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(zhí)行自己的業(yè)務代碼
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
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