ReadWriteLock几点特性

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