多线程的Lock锁——ReentrantReadWriteLock

上一篇讲到ReentrantLock是排他性，一方面保证了线程的绝对安全，但另一方面降低了效率。对于一些不需要操作实例变量的方法中，有另外一种锁可以提高运行效率，这种锁就是ReadWriteReentrantLock。

ReentrantReadWriteLock，也称为读写锁。读写锁有两个锁，一个是读操作相关的锁，也叫共享锁；另一个是写操作相关锁，也叫排他锁。

读锁之间不互斥，写锁之间互斥，读写锁之间互斥。

使用

private ReadWriteLock lock = new ReentrantReadWriteLock();

lock.readLock().lock();
.
.
.
lock.readLock().unlock();

lock.writeLock().lock();
.
.
.
lock.writeLock().unlock();

举个具体例子：

1、创建服务类

package com.cjs.ReadWriteLock_3;
 
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
 
public class Service {
    private ReadWriteLock lock = new ReentrantReadWriteLock();
 
    public void read() {
        try {
            try {
                lock.readLock().lock();
                System.out.println("获得读锁：" + Thread.currentThread().getName() + " " + System.currentTimeMillis());
                Thread.sleep(3000);
            } finally {
                lock.readLock().unlock();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
 
    public void write() {
        try {
            try {
                lock.writeLock().lock();
                System.out.println("获得写锁：" + Thread.currentThread().getName() + " " + System.currentTimeMillis());
                Thread.sleep(3000);
            } finally {
                lock.writeLock().unlock();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

2、创建两个线程类，

package com.cjs.ReadWriteLock_3;
 
public class ReadThread extends Thread {
    private Service service;
 
    @Override
    public void run() {
        service.read();
    }
 
    public ReadThread(Service service) {
        this.service = service;
    }
}

package com.cjs.ReadWriteLock_3;
 
public class WriteThread extends Thread {
    private Service service;
 
    public WriteThread(Service service) {
        this.service = service;
    }
 
    @Override
    public void run() {
        service.write();
    }
}

3、创建一个Main类，

package com.cjs.ReadWriteLock_3;
 
public class Run {
    public static void main(String[] args) {
        Service service = new Service();
        ReadThread[] readThreads = new ReadThread[10];
        WriteThread[] writeThreads = new WriteThread[10];
        for (int i = 0; i < 10; i++) {
            readThreads[i] = new ReadThread(service);
            readThreads[i].setName("ReadThread" + (i + 1));
            readThreads[i].start();
            writeThreads[i] = new WriteThread(service);
            writeThreads[i].setName("WriteThread" + (i + 1));
            writeThreads[i].start();
        }
    }
}

4、运行程序，

注意框框的两条记录，上面提及的读写锁之间的两两特性，只有当当前持有写锁时，其他线程才会进入阻塞状态，如果是持有读锁，那么还是会有可能另一个线程获得读锁。

多线程的Lock锁——ReentrantReadWriteLock

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