Java多线程Lock对象之读写锁

在之前学习过的lock对象ReentrankLock是互斥锁，每一次只有一个线程来操作。在一些特殊的情况下效率比较低的，比如说在只读环境中，多线程操作是不会影响同步的，所以多线程并发读是没问题的，JDK中提供了读写锁，这种锁在读中可以并发执行，在写中却是互斥的。看一个例子
service类：

package com.lenovo.plm.dms.p17;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class Service {
    private ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    public void read(){
        lock.readLock().lock();
        System.out.println(Thread.currentThread().getName() + " come in the read method:" + System.nanoTime());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }finally{
            lock.readLock().unlock();
        }
    }
    public void write(){
        lock.writeLock().lock();
        System.out.println(Thread.currentThread().getName() + " come in the write method:" + System.nanoTime());
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }finally{
            lock.writeLock().unlock();
        }
    }
}

线程类：

package com.lenovo.plm.dms.p17;
public class ThreadA extends Thread {
    private Service service;
    public ThreadA(Service service){
        this.service = service;
    }
    @Override
    public void run() {
        // TODO Auto-generated method stub
        super.run();
        service.read();
    }
}
package com.lenovo.plm.dms.p17;

public class ThreadB extends Thread {

    private Service service;

    public ThreadB(Service service){
        this.service = service;
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        super.run();
        service.read();
    }
}

这里有两个线程都来调用读方法。再看执行类：

package com.lenovo.plm.dms.p17;
public class Main {
    public static void main(String[] args) { 
        Service service = new Service();
        ThreadA threadA = new ThreadA(service);
        threadA.setName("A");
        ThreadB threadB = new ThreadB(service);
        threadB.setName("B");
        threadA.start();
        threadB.start();
    }
}

执行的结果：
B come in the read method:3394638905238
A come in the read method:3394638905238

因此可以判断，A,B两个线程是可以同时进入读锁的。另外，将A，B线程都调用写方法，或者将B线程换成写方法。就可以得到如下结论：
读读锁可以同时进行
读写锁是互斥进行的
写写锁是互斥进行的