线程高级篇-读写锁ReentrantReadWriteLock

转载原文:http://blog.csdn.net/john8169/article/details/53228016

读写锁:　　分为读锁和写锁,多个读锁不互斥,读锁和写锁互斥,这是有JVM本身控制的.若是代码只能读取数据,能够多人同时读,不能同时写,上读锁,

若是代码要修改数据,只能有一我的写,并且不能同时读取.上写锁.

 

线程进入写锁的前提条件:

　　没有其余线程的读锁和写锁;

线程进入读锁的前提条件:

　　没有其余线程的写锁或写请求

 

    (a).重入方面其内部的WriteLock能够获取ReadLock，可是反过来ReadLock想要得到WriteLock则永远都不要想。 
     (b).WriteLock能够降级为ReadLock，顺序是：先得到WriteLock再得到ReadLock，而后释放WriteLock，这时候线程将保持Readlock的持有。反过来ReadLock想要升级为WriteLock则不可能，为何？参看(a)，呵呵. 
     (c).ReadLock能够被多个线程持有而且在做用时排斥任何的WriteLock，而WriteLock则是彻底的互斥。这一特性最为重要，由于对于高读取频率而相对较低写入的数据结构，使用此类锁同步机制则能够提升并发量。 
     (d).无论是ReadLock仍是WriteLock都支持Interrupt，语义与ReentrantLock一致。 
     (e).WriteLock支持Condition而且与ReentrantLock语义一致，而ReadLock则不能使用Condition，不然抛出UnsupportedOperationException异常。 

 

package com.imooc.locks;

import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class Queue {
    
    private Object data = null;//共享数据,只能有一个线程写该数据,但能够多个线程读取该数据
    
    //读写锁
    ReadWriteLock rwl = new ReentrantReadWriteLock();
    
    // 至关于读操做  
    public void get() {  
        rwl.readLock().lock();  
        try {  
            System.out.println(Thread.currentThread().getName() + " be ready to read data!");  
            Thread.sleep((long) (Math.random() * 1000));  
            System.out.println(Thread.currentThread().getName() + "have read data :" + data);  
        } catch (InterruptedException e) {  
            e.printStackTrace();  
        } finally {  
            rwl.readLock().unlock();  
        }  
    }  
  
    // 至关于写操做  
    public void put(Object data) {  
        rwl.writeLock().lock();  
        try {  
            System.out.println(Thread.currentThread().getName() + " be ready to write data!");  
            Thread.sleep((long) (Math.random() * 1000));  
            this.data = data;  
            System.out.println(Thread.currentThread().getName() + " have write data: " + data);  
        } catch (InterruptedException e) {  
            e.printStackTrace();  
        } finally {  
            rwl.writeLock().unlock();  
        }  
    }  

}

 

package com.imooc.locks;

import java.util.Random;

public class ReadWriteLockTest {
    
    public static void main(String[] args) {
    
        final Queue q3 = new Queue();  
        
        for (int i = 0; i < 3; i++) {  
            new Thread() {  
                public void run() {  
                    while (true) {  
                        q3.get();  
                    }  
                }  
            }.start();  
            new Thread() {  
                public void run() {  
                    while (true) {  
                        q3.put(new Random().nextInt(10000));  
                    }  
                }  
            }.start();  
        }  
    }
} 

Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-4 be ready to read data!
Thread-0have read data :null
Thread-2have read data :null
Thread-4have read data :null
Thread-1 be ready to write data!
Thread-1 have write data: 3101
Thread-1 be ready to write data!
Thread-1 have write data: 8258
Thread-1 be ready to write data!
Thread-1 have write data: 7242
Thread-3 be ready to write data!
Thread-3 have write data: 4810
Thread-5 be ready to write data!
Thread-5 have write data: 7597
Thread-5 be ready to write data!
Thread-5 have write data: 8800
Thread-0 be ready to read data!
Thread-4 be ready to read data!
Thread-2 be ready to read data!
Thread-0have read data :8800
Thread-2have read data :8800
Thread-4have read data :8800
Thread-1 be ready to write data!
Thread-1 have write data: 6606
Thread-1 be ready to write data!
Thread-1 have write data: 5436
Thread-1 be ready to write data!
Thread-1 have write data: 3912
Thread-1 be ready to write data!
Thread-1 have write data: 7689
Thread-3 be ready to write data!
Thread-3 have write data: 3102
Thread-3 be ready to write data!
Thread-3 have write data: 466
Thread-3 be ready to write data!
Thread-3 have write data: 7377
Thread-3 be ready to write data!
Thread-3 have write data: 5461
Thread-3 be ready to write data!
Thread-3 have write data: 175
Thread-3 be ready to write data!
Thread-3 have write data: 8805
Thread-3 be ready to write data!
Thread-3 have write data: 8898
Thread-5 be ready to write data!
Thread-5 have write data: 8823
Thread-5 be ready to write data!
Thread-5 have write data: 5615
Thread-5 be ready to write data!
Thread-5 have write data: 8118
Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-4 be ready to read data!
Thread-0have read data :8118
Thread-2have read data :8118
Thread-4have read data :8118
Thread-1 be ready to write data!
Thread-1 have write data: 5314
Thread-1 be ready to write data!

　　从打印结果能够看出:

多个线程能够同时读取数据,可是只有一个线程能够写数据;