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  • 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自己控制的.如果代码只能读取数据,可以多人同时读,不能同时写,上读锁,

    如果代码要修改数据,只能有一个人写,而且不能同时读取.上写锁.

    线程进入写锁的前提条件:

      没有其他线程的读锁和写锁;

    线程进入读锁的前提条件:

      没有其他线程的写锁或写请求

        (a).重入方面其内部的WriteLock可以获取ReadLock,但是反过来ReadLock想要获得WriteLock则永远都不要想。 
         (b).WriteLock可以降级为ReadLock,顺序是:先获得WriteLock再获得ReadLock,然后释放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,为什么?参看(a),呵呵. 
         (c).ReadLock可以被多个线程持有并且在作用时排斥任何的WriteLock,而WriteLock则是完全的互斥。这一特性最为重要,因为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可以提高并发量。 
         (d).不管是ReadLock还是WriteLock都支持Interrupt,语义与ReentrantLock一致。 
         (e).WriteLock支持Condition并且与ReentrantLock语义一致,而ReadLock则不能使用Condition,否则抛出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,但可以多个线程读取该数据
        
        //读写锁
        ReadWriteLock rwl = new ReentrantReadWriteLock();
        
        // 相当于读操作  
        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();  
            }  
        }  
      
        // 相当于写操作  
        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!
    

      从打印结果可以看出:

    多个线程可以同时读取数据,但是只有一个线程可以写数据;

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  • 原文地址:https://www.cnblogs.com/Wanted-Tao/p/6379363.html
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