線程高級篇-讀寫鎖ReentrantReadWriteLock

轉載原文:http://blog.csdn.net/john8169/article/details/53228016

讀寫鎖:　　分爲讀鎖和寫鎖,多個讀鎖不互斥,讀鎖和寫鎖互斥,這是有JVM本身控制的.若是代碼只能讀取數據,能夠多人同時讀,不能同時寫,上讀鎖,

若是代碼要修改數據,只能有一我的寫,並且不能同時讀取.上寫鎖.

 

線程進入寫鎖的前提條件:

　　沒有其餘線程的讀鎖和寫鎖;

線程進入讀鎖的前提條件:

　　沒有其餘線程的寫鎖或寫請求

 

    (a).重入方面其內部的WriteLock能夠獲取ReadLock，可是反過來ReadLock想要得到WriteLock則永遠都不要想。 
     (b).WriteLock能夠降級爲ReadLock，順序是：先得到WriteLock再得到ReadLock，而後釋放WriteLock，這時候線程將保持Readlock的持有。反過來ReadLock想要升級爲WriteLock則不可能，爲何？參看(a)，呵呵. 
     (c).ReadLock能夠被多個線程持有而且在做用時排斥任何的WriteLock，而WriteLock則是徹底的互斥。這一特性最爲重要，由於對於高讀取頻率而相對較低寫入的數據結構，使用此類鎖同步機制則能夠提升併發量。 
     (d).無論是ReadLock仍是WriteLock都支持Interrupt，語義與ReentrantLock一致。 
     (e).WriteLock支持Condition而且與ReentrantLock語義一致，而ReadLock則不能使用Condition，不然拋出UnsupportedOperationException異常。 

 

package com.imooc.locks;

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

public class Queue {
    
    private Object data = null;//共享數據,只能有一個線程寫該數據,但能夠多個線程讀取該數據
    
    //讀寫鎖
    ReadWriteLock rwl = new ReentrantReadWriteLock();
    
    // 至關於讀操做  
    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();  
        }  
    }  
  
    // 至關於寫操做  
    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!

　　從打印結果能夠看出:

多個線程能夠同時讀取數據,可是隻有一個線程能夠寫數據;