鎖的升降級

本文基於ReentrantReadWriteLock來說明它自己的鎖升級的策略。

讀寫鎖總結

這里我先說明下讀寫鎖進行下總結：

要么多讀，要么一寫；讀寫鎖適用去讀多寫鎖的情況。

公平情況下，讀寫鎖都不能插隊；非公平情況下，寫鎖可以插隊，讀鎖只有在頭結點不是寫等待的情況下插隊。

 

讀鎖是否可以升級到寫鎖

　　這里我先給出結論：讀鎖不可以升級為寫鎖；若兩個線程的讀鎖都想升級寫鎖，則需要對方都釋放自己鎖，而雙方都不釋放，就會產生死鎖。

　　實現代碼如下所示

　　

package com.yang.lock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * 本實例演示讀鎖的升級
 */
public class ReentrantReadWriteUpgardeDemo {
    private static ReentrantReadWriteLock reentrantReadWriteLock=new ReentrantReadWriteLock(false);
    private static ReentrantReadWriteLock.ReadLock readLock=reentrantReadWriteLock.readLock();
    private static ReentrantReadWriteLock.WriteLock writeLock=reentrantReadWriteLock.writeLock();

    public void upgarde(){
        System.out.println(Thread.currentThread().getName()+"嘗試獲取讀鎖");
        readLock.lock();
        try{
            System.out.println(Thread.currentThread().getName()+"獲取到了讀鎖");
            System.out.println(Thread.currentThread().getName()+"阻塞獲取寫鎖");
            writeLock.lock();

        }finally {
            readLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReentrantReadWriteUpgardeDemo reentrantReadWriteUpgardeDemo=new ReentrantReadWriteUpgardeDemo();
        new Thread(()->reentrantReadWriteUpgardeDemo.upgarde(),"線程1").start();
        new Thread(()->reentrantReadWriteUpgardeDemo.upgarde(),"線程2").start();

    }
}

 

　　

 

 

 

寫鎖是否可以降級為讀鎖

　　這里我先給出結論：寫鎖是可以降級為讀鎖的；寫鎖只有一個，當寫鎖降級為讀鎖時，所有的都是讀。

　　實現代碼如下所示

　　

package com.yang.lock;

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * 本實例演示讀鎖的升級
 */
public class ReentrantReadWriteDowngradeDemo {
    private static ReentrantReadWriteLock reentrantReadWriteLock=new ReentrantReadWriteLock(false);
    private static ReentrantReadWriteLock.ReadLock readLock=reentrantReadWriteLock.readLock();
    private static ReentrantReadWriteLock.WriteLock writeLock=reentrantReadWriteLock.writeLock();

    public void downgrade(){
        System.out.println(Thread.currentThread().getName()+"嘗試獲取寫鎖");
        writeLock.lock();
        try{
            System.out.println(Thread.currentThread().getName()+"獲取到了寫鎖");
            System.out.println(Thread.currentThread().getName()+"降級獲取讀鎖");
            readLock.lock();
            try{

            }finally {
                readLock.unlock();
            }
        }finally {
            writeLock.unlock();
        }
    }

    public static void main(String[] args) {
        ReentrantReadWriteDowngradeDemo reentrantReadWriteUpgardeDemo=new ReentrantReadWriteDowngradeDemo();
        new Thread(()->reentrantReadWriteUpgardeDemo.downgrade(),"線程1").start();
        new Thread(()->reentrantReadWriteUpgardeDemo.downgrade(),"線程2").start();

    }
}

　　運行結果如下圖所示：