讀寫鎖（ReadWriteLock）的使用

Definition

讀寫鎖包含一對相關的鎖，讀鎖用於只讀操作，寫鎖用於寫操作。讀鎖可能由多個讀線程同時運行，寫鎖是唯一的。

 

Direction

1、讀鎖和寫鎖之間是互斥的，同一時間只能有一個在運行。但是可以有多個線程同時讀取數據。

2、寫入數據之前必須重新確認(ReCheck)狀態，因為其他的線程可能會拿到寫鎖再一次修改我們已經修改過的值。這是因為前一個線程拿到寫鎖之后，后面的線程會被阻塞。當前一個線程釋放寫鎖之后，被阻塞的線程會繼續運行完成被阻塞的部分代碼，所以才會出現這樣的情況。

3、當某一個線程上了寫鎖之后，自己仍然可以上讀鎖，之后在釋放寫鎖，這是一種降級(Downgrade)的處理方法。

 

Method

讀寫鎖（ReadWriteLock）包含如下兩個方法：

1.讀鎖

Lock readLock()

 

2.寫鎖

Lock writeLock()

 

Example

以下代碼在開始讀數據的時候上讀鎖，當有一個線程發現沒有數據時，釋放讀鎖，上寫鎖，開始寫入數據。

數據寫入完成后釋放寫鎖，並還原成讀鎖，實現了簡單的讀和寫之間的互斥。

示例代碼：

    private ReadWriteLock rwl = new ReentrantReadWriteLock();//定義讀寫鎖
    public Object getData(String key){
        //使用讀寫鎖的基本結構
        rwl.readLock().lock();
        Object value = null;
        try{
            value = cache.get(key);
            if(value == null){
                rwl.readLock().unlock();
                rwl.writeLock().lock();
                try{
                    // Recheck state because another thread might have
                    // acquired write lock and changed state before we did.
                    if(value == null){
                        value = "aaaa";//寫入數據
                    }
                }finally{
                    rwl.writeLock().unlock();
                }
                rwl.readLock().lock();
            }
        }finally{
            rwl.readLock().unlock();
        }
        return value;
    }

 

讀寫鎖的完整使用示例：

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

            new Thread(){
                public void run(){
                    while(true){
                        q.put(new Random().nextInt(10000));
                    }
                }        
            }.start();
        }
    }
}

class Queue{
    private Object data = null;//共享數據，只能有一個線程能寫該數據，但可以有多個線程同時讀該數據。
    ReadWriteLock rwl = new ReentrantReadWriteLock();
    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();
        }
    }
    
    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();
        }
    }
}

部分輸出結果如下：

Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-4 be ready to read data!
Thread-4have read data :null
Thread-0have read data :null
Thread-2have read data :null
Thread-3 be ready to write data!
Thread-3 have write data: 9763
Thread-3 be ready to write data!
Thread-3 have write data: 5187
Thread-3 be ready to write data!
Thread-3 have write data: 2135
Thread-1 be ready to write data!
Thread-1 have write data: 180
Thread-1 be ready to write data!
Thread-1 have write data: 5979
Thread-1 be ready to write data!
Thread-1 have write data: 9495
Thread-1 be ready to write data!
Thread-1 have write data: 6363
Thread-1 be ready to write data!
Thread-1 have write data: 31
Thread-1 be ready to write data!
Thread-1 have write data: 7530
Thread-1 be ready to write data!
Thread-1 have write data: 1354
Thread-5 be ready to write data!
Thread-5 have write data: 3580
Thread-5 be ready to write data!
Thread-5 have write data: 5534
Thread-5 be ready to write data!
Thread-5 have write data: 2522
Thread-5 be ready to write data!
Thread-5 have write data: 2926
Thread-4 be ready to read data!
Thread-0 be ready to read data!
Thread-2 be ready to read data!
Thread-2have read data :2926
Thread-0have read data :2926
Thread-4have read data :2926