ReentrantReadWriteLock的应用

说到ReentrantReadWriteLock，首先要做的是与ReentrantLock划清界限。它和后者都是单独的实现，彼此之间没有继承或实现的关系。然后就是总结这个锁机制的特性了：

     (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。

      以上就是比较重要的，或者衡量是否使用ReentrantReadWriteLock的基础了。下面还是写个小例子说明部分内容：

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @author: yanxuxin
 * @date: 2010-1-7
 */
public class ReentrantReadWriteLockSample {

    public static void main(String[] args) {
        testReadLock();
//      testWriteLock();
    }

    public static void testReadLock() {
       final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();
        support.initCache();

        Runnable runnable = new Runnable() {
            public void run() {
                support.get("test");
            }
        };

        new Thread(runnable).start();
        new Thread(runnable).start();

        new Thread(new Runnable() {
            public void run() {
                support.put("test", "test");
            }
        }).start();
    }

    public static void testWriteLock() {
       final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();
        support.initCache();

        new Thread(new Runnable() {
            public void run() {
                support.put("key1", "value1");
            }
        }).start();

        new Thread(new Runnable() {
            public void run() {
                support.put("key2", "value2");
            }
        }).start();

        new Thread(new Runnable() {
            public void run() {
                support.get("key1");
            }
        }).start();
    }
}

class ReadWriteLockSampleSupport {
    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    private final Lock readLock = lock.readLock();
    private final Lock writeLock = lock.writeLock();

    private volatile  boolean completed;
    private Map<String,String> cache;

    public void initCache() {
        readLock.lock();
        if(!completed) {
            // Must release read lock before acquiring write lock
            readLock.unlock(); // (1)
            writeLock.lock();  // (2)
            if(!completed) {
                cache = new HashMap<String,String>(32);
                completed = true;
            }
            // Downgrade by acquiring read lock before releasing write lock
            readLock.lock();    // (3)
            writeLock.unlock(); // (4) Unlock write, still hold read
        }

        System.out.println("empty? " + cache.isEmpty());
        readLock.unlock();
    }

    public String get(String key) {
        readLock.lock();
        System.out.println(Thread.currentThread().getName() + " read.");
        startTheCountdown();
        try{
            return cache.get(key);
        }
        finally{
            readLock.unlock();
        }
    }

    public String put(String key, String value) {
        writeLock.lock();
        System.out.println(Thread.currentThread().getName() + " write.");
        startTheCountdown();
        try{
            return cache.put(key, value);
        }
        finally {
            writeLock.unlock();
        }
    }

    /**
     * A simple countdown,it will stop after about 5s.
     */
    public void startTheCountdown() {
        long currentTime = System.currentTimeMillis();
        for(;;) {
            long diff = System.currentTimeMillis() - currentTime;
            if(diff > 5000) {
                break;
            }
        }
    }
}

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @author: yanxuxin
 * @date: 2010-1-7
 */
public class ReentrantReadWriteLockSample {

	public static void main(String[] args) {
		testReadLock();
//		testWriteLock();
	}
	
	public static void testReadLock() {
	   final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();
		support.initCache();
		
		Runnable runnable = new Runnable() {
			public void run() {
				support.get("test");
			}
		};
		
		new Thread(runnable).start();
		new Thread(runnable).start();
		
		new Thread(new Runnable() {
			public void run() {
				support.put("test", "test");
			}
		}).start();
	}
	
	public static void testWriteLock() {
	   final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();
		support.initCache();
		
		new Thread(new Runnable() {
			public void run() {
				support.put("key1", "value1");
			}
		}).start();
		
		new Thread(new Runnable() {
			public void run() {
				support.put("key2", "value2");
			}
		}).start();
		
		new Thread(new Runnable() {
			public void run() {
				support.get("key1");
			}
		}).start();
	}
}

class ReadWriteLockSampleSupport {
	private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
	private final Lock readLock = lock.readLock();
	private final Lock writeLock = lock.writeLock();
	
	private volatile  boolean completed;
	private Map<String,String> cache;
	
	public void initCache() {
		readLock.lock();
		if(!completed) {
			// Must release read lock before acquiring write lock
			readLock.unlock(); // (1)
			writeLock.lock();  // (2)
			if(!completed) {
				cache = new HashMap<String,String>(32);
				completed = true;
			}
			// Downgrade by acquiring read lock before releasing write lock
			readLock.lock();    // (3)
			writeLock.unlock(); // (4) Unlock write, still hold read
		}
		
		System.out.println("empty? " + cache.isEmpty());
		readLock.unlock();
	}
	
	public String get(String key) {
		readLock.lock();
		System.out.println(Thread.currentThread().getName() + " read.");
		startTheCountdown();
		try{
			return cache.get(key);
		}
		finally{
			readLock.unlock();
		}
	}
	
	public String put(String key, String value) {
		writeLock.lock();
		System.out.println(Thread.currentThread().getName() + " write.");
		startTheCountdown();
		try{
			return cache.put(key, value);
		}
		finally {
			writeLock.unlock();
		}
	}
	
	/**
	 * A simple countdown,it will stop after about 5s. 
	 */
	public void startTheCountdown() {
		long currentTime = System.currentTimeMillis();
		for(;;) {
			long diff = System.currentTimeMillis() - currentTime;
			if(diff > 5000) {
				break;
			}
		}
	}
}

    这个例子改造自JDK的API提供的示例，其中ReadWriteLockSampleSupport辅助类负责维护一个Map，当然前提是这个Map大部分的多线程下都是读取，只有很少的比例是多线程竞争修改Map的值。其中的initCache()简单的说明了特性(a),(b).在这个方法中如果把注释(1)和(2)处的代码调换位置，就会发现轻而易举的死锁了，当然是因为特性(1)的作用了。而注释(3)，(4)处的代码位置则再次证明了特性(a)，并且有力的反映了特性(b)--WriteLock在cache初始化完毕之后，降级为ReadLock。另外get(),put()方法在线程获取锁之后会在方法中呆上近5s的时间。

     ReentrantReadWriteLockSample中的两个静态测试方法则分别测试了ReadLock和WriteLock的排斥性。testReadLock()中，开启三个线程，前两者试图获取ReadLock而后者去获取WriteLock。执行结果可以看到：ReadWriteLockSampleSupport的get()方法中的打印结果在前两个线程中几乎同时显示，而put()中的打印结果则要等上近5s。这就说明了，ReadLock可以多线程持有并且排斥WriteLock的持有线程。testWriteLock()中，也开启三个线程。前两个是去获取WriteLock，最后一个获取ReadLock。执行的结果是三个打印结果都有近5s的间隔时间，这说明了WriteLock是独占的，比较独！

    这篇ReentrantReadWriteLock的总结写的有点迟了，主要是最近对js和ajax很有兴趣，突然觉得css也很好玩。看着网上很多人对技术的狂热和个人规划，我想对我而言：不迷恋技术而是作为兴趣，不管是J2EE还是Web前端，不管是移动设备的三方开发还是专业的视频剪辑技术，我都希望很自然的感兴趣了，有条件了就去狠狠的玩玩。我想我迷恋的只是高性能的计算机和互联网，哈哈