java细粒度锁

java中的几种锁：synchronized，reentrantlock，reentrantreadwritelock已基本可以满足编程需求，但其粒度都太大，同一时刻只有一个线程能进入同步块，这对于某些高并发的场景并不适用。

下面来提供几个更细的粒度锁：

1. 分段锁

借鉴concurrentHashMap的分段思想，先生成一定数量的锁，具体使用的时候再根据key来返回对应的lock。这是几个实现里最简单，性能最高，也是最终被采用的锁策略，代码如下：

/**
 * 分段锁，系统提供一定数量的原始锁，根据传入对象的哈希值获取对应的锁并加锁
 * 注意：要锁的对象的哈希值如果发生改变，有可能导致锁无法成功释放!!!
 */
public class SegmentLock {
    private Integer segments = 16;//默认分段数量
    private final HashMap lockMap = new HashMap<>();
 
    public SegmentLock() {
        init(null, false);
    }
 
    public SegmentLock(Integer counts, boolean fair) {
        init(counts, fair);
    }
 
    private void init(Integer counts, boolean fair) {
        if (counts != null) {
            segments = counts;
        }
        for (int i = 0; i < segments; i++) {
            lockMap.put(i, new ReentrantLock(fair));
        }
    }
 
    public void lock(T key) {
        ReentrantLock lock = lockMap.get(key.hashCode() % segments);
        lock.lock();
    }
 
    public void unlock(T key) {
        ReentrantLock lock = lockMap.get(key.hashCode() % segments);
        lock.unlock();
    }
}

2. 哈希锁

上述分段锁的基础上发展起来的第二种锁策略，目的是实现真正意义上的细粒度锁。每个哈希值不同的对象都能获得自己独立的锁。在测试中，在被锁住的代码执行速度飞快的情况下，效率比分段锁慢 30% 左右。如果有长耗时操作，感觉表现应该会更好。代码如下：

public class HashLock {
    private boolean isFair = false;
    private final SegmentLock segmentLock = new SegmentLock<>();//分段锁
    private final ConcurrentHashMap lockMap = new ConcurrentHashMap<>();
 
    public HashLock() {
    }
 
    public HashLock(boolean fair) {
        isFair = fair;
    }
 
    public void lock(T key) {
        LockInfo lockInfo;
        segmentLock.lock(key);
        try {
            lockInfo = lockMap.get(key);
            if (lockInfo == null) {
                lockInfo = new LockInfo(isFair);
                lockMap.put(key, lockInfo);
            } else {
                lockInfo.count.incrementAndGet();
            }
        } finally {
            segmentLock.unlock(key);
        }
        lockInfo.lock.lock();
    }
 
    public void unlock(T key) {
        LockInfo lockInfo = lockMap.get(key);
        if (lockInfo.count.get() == 1) {
            segmentLock.lock(key);
            try {
                if (lockInfo.count.get() == 1) {
                    lockMap.remove(key);
                }
            } finally {
                segmentLock.unlock(key);
            }
        }
        lockInfo.count.decrementAndGet();
        lockInfo.unlock();
    }
 
    private static class LockInfo {
        public ReentrantLock lock;
        public AtomicInteger count = new AtomicInteger(1);
 
        private LockInfo(boolean fair) {
            this.lock = new ReentrantLock(fair);
        }
 
        public void lock() {
            this.lock.lock();
        }
 
        public void unlock() {
            this.lock.unlock();
        }
    }
}

3. 弱引用锁

哈希锁因为引入的分段锁来保证锁创建和销毁的同步，总感觉有点瑕疵，所以写了第三个锁来寻求更好的性能和更细粒度的锁。这个锁的思想是借助java的弱引用来创建锁，把锁的销毁交给jvm的垃圾回收，来避免额外的消耗。

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有点遗憾的是因为使用了ConcurrentHashMap作为锁的容器，所以没能真正意义上的摆脱分段锁。这个锁的性能比 HashLock 快10% 左右。锁代码：

/**
 * 弱引用锁，为每个独立的哈希值提供独立的锁功能
 */
public class WeakHashLock {
    private ConcurrentHashMap> lockMap = new ConcurrentHashMap<>();
    private ReferenceQueue queue = new ReferenceQueue<>();
 
    public ReentrantLock get(T key) {
        if (lockMap.size() > 1000) {
            clearEmptyRef();
        }
        WeakReference lockRef = lockMap.get(key);
        ReentrantLock lock = (lockRef == null ? null : lockRef.get());
        while (lock == null) {
            lockMap.putIfAbsent(key, new WeakLockRef<>(new ReentrantLock(), queue, key));
            lockRef = lockMap.get(key);
            lock = (lockRef == null ? null : lockRef.get());
            if (lock != null) {
                return lock;
            }
            clearEmptyRef();
        }
        return lock;
    }
 
    @SuppressWarnings("unchecked")
    private void clearEmptyRef() {
        Reference ref;
        while ((ref = queue.poll()) != null) {
            WeakLockRef weakLockRef = (WeakLockRef) ref;
            lockMap.remove(weakLockRef.key);
        }
    }
 
    private static final class WeakLockRef extends WeakReference {
        final T key;
 
        private WeakLockRef(K referent, ReferenceQueue q, T key) {
            super(referent, q);
            this.key = key;
        }
    }
}

4.基于KEY（主键）的互斥锁

KeyLock是对所需处理的数据的KEY（主键）进行加锁，只要是对不同key操作，其就可以并行处理，大大提高了线程的并行度

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KeyLock有如下几个特性：

    1、细粒度，高并行性
    2、可重入
    3、公平锁
    4、加锁开销比ReentrantLock大，适用于处理耗时长、key范围大的场景

public class KeyLock {
	// 保存所有锁定的KEY及其信号量
	private final ConcurrentMap map = new ConcurrentHashMap();
	// 保存每个线程锁定的KEY及其锁定计数
	private final ThreadLocal> local = new ThreadLocal>() {
		@Override
		protected Map initialValue() {
			return new HashMap();
		}
	};

	/**
	 * 锁定key，其他等待此key的线程将进入等待，直到调用{@link #unlock(K)}
	 * 使用hashcode和equals来判断key是否相同，因此key必须实现{@link #hashCode()}和
	 * {@link #equals(Object)}方法
	 * 
	 * @param key
	 */
	public void lock(K key) {
		if (key == null)
			return;
		LockInfo info = local.get().get(key);
		if (info == null) {
			Semaphore current = new Semaphore(1);
			current.acquireUninterruptibly();
			Semaphore previous = map.put(key, current);
			if (previous != null)
				previous.acquireUninterruptibly();
			local.get().put(key, new LockInfo(current));
		} else {
			info.lockCount++;
		}
	}
	
	/**
	 * 释放key，唤醒其他等待此key的线程
	 * @param key
	 */
	public void unlock(K key) {
		if (key == null)
			return;
		LockInfo info = local.get().get(key);
		if (info != null && --info.lockCount == 0) {
			info.current.release();
			map.remove(key, info.current);
			local.get().remove(key);
		}
	}

	/**
	 * 锁定多个key
	 * 建议在调用此方法前先对keys进行排序，使用相同的锁定顺序，防止死锁发生
	 * @param keys
	 */
	public void lock(K[] keys) {
		if (keys == null)
			return;
		for (K key : keys) {
			lock(key);
		}
	}

	/**
	 * 释放多个key
	 * @param keys
	 */
	public void unlock(K[] keys) {
		if (keys == null)
			return;
		for (K key : keys) {
			unlock(key);
		}
	}

	private static class LockInfo {
		private final Semaphore current;
		private int lockCount;

		private LockInfo(Semaphore current) {
			this.current = current;
			this.lockCount = 1;
		}
	}
}

KeyLock使用示例：

private int[] accounts;  
private KeyLock lock = new KeyLock();  
  
public boolean transfer(int from, int to, int money) {  
    Integer[] keys = new Integer[] {from, to};  
    Arrays.sort(keys); //对多个key进行排序，保证锁定顺序防止死锁  
    lock.lock(keys);  
    try {  
        //处理不同的from和to的线程都可进入此同步块  
        if (accounts[from] < money)  
            return false;  
        accounts[from] -= money;  
        accounts[to] += money;  
        return true;  
    } finally {  
        lock.unlock(keys);  
    }  
}

测试代码如下：

//场景：多线程并发转账  
public class Test {  
    private final int[] account; // 账户数组，其索引为账户ID，内容为金额  
  
    public Test(int count, int money) {  
        account = new int[count];  
        Arrays.fill(account, money);  
    }  
  
    boolean transfer(int from, int to, int money) {  
        if (account[from] < money)  
            return false;  
        account[from] -= money;  
        try {  
            Thread.sleep(2);  
        } catch (Exception e) {  
        }  
        account[to] += money;  
        return true;  
    }  
      
    int getAmount() {  
        int result = 0;  
        for (int m : account)  
            result += m;  
        return result;  
    }  
  
    public static void main(String[] args) throws Exception {  
        int count = 100;        //账户个数  
        int money = 10000;      //账户初始金额  
        int threadNum = 8;      //转账线程数  
        int number = 10000;     //转账次数  
        int maxMoney = 1000;    //随机转账最大金额  
        Test test = new Test(count, money);  
          
        //不加锁  
//      Runner runner = test.new NonLockRunner(maxMoney, number);  
        //加synchronized锁  
//      Runner runner = test.new SynchronizedRunner(maxMoney, number);  
        //加ReentrantLock锁  
//      Runner runner = test.new ReentrantLockRunner(maxMoney, number);  
        //加KeyLock锁  
        Runner runner = test.new KeyLockRunner(maxMoney, number);  
          
        Thread[] threads = new Thread[threadNum];  
        for (int i = 0; i < threadNum; i++)  
            threads[i] = new Thread(runner, "thread-" + i);  
        long begin = System.currentTimeMillis();  
        for (Thread t : threads)  
            t.start();  
        for (Thread t : threads)  
            t.join();  
        long time = System.currentTimeMillis() - begin;  
        System.out.println("类型：" + runner.getClass().getSimpleName());  
        System.out.printf("耗时：%dms\n", time);  
        System.out.printf("初始总金额：%d\n", count * money);  
        System.out.printf("终止总金额：%d\n", test.getAmount());  
    }  
  
    // 转账任务  
    abstract class Runner implements Runnable {  
        final int maxMoney;  
        final int number;  
        private final Random random = new Random();  
        private final AtomicInteger count = new AtomicInteger();  
  
        Runner(int maxMoney, int number) {  
            this.maxMoney = maxMoney;  
            this.number = number;  
        }  
  
        @Override  
        public void run() {  
            while(count.getAndIncrement() < number) {  
                int from = random.nextInt(account.length);  
                int to;  
                while ((to = random.nextInt(account.length)) == from)  
                    ;  
                int money = random.nextInt(maxMoney);  
                doTransfer(from, to, money);  
            }  
        }  
  
        abstract void doTransfer(int from, int to, int money);  
    }  
  
    // 不加锁的转账  
    class NonLockRunner extends Runner {  
        NonLockRunner(int maxMoney, int number) {  
            super(maxMoney, number);  
        }  
  
        @Override  
        void doTransfer(int from, int to, int money) {  
            transfer(from, to, money);  
        }  
    }  
  
    // synchronized的转账  
    class SynchronizedRunner extends Runner {  
        SynchronizedRunner(int maxMoney, int number) {  
            super(maxMoney, number);  
        }  
  
        @Override  
        synchronized void doTransfer(int from, int to, int money) {  
            transfer(from, to, money);  
        }  
    }  
  
    // ReentrantLock的转账  
    class ReentrantLockRunner extends Runner {  
        private final ReentrantLock lock = new ReentrantLock();  
  
        ReentrantLockRunner(int maxMoney, int number) {  
            super(maxMoney, number);  
        }  
  
        @Override  
        void doTransfer(int from, int to, int money) {  
            lock.lock();  
            try {  
                transfer(from, to, money);  
            } finally {  
                lock.unlock();  
            }  
        }  
    }  
  
    // KeyLock的转账  
    class KeyLockRunner extends Runner {  
        private final KeyLock lock = new KeyLock();  
  
        KeyLockRunner(int maxMoney, int number) {  
            super(maxMoney, number);  
        }  
  
        @Override  
        void doTransfer(int from, int to, int money) {  
            Integer[] keys = new Integer[] {from, to};  
            Arrays.sort(keys);  
            lock.lock(keys);  
            try {  
                transfer(from, to, money);  
            } finally {  
                lock.unlock(keys);  
            }  
        }  
    }  
}

测试结果：

（8线程对100个账户随机转账总共10000次）：

       类型：NonLockRunner（不加锁）
       耗时：2482ms
       初始总金额：1000000
       终止总金额：998906（无法保证原子性）

       类型：SynchronizedRunner（加synchronized锁）
       耗时：20872ms
       初始总金额：1000000
       终止总金额：1000000

       类型：ReentrantLockRunner（加ReentrantLock锁）
       耗时：21588ms
       初始总金额：1000000
       终止总金额：1000000

       类型：KeyLockRunner（加KeyLock锁）
       耗时：2831ms
       初始总金额：1000000
       终止总金额：1000000