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<T> {
    private Integer segments = 16;//默认分段数量
    private final HashMap<Integer, ReentrantLock> 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<T> {
    private boolean isFair = false;
    private final SegmentLock<T> segmentLock = new SegmentLock<>();//分段锁
    private final ConcurrentHashMap<T, LockInfo> 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。

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<T> {
    private ConcurrentHashMap<T, WeakLockRef<T, ReentrantLock>> lockMap = new ConcurrentHashMap<>();
    private ReferenceQueue<ReentrantLock> queue = new ReferenceQueue<>();

    public ReentrantLock get(T key) {
        if (lockMap.size() > 1000) {
            clearEmptyRef();
        }
        WeakReference<ReentrantLock> 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<? extends ReentrantLock> ref;
        while ((ref = queue.poll()) != null) {
            WeakLockRef<T, ? extends ReentrantLock> weakLockRef = (WeakLockRef<T, ? extends ReentrantLock>) ref;
            lockMap.remove(weakLockRef.key);
        }
    }

    private static final class WeakLockRef<T, K> extends WeakReference<K> {
        final T key;

        private WeakLockRef(K referent, ReferenceQueue<? super K> 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

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

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<Integer> lock = new KeyLock<Integer>();

        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

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