JAVA细粒度锁实现的几种方式

 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()>>>1) % segments);        lock.lock();    }    public void unlock(T key) {        ReentrantLock lock = lockMap.get((key.hashCode()>>>1) % 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;        }    }}

后记


    最开始想借助 locksupport 和 AQS 来实现细粒度锁，写着写着发现正在实现的东西和java 原生的锁区别不大，于是放弃改为对java自带锁的封装，浪费了不少时间。

    实际上在实现了这些细粒度锁之后，又有了新的想法，比如可以通过分段思想将数据提交给专门的线程来处理，可以减少大量线程的阻塞时间，留待日后探索...