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; } }}| 欢迎光临 黑马程序员技术交流社区 (http://bbs.itheima.com/) | 黑马程序员IT技术论坛 X3.2 |