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Java 細粒度鎖實現方案：分段鎖、哈希鎖與弱引用鎖深度解析

編輯：Java 相關知識

Java 細粒度鎖實現方案深度解析

高併發場景下需加鎖保證業務邏輯正確性，但鎖粒度過大會嚴重影響效能。Java 原生鎖粒度較粗，難以滿足按業務鍵（如時間戳、ID）加鎖需求。本文實現三種細粒度鎖策略：分段鎖、哈希鎖、弱引用鎖。

1. 分段鎖

借鑑 ConcurrentHashMap 分段思想，預創建固定數量鎖，根據 key 的 hash 值映射到對應鎖。最簡單、效能最高，最終採用方案。

public class SegmentLock<T> {
    private Integer segments = 16;
    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();
    }
}

注意：被鎖物件的 hashCode 若發生變化，會導致鎖無法釋放。

2. 哈希鎖

基於分段鎖演進，目標實現真正細粒度：每個 hash 值不同的物件擁有獨立鎖。測試中，鎖住代碼執行極快時比分段鎖慢約 30%，若有長耗時操作表現應更好。

public class HashLock<T> {
    private boolean isFair = false;
    private final SegmentLock<T> segmentLock = new SegmentLock<>();
    private final ConcurrentHashMap<T, LockInfo> lockMap = new ConcurrentHashMap<>();
    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. 弱引用鎖

利用 Java 弱引用創建鎖，將鎖銷毀交給 JVM GC，避免額外開銷。因使用 ConcurrentHashMap 作為容器，未能完全擺脫分段鎖。效能比 HashLock 快約 10%。

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;
        }
    }
}