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简介
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，加锁后性能受到太大的影响。这对于某些高并发的场景并不适用。本文实现了一个基于KEY（主键）的互斥锁，具有更细的粒度，在缓存或其他基于KEY的场景中有很大的用处。下面将讲解这个锁的设计和实现
分段锁
/**
* Created by hhbbz on 2018/7/13.
* @Explain: key锁（要保证key的hashCode不变,否则无法释放锁。即加锁之后不要手动更改lockMap）
*/
@Component
public class LoadKeyLock<T> {
//默认分段数量
private Integer segments = 16;
private final HashMap<Integer, ReentrantLock> lockMap = new HashMap<>();
public LoadKeyLock() {
init(null, false);
}
public LoadKeyLock(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();
}
}
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();
}
}
}
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;
}
}
}
适合耗时长场景的互斥key锁
一个细粒度的锁，在某些场景能比synchronized，ReentrantLock等获得更高的并行度更好的性能
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;
}
}
}
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