最近在工作上碰見了一些高並發的場景需要加鎖來保證業務邏輯的正確性，並且要求加鎖後性能不能受到太大的影響。初步的想法是透過資料的時間戳，id等關鍵字來加鎖，從而保證不同類型資料處理的並發性。而java本身api提供的鎖定粒度太大，很難同時滿足這些需求，於是自己動手寫了幾個簡單的擴充…
/**
* 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();
}
}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();
}
}
}引用
鎖定哈希鎖定因為引入的分段鎖定來保證鎖定創建和銷毀的同步，總感覺有點瑕疵，所以寫了第三個鎖來尋求更好的性能和更細粒度的鎖。這個鎖的想法是藉助java的弱引用來創建鎖，把鎖的銷毀交給jvm的垃圾回收，來避免額外的消耗。 ######有點遺憾的是因為使用了ConcurrentHashMap作為鎖的容器，所以沒能真正意義上的擺脫分段鎖定。這個鎖的效能比 HashLock 快10% 左右。鎖定程式碼：###/**
* 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;
}
}
}以上是詳解Java細粒度鎖實現的3種方式的範例程式碼的詳細內容。更多資訊請關注PHP中文網其他相關文章！
