KeyLock 工具

群友分享的一个 es 的 key lock 工具

/**
 * @author tys
 * @version 1.0
 * @date 2022/8/18 5:00 PM
 */
/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you under
 * the Apache License, Version 2.0 (the "License"); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
import org.elasticsearch.common.lease.Releasable;
import org.elasticsearch.common.util.concurrent.ConcurrentCollections;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.ReentrantLock;
/**
 * This class manages locks. Locks can be accessed with an identifier and are
 * created the first time they are acquired and removed if no thread hold the
 * lock. The latter is important to assure that the list of locks does not grow
 * infinitely.
 * Note: this lock is reentrant
 *
 * */
public final class KeyedLock<T> {
    private final ConcurrentMap<T, KeyLock> map = ConcurrentCollections.newConcurrentMapWithAggressiveConcurrency();
    private final boolean                                                                      fair;
    /**
     * Creates a new lock
     * @param fair Use fair locking, ie threads get the lock in the order they requested it
     */
    public KeyedLock(boolean fair) {
        this.fair = fair;
    }
    /**
     * Creates a non-fair lock
     */
    public KeyedLock() {
        this(false);
    }
    /**
     * Acquires a lock for the given key. The key is compared by it's equals method not by object identity. The lock can be acquired
     * by the same thread multiple times. The lock is released by closing the returned {@link Releasable}.
     */
    public Releasable acquire(T key) {
        while (true) {
            KeyLock perNodeLock = map.get(key);
            if (perNodeLock == null) {
                ReleasableLock newLock = tryCreateNewLock(key);
                if (newLock != null) {
                    return newLock;
                }
            } else {
                assert perNodeLock != null;
                int i = perNodeLock.count.get();
                if (i > 0 && perNodeLock.count.compareAndSet(i, i + 1)) {
                    perNodeLock.lock();
                    return new ReleasableLock(key, perNodeLock);
                }
            }
        }
    }
    /**
     * Tries to acquire the lock for the given key and returns it. If the lock can't be acquired null is returned.
     */
    public Releasable tryAcquire(T key) {
        final KeyLock perNodeLock = map.get(key);
        if (perNodeLock == null) {
            return tryCreateNewLock(key);
        }
        if (perNodeLock.tryLock()) { // ok we got it - make sure we increment it accordingly otherwise release it again
            int i;
            while ((i = perNodeLock.count.get()) > 0) {
                // we have to do this in a loop here since even if the count is > 0
                // there could be a concurrent blocking acquire that changes the count and then this CAS fails. Since we already got
                // the lock we should retry and see if we can still get it or if the count is 0. If that is the case and we give up.
                if (perNodeLock.count.compareAndSet(i, i + 1)) {
                    return new ReleasableLock(key, perNodeLock);
                }
            }
            perNodeLock.unlock(); // make sure we unlock and don't leave the lock in a locked state
        }
        return null;
    }
    private ReleasableLock tryCreateNewLock(T key) {
        KeyLock newLock = new KeyLock(fair);
        newLock.lock();
        KeyLock keyLock = map.putIfAbsent(key, newLock);
        if (keyLock == null) {
            return new ReleasableLock(key, newLock);
        }
        return null;
    }
    /**
     * Returns <code>true</code> iff the caller thread holds the lock for the given key
     */
    public boolean isHeldByCurrentThread(T key) {
        KeyLock lock = map.get(key);
        if (lock == null) {
            return false;
        }
        return lock.isHeldByCurrentThread();
    }
    private void release(T key, KeyLock lock) {
        assert lock == map.get(key);
        final int decrementAndGet = lock.count.decrementAndGet();
        lock.unlock();
        if (decrementAndGet == 0) {
            map.remove(key, lock);
        }
        assert decrementAndGet >= 0 : decrementAndGet + " must be >= 0 but wasn't";
    }
    private final class ReleasableLock implements Releasable {
        final T                                                          key;
        final KeyLock lock;
        final AtomicBoolean                                              closed = new AtomicBoolean();
        private ReleasableLock(T key, KeyLock lock) {
            this.key = key;
            this.lock = lock;
        }
        @Override
        public void close() {
            if (closed.compareAndSet(false, true)) {
                release(key, lock);
            }
        }
    }
    @SuppressWarnings("serial")
    private static final class KeyLock extends ReentrantLock {
        KeyLock(boolean fair) {
            super(fair);
        }
        private final AtomicInteger count = new AtomicInteger(1);
    }
    /**
     * Returns <code>true</code> if this lock has at least one locked key.
     */
    public boolean hasLockedKeys() {
        return map.isEmpty() == false;
    }
}

重点总结：

1. ConcurrentMap.putIfAbsent用于 keylock 的创建
2. AtomicInteger.compareAndSet 用于在同步阻塞获取锁的时候，判定哪一个线程能获取到锁
3. 释放锁的时候，根据 AtomicInteger 的值判定，如果等于 0 的时候，就从 map 中移除锁