hutool/hutool-cache/src/main/java/cn/hutool/cache/impl/StampedCache.java

package cn.hutool.cache.impl;

import cn.hutool.core.collection.CopiedIter;
import cn.hutool.core.thread.ThreadUtil;

import java.util.Iterator;
import java.util.concurrent.locks.StampedLock;

/**
 * 使用{@link StampedLock}保护的缓存，使用读写乐观锁
 *
 * @param <K> 键类型
 * @param <V> 值类型
 * @author looly
 * @since 5.7.15
 */
public abstract class StampedCache<K, V> extends AbstractCache<K, V> {
	private static final long serialVersionUID = 1L;

	// 乐观锁，此处使用乐观锁解决读多写少的场景
	// get时乐观读，再检查是否修改，修改则转入悲观读重新读一遍，可以有效解决在写时阻塞大量读操作的情况。
	// see: https://www.cnblogs.com/jiagoushijuzi/p/13721319.html
	protected final StampedLock lock = new StampedLock();

	@Override
	public void put(K key, V object, long timeout) {
		final long stamp = lock.writeLock();
		try {
			putWithoutLock(key, object, timeout);
		} finally {
			lock.unlockWrite(stamp);
		}
	}

	@Override
	public boolean containsKey(K key) {
		return null != get(key, false, false);
	}

	@Override
	public V get(K key, boolean isUpdateLastAccess) {
		return get(key, isUpdateLastAccess, true);
	}

	@Override
	public Iterator<CacheObj<K, V>> cacheObjIterator() {
		CopiedIter<CacheObj<K, V>> copiedIterator;
		final long stamp = lock.readLock();
		try {
			copiedIterator = CopiedIter.copyOf(cacheObjIter());
		} finally {
			lock.unlockRead(stamp);
		}
		return new CacheObjIterator<>(copiedIterator);
	}

	@Override
	public final int prune() {
		final long stamp = lock.writeLock();
		try {
			return pruneCache();
		} finally {
			lock.unlockWrite(stamp);
		}
	}

	@Override
	public void remove(K key) {
		final long stamp = lock.writeLock();
		CacheObj<K, V> co;
		try {
			co = removeWithoutLock(key);
		} finally {
			lock.unlockWrite(stamp);
		}
		if (null != co) {
			onRemove(co.key, co.obj);
		}
	}

	@Override
	public void clear() {
		final long stamp = lock.writeLock();
		try {
			cacheMap.clear();
		} finally {
			lock.unlockWrite(stamp);
		}
	}

	/**
	 * 获取值
	 *
	 * @param key                键
	 * @param isUpdateLastAccess 是否更新最后修改时间
	 * @param isUpdateCount      是否更新命中数，get时更新，contains时不更新
	 * @return 值或null
	 */
	private V get(K key, boolean isUpdateLastAccess, boolean isUpdateCount) {
		// 尝试读取缓存，使用乐观读锁
		long stamp = lock.tryOptimisticRead();
		CacheObj<K, V> co = getWithoutLock(key);
		if (false == lock.validate(stamp)) {
			// 有写线程修改了此对象，悲观读
			stamp = lock.readLock();
			try {
				co = getWithoutLock(key);
			} finally {
				lock.unlockRead(stamp);
			}
		}

		// 未命中
		if (null == co) {
			if (isUpdateCount) {
				missCount.increment();
			}
			return null;
		} else if (false == co.isExpired()) {
			if (isUpdateCount) {
				hitCount.increment();
			}
			return co.get(isUpdateLastAccess);
		}

		// 悲观锁，二次检查
		return getOrRemoveExpired(key, isUpdateCount);
	}

	/**
	 * 同步获取值，如果过期则移除之
	 *
	 * @param key           键
	 * @param isUpdateCount 是否更新命中数，get时更新，contains时不更新
	 * @return 有效值或null
	 */
	private V getOrRemoveExpired(K key, boolean isUpdateCount) {
		final long stamp = lock.writeLock();
		CacheObj<K, V> co;
		try {
			co = getWithoutLock(key);
			if (null == co) {
				return null;
			}
			if (false == co.isExpired()) {
				// 首先尝试获取值，如果值存在且有效，返回之
				if (isUpdateCount) {
					hitCount.increment();
				}
				return co.getValue();
			}

			// 无效移除
			co = removeWithoutLock(key);
		} finally {
			lock.unlockWrite(stamp);
		}
		if (null != co) {
			onRemove(co.key, co.obj);
		}
		return null;
	}
}