多线程下使用缓存+锁Lock, 出现“锁失效” + “缓存未命中竞争”的缓存击穿情况，双重检查缓存解决问题

多线程情况下，想通过缓存+同步锁的机制去避免多次重复处理逻辑，尤其是I/0操作，但是在实际的操作过程中发现多次访问的日志

python 复制代码

2025-06-05 17:30:27.683 [ForkJoinPool.commonPool-worker-3] INFO Rule - [vagueNameMilvusReacll,285] - embedding time-consuming:503 
2025-06-05 17:30:29.693 [ForkJoinPool.commonPool-worker-3] INFO Rule - [vagueNameMilvusReacll,314] - milvus time-consuming:2010 
2025-06-05 17:30:29.701 [ForkJoinPool.commonPool-worker-3] INFO Rule - [vagueNameMilvusReacll,358] - vagueName time-consuming:2534 

2025-06-05 17:30:30.135 [ForkJoinPool.commonPool-worker-11] INFO Rule - [vagueNameMilvusReacll,285] - embedding time-consuming:434 
2025-06-05 17:30:30.363 [ForkJoinPool.commonPool-worker-11] INFO Rule - [vagueNameMilvusReacll,314] - milvus time-consuming:228 
2025-06-05 17:30:30.369 [ForkJoinPool.commonPool-worker-11] INFO Rule - [vagueNameMilvusReacll,358] - vagueName time-consuming:3202 

2025-06-05 17:30:30.750 [ForkJoinPool.commonPool-worker-8] INFO Rule - [vagueNameMilvusReacll,285] - embedding time-consuming:381 
2025-06-05 17:30:31.021 [ForkJoinPool.commonPool-worker-8] INFO Rule - [vagueNameMilvusReacll,314] - milvus time-consuming:270 
2025-06-05 17:30:31.022 [ForkJoinPool.commonPool-worker-8] INFO Rule - [vagueNameMilvusReacll,358] - vagueName time-consuming:3855

代码如下：

java 复制代码

public final static Map<String, Lock> keyLockMap = new ConcurrentHashMap<>();


Rule cacheRule = (Rule) CacheMap.get(nodeValue);
if (cacheRule != null) {
    // 返回缓存
}

Lock lock = keyLockMap.computeIfAbsent(nodeValue, k -> new ReentrantLock());
lock.lock();
try {

}finally {
   lock.unlock();
   // 释放锁资源，避免 map 持有无用锁对象太久
   keyLockMap.remove(nodeValue);
}

实际的问题：

在加锁之前做了第一次缓存检查（没问题），但在加锁之后没有再次检查缓存是否被其他线程填充过！

这就导致多个线程可能都进入了 lock.lock() 后的代码块，并且都执行了实际查询逻辑。

解决方案：双重检查缓存（Double-Checked Caching）

python 复制代码

Rule cacheRule = (Rule) CacheMap.get(nodeValue);
if (cacheRule != null) {
    rule.setKey(cacheRule.getKey());
    rule.setValue(cacheRule.getValue());
    return;
}

Lock lock = keyLockMap.computeIfAbsent(nodeValue, k -> new ReentrantLock());
lock.lock();
try {
    // 【关键】第二次检查缓存
    cacheRule = (Rule) CacheMap.get(nodeValue);
    if (cacheRule != null) {
        rule.setKey(cacheRule.getKey());
        rule.setValue(cacheRule.getValue());
        return;
    }

    // 真正执行 Milvus 请求...
    // ...
    // 最后更新缓存
    CacheMap.put(nodeValue, rule);
} finally {
    lock.unlock();
    keyLockMap.remove(nodeValue); // 可选释放锁对象
}

这个可能出现锁失效的情况

keyLockMap.remove(nodeValue); // 可选释放锁对象

当T1 进入的时候处理完逻辑后，放入缓存，然后删除锁

sleep(xxx)

当T2 进入的时候处理逻辑，发现没有锁，上锁，访问缓存

发现问题了，如果finally 及时删除锁，可能会出现下一个线程重新建立锁对象，然后多了查询缓存的性能消耗。

为了避免这种情况存在

建立LockManager 类管理锁对象，同时对锁进行ttl 保留时间定期任务删除对应的key

java 复制代码

import lombok.extern.slf4j.Slf4j;

import java.util.Map;
import java.util.concurrent.*;
import java.util.concurrent.locks.ReentrantLock;

@Slf4j
public class LockManager {
    private final Map<String, ReentrantLock> lockMap = new ConcurrentHashMap<>();
    private final Map<String, Long> lastAccessTime = new ConcurrentHashMap<>();
    private static final long TTL = TimeUnit.MINUTES.toMillis(5); // 锁保留5分钟

    private final ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();

    public LockManager() {
        startCleanupTask();
    }

    // 获取锁，并更新最后访问时间
    public ReentrantLock getLock(String key) {
        lastAccessTime.put(key, System.currentTimeMillis());
        return lockMap.computeIfAbsent(key, k -> new ReentrantLock());
    }

    // 清理任务：扫描并移除超时的锁对象
    private void startCleanupTask() {
        scheduler.scheduleAtFixedRate(() -> {
            log.info("------------start check expired lock---------------");
            long now = System.currentTimeMillis();
            lastAccessTime.forEach((key, timestamp) -> {
                if (now - timestamp > TTL) {
                    lockMap.remove(key);
                    lastAccessTime.remove(key);
                    log.info("Removed expired lock for key: {}", key);
                }
            });
        }, 1, 1, TimeUnit.MINUTES); // 每分钟执行一次清理
    }

    public void shutdown() {
        scheduler.shutdownNow();
    }
}

java 复制代码

@Configuration
public class AppConfig {

    @Bean(destroyMethod = "shutdown")
    public LockManager lockManager() {
        return new LockManager();
    }
}

注入使用

java 复制代码

        Lock lock = lockManager.getLock(nodeValue);