Guava CacheBuilder不会调用remove listener

Question

我想:在超时到期时删除实体时收到通知.

我试过:设置删除监听器.

问题: 似乎删除侦听器无法正常工作.它只在我将新项目放入缓存时才有效(参见下面的代码)

问题: 如何在不放置新项目的情况下使删除侦听器工作？

码:

我的加载缓存:

LoadingCache<String, Integer> ints = CacheBuilder.newBuilder()
            .maximumSize(10000)
            .expireAfterAccess(ACCESS_TIMEOUT, TimeUnit.MILLISECONDS)
            .removalListener(
                    new RemovalListener() {
                        //PROBLEM: THIS METHOD IS NEVER CALLED!!!
                        public void onRemoval(RemovalNotification notification) {
                            if (notification.getCause() == RemovalCause.EXPIRED) {
                                System.out.println("Value " + notification.getValue() + " has been expired");
                            } else {
                                System.out.println("Just removed for some reason");
                            }
                        }
                    }
            )
            .build(
                    new CacheLoader<String, Integer>() {
                        public Integer load(String key) throws Exception {
                            return new Integer(-1);
                        }
                    });

我如何在单独的线程中使用缓存:

cache.put("key", 100);
Thread.sleep(ACCESS_TIMEOUT / 2);
System.out.println(cache.getIfPresent(key)); //returns 100
Thread.sleep(ACCESS_TIMEOUT * 5);
//CRUCIAL STRING: cache.put("key2", 200); //invoke removal listener
System.out.println(cache.getIfPresent(key)); //return null
//And again: the problem is that entity has been already expired, but removal listener isn't called untill I add new item to the cache.

PS:如果需要,我可以在GitHub上分享完整的演示,告诉我

Answer 1

这是因为当超时值到期时,Guava无法确保自动驱逐值.然而,它在一系列读写操作期间执行此操作.

每它的文档在这里:

使用CacheBuilder构建的缓存不会"自动"执行清理和逐出值,或者在值到期后立即执行或逐出任何类型.相反,它在写入操作期间执行少量维护,或者在写入很少的情况下偶尔执行读取操作.

原因如下:如果我们想要连续执行缓存维护,我们需要创建一个线程,其操作将与共享锁的用户操作竞争.此外,某些环境会限制线程的创建,这会使CacheBuilder在该环境中无法使用.

要验证您onRemoval的到期日期,请cache#cleanUp在第二次阅读操作之前立即致电,并且应该拨打您的电话onRemoval.