java - 为什么设置 expireAfterAccess 不起作用？

Question

为什么 timedCachetest 在最后一行失败？为什么缓存在 60 秒后不为空？

package com.test.cache;

import java.util.concurrent.TimeUnit;

import junit.framework.Assert;

import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.google.common.cache.Cache;
import com.google.common.cache.CacheBuilder;

public class CacheTest {
    private static final int MAXIMUM_SIZE = 10;
    private static final int CONCURRENCY_LEVEL = 1;
    private static final long EXPIRE_AFTER_ACCESS = 60;
    Cache<String, Thing> cache;
    private static TimeUnit unit = TimeUnit.SECONDS;
    private static long sec = 1000;

    @Before
    public void setUp() throws Exception {
        cache = CacheBuilder.newBuilder().maximumSize(MAXIMUM_SIZE).concurrencyLevel(CONCURRENCY_LEVEL).expireAfterAccess(EXPIRE_AFTER_ACCESS, unit).build();
    }

    @After
    public void tearDown() {
        cache = null;
    }

    @Test
    public void simpleCachetest() {
        String key = "key";
        Integer hc = key.hashCode();
        Thing thing = new Thing(key);
        cache.put(key, thing);
        thing = cache.getIfPresent(key);
        Assert.assertNotNull(thing);
        Assert.assertEquals(hc, thing.getValue());
        Assert.assertEquals(key, thing.getName());
        Assert.assertEquals(1, cache.size());
    }

    @Test
    public void timedCachetest() {
        String key = "key";
        Thing thing = new Thing(key);
        Assert.assertEquals(0, cache.size());
        cache.put(key, thing);
        Assert.assertEquals(1, cache.size());
        try {
            thing = cache.getIfPresent(key);
            long millis = 100 * sec;
            Thread.sleep(millis);
            // cache.invalidateAll();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        Assert.assertNotNull(thing);
        Assert.assertEquals(key, thing.getName());
        Assert.assertEquals(0, cache.size());
    }

    class Thing {
        public Thing(String key) {
            this.name = key;
            this.value = key.hashCode();
        }

        public String getName() {
            return name;
        }

        public Integer getValue() {
            return value;
        }

        private String name;
        private Integer value;
    }
}

Answer 1

它在 CacheBuilder Javadoc 中说明:

If expireAfterWrite or expireAfterAccess is requested entries may be evicted on each cache modification, on occasional cache accesses, or on calls to Cache.cleanUp(). Expired entries may be counted in Cache.size(), but will never be visible to read or write operations.

CacheBuilder 缓存会在特定请求时进行维护，或者当它可以作为缓存突变的一部分进行维护时，或者偶尔在读取时进行维护。它不是启动一个线程做缓存维护，因为a)线程比较重量级，b)有些环境限制线程的创建。