我有一个 LoadingCache<K,V> 使用 CacheBuilder 创建:

LoadingCache<K,V> myCache = CacheBuilder.newBuilder()
    .expireAfterAccess(1, TimeUnit.MINUTES)
    .maximumSize(500)
    .build(someCacheLoader);

我需要定期迭代缓存中的所有条目(键和值)。我知道我可以使用 LoadingCache#asMap() 来完成它，或者:

• 遍历 map View 的 Map#entrySet() , 或
• 仅遍历 Map#keySet() ，并明确地 Map#get(Object) 值(value)观。

在前一种情况下:

for (Map.Entry<K, V> entry : myCache.asMap().entrySet()) {
    K key = entry.getKey();
    V value = entry.getValue();

    doSomeWorkOn(key, value);
}

这会更新缓存中每个条目的访问时间吗？我已经阅读了 CacheBuilder#expireAfterAccess(long, TimeUnit) 的 JavaDoc非常接近，但在这种情况下发现它是模棱两可的/不清楚的:

Specifies that each entry should be automatically removed from the cache once a fixed duration has elapsed after the entry's creation, the most recent replacement of its value, or its last access. Access time is reset by all cache read and write operations (including Cache.asMap().get(Object) and Cache.asMap().put(K, V)), but not by operations on the collection-views of Cache.asMap.

显然，我提到的第二种迭代方式确实重置了访问时间，但我想知道使用第一种方式的行为是什么。

最佳答案

我会这样解释:

(...) but not by operations on the collection-views of Cache.asMap

引用 entrySet、keySet 和 values。这些是 Map 的三个 Collection View 。因此，使用它们不应导致访问。

这里是 JUnit (+Mockito) 测试，显示了每种情况下的行为。通过 entrySet 或 values 读取值不会阻止删除条目(也不会从 entrySet 读取键> 或 keySet)。如文档所述，使用 asMap().get() 读取确实算作一次访问。

设置

private Ticker ticker = Mockito.mock(Ticker.class);

@SuppressWarnings({"unchecked"})
private RemovalListener<String, String> removalListener = Mockito.mock(RemovalListener.class);

private Cache<String, String> cache = CacheBuilder.newBuilder()
            .expireAfterAccess(5, TimeUnit.SECONDS)
            .removalListener(removalListener)
            .ticker(ticker)
            .build();

入口集

@Test
public void testEntrySetAccessDoesNotCountAsAccess() {
    //write
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(0));
    cache.put("foo", "bar");

    //read
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(4));
    cache.asMap().entrySet().iterator().next().getValue();
    cache.asMap().entrySet().iterator().next().getKey();

    //maintenance
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(6));
    cache.cleanUp();

    verify(removalListener).onRemoval(Mockito.<RemovalNotification<String,String>>any());
}

键集

@Test
public void testKeySetAccessDoesNotCountAsAccess() {
    //write
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(0));
    cache.put("foo", "bar");

    //read
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(4));
    cache.asMap().keySet().iterator().next();

    //maintenance
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(6));
    cache.cleanUp();

    verify(removalListener).onRemoval(Mockito.<RemovalNotification<String,String>>any());
}

值

@Test
public void testValuesAccessDoesNotCountAsAccess() {
    //write
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(0));
    cache.put("foo", "bar");

    //read
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(4));
    cache.asMap().values().iterator().next();

    //maintenance
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(6));
    cache.cleanUp();

    verify(removalListener).onRemoval(Mockito.<RemovalNotification<String,String>>any());
}

asMap().get()

@Test
public void testMapGetAccessCountsAsAccess() {
    //write
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(0));
    cache.put("foo", "bar");

    //read
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(4));
    cache.asMap().get("foo");

    //maintenance
    when(ticker.read()).thenReturn(TimeUnit.SECONDS.toNanos(6));
    cache.cleanUp();

    verify(removalListener, never()).onRemoval(Mockito.<RemovalNotification<String,String>>any());
}

关于java - 遍历 Guava LoadingCache 的 map View 条目集是否会重置用于过期的访问时间？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/14371888/