Caches

A LoadingCache is a Cache built with an attached CacheLoader. Creating a CacheLoader is typically as easy as implementing the method V load(K key) throws Exception.

LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
       .maximumSize(1000)
       .expireAfterWrite(10, TimeUnit.MINUTES)
       .removalListener(MY_LISTENER)
       .build(
           new CacheLoader<Key, Graph>() {
             public Graph load(Key key) throws AnyException {
               return createExpensiveGraph(key);
             }
           });

主要的类

• CacheLoader: 负责加载缓存
• CacheBuilder: 生成Cache实例的创建者，配置各项Cache属性
• RemovalListener: Cache数据移除事件的监听器
• CacheStats: Cache的统计功能

Eviction 数据回收的方式

• 缓存容量策略
  • CacheBuilder.maximumSize(long): 基于缓存元素的数量
• 缓存元素权重
  • CacheBuilder.weigher(Weigher)
  • CacheBuilder.maximumWeight(long): 基于缓存元素的权重限制
• 根据时间策略
  • expireAfterAccess(long, TimeUnit): 最后一次访问操作后，超时失效
  • expireAfterWrite(long, TimeUnit) : 最后一次写操作后，超时失效
• 指定引用级别,默认key,value都是强引用
  • CacheBuilder.weakKeys()
  • CacheBuilder.weakValues()
  • CacheBuilder.softValues()

Explicit Removals

• individually, using Cache.invalidate(key)
• in bulk, using Cache.invalidateAll(keys)
• to all entries, using Cache.invalidateAll()

Refresh 缓存数据的刷新

• LoadingCache.refresh(K)
• CacheLoader.reload(K, V)
• CacheBuilder.refreshAfterWrite(long, TimeUnit)

// Some keys don't need refreshing, and we want refreshes to be done asynchronously.
LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
       .maximumSize(1000)
       .refreshAfterWrite(1, TimeUnit.MINUTES)
       .build(
           new CacheLoader<Key, Graph>() {
             public Graph load(Key key) { // no checked exception
               return getGraphFromDatabase(key);
             }

             public ListenableFuture<Graph> reload(final Key key, Graph prevGraph) {
               if (neverNeedsRefresh(key)) {
                 return Futures.immediateFuture(prevGraph);
               } else {
                 // asynchronous!
                 ListenableFutureTask<Graph> task = ListenableFutureTask.create(new Callable<Graph>() {
                   public Graph call() {
                     return getGraphFromDatabase(key);
                   }
                 });
                 executor.execute(task);
                 return task;
               }
             }
           });

Statistics 统计功能 By using CacheBuilder.recordStats(), you can turn on statistics collection for Guava caches. The Cache.stats() method returns a CacheStats object, which provides statistics such as

• hitRate(), which returns the ratio of hits to requests
• averageLoadPenalty(), the average time spent loading new values, in nanoseconds
• evictionCount(), the number of cache evictions