Guava Cache 研究

，Guava Cache其核心数据结构大体上和ConcurrentHashMap一致，具体细节上会有些区别。功能上，ConcurrentMap会一直保存所有添加的元素，直到显式地移除.相对地,Guava Cache为了限制内存占用,通常都设定为自动回收元素.在某些场景下,尽管它不回收元素,也是很有用的,因为它会自动加载缓存.

class LocalCache<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>

子类 LocalLoadingCache

  static class LocalLoadingCache<K, V>
      extends LocalManualCache<K, V> implements LoadingCache<K, V> {

    LocalLoadingCache(CacheBuilder<? super K, ? super V> builder,
        CacheLoader<? super K, V> loader) {
      super(new LocalCache<K, V>(builder, checkNotNull(loader)));
    }

在使用get方法的时候，如果缓存不存在该key或者key过期等，则调用getOrLoad 方法加载数据；

// LoadingCache methods

    @Override
    public V get(K key) throws ExecutionException {
      return localCache.getOrLoad(key);
    }

在LocalCache中确实提供了两种类，一个是支持自动加载value的LocalLoadingCache 和只能在键值找不到的时候手动调用获取值方法的LocalManualCache。

/**
   * Builds a cache, which either returns an already-loaded value for a given key or atomically
   * computes or retrieves it using the supplied {@code CacheLoader}. If another thread is currently
   * loading the value for this key, simply waits for that thread to finish and returns its
   * loaded value. Note that multiple threads can concurrently load values for distinct keys.
   *
   * <p>This method does not alter the state of this {@code CacheBuilder} instance, so it can be
   * invoked again to create multiple independent caches.
   *
   * @param loader the cache loader used to obtain new values
   * @return a cache having the requested features
   */
  public <K1 extends K, V1 extends V> LoadingCache<K1, V1> build(
      CacheLoader<? super K1, V1> loader) {
    checkWeightWithWeigher();
    return new LocalCache.LocalLoadingCache<K1, V1>(this, loader);
  }

  /**
   * Builds a cache which does not automatically load values when keys are requested.
   *
   * <p>Consider {@link #build(CacheLoader)} instead, if it is feasible to implement a
   * {@code CacheLoader}.
   *
   * <p>This method does not alter the state of this {@code CacheBuilder} instance, so it can be
   * invoked again to create multiple independent caches.
   *
   * @return a cache having the requested features
   * @since 11.0
   */
  public <K1 extends K, V1 extends V> Cache<K1, V1> build() {
    checkWeightWithWeigher();
    checkNonLoadingCache();
    return new LocalCache.LocalManualCache<K1, V1>(this);
  }

Guava Cache的实现,核心数据结构和算法都是和JDK 1.6版本的ConcurrentHashMap一致

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