本文介绍LruBlockCache如何获取缓存数据。

        缓存数据的获取是在方法getBlock()中实现的，代码如下：

  /**
   * Get the buffer of the block with the specified name.
   * @param cacheKey block's cache key
   * @param caching true if the caller caches blocks on cache misses
   * @param repeat Whether this is a repeat lookup for the same block
   *        (used to avoid double counting cache misses when doing double-check locking)
   * @param updateCacheMetrics Whether to update cache metrics or not
   * @return buffer of specified cache key, or null if not in cache
   */
  @Override
  public Cacheable getBlock(BlockCacheKey cacheKey, boolean caching, boolean repeat,
      boolean updateCacheMetrics) {
    LruCachedBlock cb = map.get(cacheKey);
    if (cb == null) {
      if (!repeat && updateCacheMetrics) stats.miss(caching, cacheKey.isPrimary());
      // If there is another block cache then try and read there.
      // However if this is a retry ( second time in double checked locking )
      // And it's already a miss then the l2 will also be a miss.
      if (victimHandler != null && !repeat) {
        Cacheable result = victimHandler.getBlock(cacheKey, caching, repeat, updateCacheMetrics);

        // Promote this to L1.
        if (result != null && caching) {
          cacheBlock(cacheKey, result, /* inMemory = */ false, /* cacheData = */ true);
        }
        return result;
      }
      return null;
    }
    if (updateCacheMetrics) stats.hit(caching, cacheKey.isPrimary());
    cb.access(count.incrementAndGet());
    return cb.getBuffer();
  }

        1、首先，从LruBlockCache的map中直接获取；

        2、如果map中没有，则在victimHandler存在且!repeat的情况下，通过victimHandler的getBlock()方法获取并缓存到LruBlockCache中，即综合考虑第二种缓存模式，并同步到第一种缓存中；

        3、如果1或2能够获取到数据，更新统计数据，且通过缓存块的access方法，更新访问时间accessTime，将可能的BlockPriority.SINGLE升级为BlockPriority.MULTI；

        4、返回。