HBase-1.2.4LruBlockCache实现分析（二）

来源：互联网发布：乃木坂网络直播电视剧编辑：程序博客网时间：2024/05/17 06:19

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();  }

实现逻辑如下：

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。

1 0