Apache Zeppelin系列教程第八篇——LRU算法在Apache Zeppelin中的应用

private class LRUCache extends LinkedHashMap<String, Note> {
      private static final long serialVersionUID = 1L;
      public LRUCache() {
        super(NoteCache.this.threshold, 0.5f, true /* lru by access mode */);
      }
      @Override
      protected boolean removeEldestEntry(java.util.Map.Entry<String, Note> eldest) {
        if (size() <= NoteCache.this.threshold) {
          return false;
        }
        final Note eldestNote = eldest.getValue();
        final Lock lock = eldestNote.getLock().writeLock();
        if (lock.tryLock()) { // avoid eviction in case the note is in use
          try {
            return true;
          } finally {
            lock.unlock();
          }
        } else {
          LOGGER.info("Can not evict note {}, because the write lock can not be acquired. {} notes currently loaded.",
              eldestNote.getId(), size());
          cleanupCache();
          return false;
        }
      }
      private void cleanupCache() {
        Iterator<Map.Entry<String, Note>> iterator = this.entrySet().iterator();
        int count = 0;
        // if size >= shrinked_size and have next() try remove
        while ((this.size() - 1) >= NoteCache.this.threshold && iterator.hasNext()) {
          Map.Entry<String, Note> noteEntry = iterator.next();
          final Note note = noteEntry.getValue();
          final Lock lock = note.getLock().writeLock();
          if (lock.tryLock()) { // avoid eviction in case the note is in use
            try {
              iterator.remove(); // remove LRU element from LinkedHashMap
              LOGGER.debug("Remove note {} from LRU Cache", note.getId());
              ++count;
            } finally {
              lock.unlock();
            }
          }
        }
        LOGGER.info("The cache cleanup removes {} entries", count);
      }
    }
  }