LruCache.java

/*
 *    Copyright 2009-2014 the original author or authors.
 *
 *    Licensed under the Apache License, Version 2.0 (the "License");
 *    you may not use this file except in compliance with the License.
 *    You may obtain a copy of the License at
 *
 *       http://www.apache.org/licenses/LICENSE-2.0
 *
 *    Unless required by applicable law or agreed to in writing, software
 *    distributed under the License is distributed on an "AS IS" BASIS,
 *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *    See the License for the specific language governing permissions and
 *    limitations under the License.
 */
package org.apache.ibatis.cache.decorators;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;

import org.apache.ibatis.cache.Cache;

/**
 * Lru (first in, first out) cache decorator
 *
 * @author Clinton Begin
 */
/*
 * 最近最少使用缓存
 * 基于 LinkedHashMap 覆盖其 removeEldestEntry 方法实现。
 */
public class LruCache implements Cache {

  private final Cache delegate;
  //额外用了一个map才做lru，但是委托的Cache里面其实也是一个map，这样等于用2倍的内存实现lru功能
  private Map<Object, Object> keyMap;
  private Object eldestKey;

  public LruCache(Cache delegate) {
    this.delegate = delegate;
    setSize(1024);
  }

  @Override
  public String getId() {
    return delegate.getId();
  }

  @Override
  public int getSize() {
    return delegate.getSize();
  }

  public void setSize(final int size) {
    keyMap = new LinkedHashMap<Object, Object>(size, .75F, true) {
      private static final long serialVersionUID = 4267176411845948333L;

      //核心就是覆盖 LinkedHashMap.removeEldestEntry方法,
      //返回true或false告诉 LinkedHashMap要不要删除此最老键值
      //LinkedHashMap内部其实就是每次访问或者插入一个元素都会把元素放到链表末尾，
      //这样不经常访问的键值肯定就在链表开头啦
      @Override
      protected boolean removeEldestEntry(Map.Entry<Object, Object> eldest) {
        boolean tooBig = size() > size;
        if (tooBig) {
            //这里没辙了，把eldestKey存入实例变量
          eldestKey = eldest.getKey();
        }
        return tooBig;
      }
    };
  }

  @Override
  public void putObject(Object key, Object value) {
    delegate.putObject(key, value);
    //增加新纪录后，判断是否要将最老元素移除
    cycleKeyList(key);
  }

  @Override
  public Object getObject(Object key) {
      //get的时候调用一下LinkedHashMap.get，让经常访问的值移动到链表末尾
    keyMap.get(key); //touch
    return delegate.getObject(key);
  }

  @Override
  public Object removeObject(Object key) {
    return delegate.removeObject(key);
  }

  @Override
  public void clear() {
    delegate.clear();
    keyMap.clear();
  }

  @Override
  public ReadWriteLock getReadWriteLock() {
    return null;
  }

  private void cycleKeyList(Object key) {
    keyMap.put(key, key);
    //keyMap是linkedhashmap，最老的记录已经被移除了，然后这里我们还需要移除被委托的那个cache的记录
    if (eldestKey != null) {
      delegate.removeObject(eldestKey);
      eldestKey = null;
    }
  }

}