缓存工具

缓存设计

缓存接口

/**
 *    Copyright 2009-2015 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;

import java.util.concurrent.locks.ReadWriteLock;

/**
 * SPI for cache providers.
 * 
 * One instance of cache will be created for each namespace.
 * 
 * The cache implementation must have a constructor that receives the cache id as an String parameter.
 * 
 * MyBatis will pass the namespace as id to the constructor.
 * 
 * <pre>
 * public MyCache(final String id) {
 *  if (id == null) {
 *    throw new IllegalArgumentException("Cache instances require an ID");
 *  }
 *  this.id = id;
 *  initialize();
 * }
 * </pre>
 *
 * @author Clinton Begin
 */

public interface Cache {

  /**
   * @return The identifier of this cache
   */
  String getId();

  /**
   * @param key Can be any object but usually it is a {@link CacheKey}
   * @param value The result of a select.
   */
  void putObject(Object key, Object value);

  /**
   * @param key The key
   * @return The object stored in the cache.
   */
  Object getObject(Object key);

  /**
   * As of 3.3.0 this method is only called during a rollback 
   * for any previous value that was missing in the cache.
   * This lets any blocking cache to release the lock that 
   * may have previously put on the key.
   * A blocking cache puts a lock when a value is null 
   * and releases it when the value is back again.
   * This way other threads will wait for the value to be 
   * available instead of hitting the database.
   *
   * 
   * @param key The key
   * @return Not used
   */
  Object removeObject(Object key);

  /**
   * Clears this cache instance
   */  
  void clear();

  /**
   * Optional. This method is not called by the core.
   * 
   * @return The number of elements stored in the cache (not its capacity).
   */
  int getSize();
  
  /** 
   * Optional. As of 3.2.6 this method is no longer called by the core.
   *  
   * Any locking needed by the cache must be provided internally by the cache provider.
   * 
   * @return A ReadWriteLock 
   */
  ReadWriteLock getReadWriteLock();

}

我们来讨论一下，说到MyBatis，我们的第一印象是什么？毫无疑问是数据库交互操作，那么问题来了，如果要用到缓存怎么办？还好MyBatis也是支持。

俗话说的好，一个人只会在一方面精通，不可能都做得好，那有人是专门做缓存的，怎么去获取数据呢？

MyBatis就提供了这样一个Cache接口。谁做得好，谁就去实现就行了。

都知道Spring强大，面试的时候，都会问，那么我们为什么要用Spring呢？我们在用的时候，也感觉Spring好厉害，好像都在用。

Spring 需要集成很多别的框架，才能帮助我们完成整个系统，说白了，Spring就是一个容器。

所以我们在系统设计的时候，也可采取这样的方式，把自己当做一个容器，先把需要的东西写成接口，不管我自己能不能实现，这都不重要。