如何理解Spring对缓存的支持

1.Spring缓存机制的理解

在Spring缓存机制中，包括了两个方面的缓存操作：1.缓存某个方法返回的结果；2.在某个方法执行前或后清空缓存。

下面写两个类来模拟Spring的缓存机制:
package com.sin90lzc.java.test;

/**
* 一个简单的Dao接口,我们要对这个接口的方法提供缓存的功能
* @author Tim
*
*/
public interface Dao {
    Object select();
    void save(Object obj);
}


package com.sin90lzc.java.test;

import java.util.HashMap;
import java.util.Map;
/**
* 实现缓存功能，在Spring中，该类可以看作是Advice
* @author Tim
*
*/
public class Cache {
    private static final Map<String, Object> cache = new HashMap<String, Object>();
   
    /**
     * 把对象添加到缓存中去
     * @param key
     * @param value
     */
    public void checkIn(String key, Object value) {
        if (!cache.containsKey(key)) {
            cache.put(key, value);
        }
    }

    /**
     * 从缓存中找对象
     * @param key
     * @return
     */
    public Object checkOut(String key) {
        if (cache.containsKey(key)) {
            return cache.get(key);
        }
        return null;
    }
   
    /**
     * 在方法执行前清除缓存
     */
    public void clearCacheBeforeMethod(){
        cache.clear();
    }
   
    /**
     * 在方法执行后清除缓存
     */
    public void clearCacheAfterMethod(){
        cache.clear();
    }
}


package com.sin90lzc.java.test;

/**
* Dao的代理对象，它不仅仅完成主要的查询，存储操作，还实现了缓存
* @author Tim
*
*/
public class DaoProxy implements Dao {
    private Cache cache;//该对象实现了缓存的功能
    private Dao daoImpl;//完成主要查询，存储操作的Dao实现类,注意，这是一个Dao的实现类

    public Object select() {
        //在执行方法前，尝试从缓存中取出结果并返回，如果没找到，则执行实际的操作。
        Object obj = cache.checkOut("DaoProxy.select");
        boolean hasCache = false;
        if (obj != null) {
            hasCache = true;
            return obj;
        }
       
        //实际的查询操作
        obj = daoImpl.select();
       
        //如果在缓存中找不到该方法的结果，缓存该结果
        if (!hasCache) {
            cache.checkIn("DaoProxy.select", obj);
        }
        return obj;
    }

    @Override
    public void save(Object obj) {
        //在执行方法前清除缓存
        cache.clearCacheBeforeMethod();
        //实际的操作
        daoImpl.save(obj);
        //在执行方法后清除缓存
        cache.clearCacheAfterMethod();
    }

    public void setCache(Cache cache) {
        this.cache = cache;
    }

    public void setDao(Dao dao) {
        this.daoImpl = dao;
    }

}

从代码中可以看到，真正完成缓存功能的类是Cache，真正完成Dao（数据的增删查改）功能的类是Dao的实现类，这就是实现了实际业务(Dao)与功能（缓存）的分离。实际的Dao操作与缓存功能是如何结合起来的呢？这就是通过代理对象。我们可以注意到ProxyDao的真正身份也是一个Dao，是它把Dao操作与缓存结合起来的。这三个类恰恰说明了AOP的本质。

2.EHCache

Spring仅仅是提供了对缓存的支持，但它并没有任何的缓存功能的实现，spring使用的是第三方的缓存框架来实现缓存的功能。其中，spring对EHCache提供了很好的支持。下面我们以EHCache为例来介绍spring的缓存配置。

在介绍Spring的缓存配置之前，我们先看一下EHCache是如何配置。

<?xml version="1.0" encoding="UTF-8" ?>
<ehcache>
    <!-- 定义默认的缓存区，如果在未指定缓存区时，默认使用该缓存区 -->
    <defaultCache maxElementsInMemory="500" eternal="true"
        overflowToDisk="false" memoryStoreEvictionPolicy="LFU">
    </defaultCache>
    <!-- 定义名字为"dao.select"的缓存区 -->
    <cache name="dao.select" maxElementsInMemory="500" eternal="true"
        overflowToDisk="false" memoryStoreEvictionPolicy="LFU" />
</ehcache>

3.Spring缓存机制中的Advice

由于Spring的缓存机制是基于Spring的AOP，那么在Spring Cache中应该存在着一个Advice。没错，在Spring Cache中的Advice是存在的，它就是org.springframework.cache.Cache。我们看一下它的接口定义：

/*
* Copyright 2002-2011 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.springframework.cache;

/**
* Interface that defines the common cache operations.
*
* <b>Note:</b> Due to the generic use of caching, it is recommended that
* implementations allow storage of <tt>null</tt> values (for example to
* cache methods that return {@code null}).
*
* @author Costin Leau
* @since 3.1
*/
public interface Cache {

    /**
     * Return the cache name.
     */
    String getName();

    /**
     * Return the the underlying native cache provider.
     */
    Object getNativeCache();

    /**
     * Return the value to which this cache maps the specified key. Returns
     * <code>null</code> if the cache contains no mapping for this key.
     * @param key key whose associated value is to be returned.
     * @return the value to which this cache maps the specified key,
     * or <code>null</code> if the cache contains no mapping for this key
     */
    ValueWrapper get(Object key);

    /**
     * Associate the specified value with the specified key in this cache.
     * <p>If the cache previously contained a mapping for this key, the old
     * value is replaced by the specified value.
     * @param key the key with which the specified value is to be associated
     * @param value the value to be associated with the specified key
     */
    void put(Object key, Object value);

    /**
     * Evict the mapping for this key from this cache if it is present.
     * @param key the key whose mapping is to be removed from the cache
     */
    void evict(Object key);

    /**
     * Remove all mappings from the cache.
     */
    void clear();


    /**
     * A (wrapper) object representing a cache value.
     */
    interface ValueWrapper {

        /**
         * Return the actual value in the cache.
         */
        Object get();
    }

}
evict,put方法就是Advice的功能方法，或者可以这样去理解。

但spring并不是直接使用org.springframework.cache.Cache，spring把Cache对象交给org.springframework.cache.CacheManager来管理，下面是org.springframework.cache.CacheManager接口的定义：

/*
* Copyright 2002-2011 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.springframework.cache;

import java.util.Collection;

/**
* A manager for a set of {@link Cache}s.
*
* @author Costin Leau
* @since 3.1
*/
public interface CacheManager {

    /**
     * Return the cache associated with the given name.
     * @param name cache identifier (must not be {@code null})
     * @return associated cache, or {@code null} if none is found
     */
    Cache getCache(String name);

    /**
     * Return a collection of the caches known by this cache manager.
     * @return names of caches known by the cache manager.
     */
    Collection<String> getCacheNames();

}
在spring对EHCache的支持中，org.springframework.cache.ehcache.EhCacheManager就是org.springframework.cache.CacheManager的一个实现
<!--
        该Bean是一个org.springframework.cache.CacheManager对象
        属性cacheManager是一个net.sf.ehcache.CacheManager对象
     -->
    <bean id="cacheManager" class="org.springframework.cache.ehcache.EhCacheCacheManager">
        <property name="cacheManager">
            <bean class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean">
                <property name="configLocation" value="classpath:ehcache-config.xml"></property>
            </bean>
        </property>
    </bean>

4.基于xml配置方式配置缓存

在上一节中，我们得到了cacheManagr，那么我们就可以对某些方法配置缓存了。下面是基于xml方式的缓存配置
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:cache="http://www.springframework.org/schema/cache"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:aop="http://www.springframework.org/schema/aop" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
     http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
     http://www.springframework.org/schema/context
     http://www.springframework.org/schema/context/spring-context-3.1.xsd
     http://www.springframework.org/schema/cache
     http://www.springframework.org/schema/cache/spring-cache-3.1.xsd
     http://www.springframework.org/schema/aop
     http://www.springframework.org/schema/aop/spring-aop-3.1.xsd
     ">

    <context:component-scan base-package="com.sin90lzc"></context:component-scan>
    <bean id="cacheManager" class="org.springframework.cache.ehcache.EhCacheCacheManager">
        <property name="cacheManager">
            <bean class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean">
                <property name="configLocation" value="classpath:ehcache-config.xml"></property>
            </bean>
        </property>
    </bean>

    <cache:advice id="cacheAdvice" cache-manager="cacheManager">
        <cache:caching>
            <cache:cacheable cache="dao.select" method="select"
                key="#id" />
            <cache:cache-evict cache="dao.select" method="save"
                key="#obj" />
        </cache:caching>
    </cache:advice>

    <aop:config>
        <aop:advisor advice-ref="cacheAdvice" pointcut="execution(* com.sin90lzc.train.spring_cache.simulation.DaoImpl.*(..))"/>
    </aop:config>
</beans>
5.注解驱动的缓存配置
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:cache="http://www.springframework.org/schema/cache"
    xmlns:context="http://www.springframework.org/schema/context"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
     http://www.springframework.org/schema/beans/spring-beans-3.1.xsd
     http://www.springframework.org/schema/context
     http://www.springframework.org/schema/context/spring-context-3.1.xsd
     http://www.springframework.org/schema/cache
     http://www.springframework.org/schema/cache/spring-cache-3.1.xsd">

    <context:component-scan base-package="com.sin90lzc"></context:component-scan>

    <!--
        该Bean是一个org.springframework.cache.CacheManager对象
        属性cacheManager是一个net.sf.ehcache.CacheManager对象
     -->
    <bean id="cacheManager" class="org.springframework.cache.ehcache.EhCacheCacheManager">
        <property name="cacheManager">
            <bean class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean">
                <property name="configLocation" value="classpath:ehcache-config.xml"></property>
            </bean>
        </property>
    </bean>
   
    <cache:annotation-driven />


</beans>
package com.sin90lzc.train.spring_cache.simulation;

import org.springframework.cache.annotation.CacheEvict;
import org.springframework.cache.annotation.Cacheable;
import org.springframework.stereotype.Component;


@Component
public class DaoImpl implements Dao {

    /**
     * value定义了缓存区（缓存区的名字），每个缓存区可以看作是一个Map对象
     * key作为该方法结果缓存的唯一标识，
     */
    @Cacheable(value = { "dao.select" },key="#id")
    @Override
    public Object select(int id) {
        System.out.println("do in function select()");
        return new Object();
    }

    @CacheEvict(value = { "dao.select" }, key="#obj")
    @Override
    public void save(Object obj) {
        System.out.println("do in function save(obj)");
    }

}