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)"); } }
