【1】一级缓存
JPA默认情况下和MyBatis一样开启一级缓存。JPA是针对与entityManager，Mybatis是针对于namespace。
示例代码如下：
@Test public void testSecondLevelCache(){ Customer customer1 = entityManager.find(Customer.class, 1); Customer customer2 = entityManager.find(Customer.class, 1); }
控制台输出如下(将只打印一条SQL语句)：
【2】二级缓存
JPA的二级缓存是跨entityManager的，JPA的二级缓存需要显式配置。
persistence.xml配置如下：
<!--ENABLE_SELECTIVE表示只在使用了缓存注解(@Cacheable)的上面使用二级缓存--> <shared-cache-mode>ENABLE_SELECTIVE</shared-cache-mode> <properties> <!-- 配置 JPA 实现产品的基本属性. 配置 hibernate 的基本属性 --> <property name="hibernate.format_sql" value="true"/> <property name="hibernate.show_sql" value="true"/> <property name="hibernate.hbm2ddl.auto" value="update"/> <!-- 二级缓存相关 --> <property name="hibernate.cache.use_second_level_cache" value="true"/> <property name="hibernate.cache.region.factory_class" value="org.hibernate.cache.ehcache.EhCacheRegionFactory"/> <property name="hibernate.cache.use_query_cache" value="true"/> </properties>
这里使用的缓存产品为ehcache，将ehcache.xml放在src资源路径下。
ehcache.xml示例如下：
<ehcache> <!-- Sets the path to the directory where cache .data files are created. If the path is a Java System Property it is replaced by its value in the running VM. The following properties are translated: user.home - User's home directory user.dir - User's current working directory java.io.tmpdir - Default temp file path --> <diskStore path="java.io.tmpdir"/> <!--Default Cache configuration. These will applied to caches programmatically created through the CacheManager. The following attributes are required for defaultCache: maxInMemory - Sets the maximum number of objects that will be created in memory eternal - Sets whether elements are eternal. If eternal, timeouts are ignored and the element is never expired. timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used if the element is not eternal. Idle time is now - last accessed time timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used if the element is not eternal. TTL is now - creation time overflowToDisk - Sets whether elements can overflow to disk when the in-memory cache has reached the maxInMemory limit. --> <defaultCache maxElementsInMemory="10000" eternal="false" timeToIdleSeconds="120" timeToLiveSeconds="120" overflowToDisk="true" /> <!--Predefined caches. Add your cache configuration settings here. If you do not have a configuration for your cache a WARNING will be issued when the CacheManager starts The following attributes are required for defaultCache: name - Sets the name of the cache. This is used to identify the cache. It must be unique. maxInMemory - Sets the maximum number of objects that will be created in memory eternal - Sets whether elements are eternal. If eternal, timeouts are ignored and the element is never expired. timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only used if the element is not eternal. Idle time is now - last accessed time timeToLiveSeconds - Sets the time to live for an element before it expires. Is only used if the element is not eternal. TTL is now - creation time overflowToDisk - Sets whether elements can overflow to disk when the in-memory cache has reached the maxInMemory limit. --> <!-- Sample cache named sampleCache1 This cache contains a maximum in memory of 10000 elements, and will expire an element if it is idle for more than 5 minutes and lives for more than 10 minutes. If there are more than 10000 elements it will overflow to the disk cache, which in this configuration will go to wherever java.io.tmp is defined on your system. On a standard Linux system this will be /tmp" --> <cache name="sampleCache1" maxElementsInMemory="10000" eternal="false" timeToIdleSeconds="300" timeToLiveSeconds="600" overflowToDisk="true" /> <!-- Sample cache named sampleCache2 This cache contains 1000 elements. Elements will always be held in memory. They are not expired. --> <cache name="sampleCache2" maxElementsInMemory="1000" eternal="true" timeToIdleSeconds="0" timeToLiveSeconds="0" overflowToDisk="false" /> --> <!-- Place configuration for your caches following --> </ehcache>
Customer类修改如下：
@Cacheable(true)//这里表示启用二级缓存 @Table(name="JPA_CUTOMERS") @Entity public class Customer { //... }
测试代码如下：
@Test public void testSecondLevelCache(){ Customer customer1 = entityManager.find(Customer.class, 1); transaction.commit(); entityManager.close(); entityManager = entityManagerFactory.createEntityManager(); transaction = entityManager.getTransaction(); transaction.begin(); Customer customer2 = entityManager.find(Customer.class, 1); }
控制台输出如下(将只会有一条语句)：
综上可知，JPA的一级(二级)缓存和Mybatis的一级(二级)缓存相当类似。
只不过这里JPA的二级缓存以entityManager来区分，Mybatis的二级缓存以namespace来区分。
【3】配置二级缓存的策略
<shared-cache-mode>节点：若 JPA 实现支持二级缓存，该节点可配置在当前的持久化单元中是否启用二级缓存，可配置如下属性值：
ALL：所有的实体类都被缓存； NONE：所有的实体类都不被缓存； ENABLE_SELECTIVE：标识 @Cacheable(true) 注解的实体类将被缓存； DISABLE_SELECTIVE：缓存除标识 @Cacheable(false) 以外的所有实体类； UNSPECIFIED：默认值，JPA 产品默认值将被使用。
示例如下：
<?xml version="1.0" encoding="UTF-8"?> <persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"> <persistence-unit name="jpa-1" transaction-type="RESOURCE_LOCAL"> <!-- 配置使用什么 ORM 产品来作为 JPA 的实现 1. 实际上配置的是 javax.persistence.spi.PersistenceProvider 接口的实现类 2. 若 JPA 项目中只有一个 JPA 的实现产品, 则也可以不配置该节点. --> <provider>org.hibernate.ejb.HibernatePersistence</provider> <!-- 配置二级缓存的策略 ALL：所有的实体类都被缓存 NONE：所有的实体类都不被缓存. ENABLE_SELECTIVE：标识 @Cacheable(true) 注解的实体类将被缓存 DISABLE_SELECTIVE：缓存除标识 @Cacheable(false) 以外的所有实体类 UNSPECIFIED：默认值，JPA 产品默认值将被使用 --> <shared-cache-mode>ENABLE_SELECTIVE</shared-cache-mode> <properties> <!-- 连接数据库的基本信息 --> <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/> <property name="javax.persistence.jdbc.url" value="jdbc:mysql:///jpa"/> <property name="javax.persistence.jdbc.user" value="root"/> <property name="javax.persistence.jdbc.password" value="123456"/> <!-- 配置 JPA 实现产品的基本属性. 配置 hibernate 的基本属性 --> <property name="hibernate.format_sql" value="true"/> <property name="hibernate.show_sql" value="true"/> <property name="hibernate.hbm2ddl.auto" value="update"/> <!-- 二级缓存相关 --> <property name="hibernate.cache.use_second_level_cache" value="true"/> <property name="hibernate.cache.region.factory_class" value="org.hibernate.cache.ehcache.EhCacheRegionFactory"/> <property name="hibernate.cache.use_query_cache" value="true"/> </properties> </persistence-unit> </persistence>
