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JPA + SpringData 操作数据库原来可以这么简单 ---- 深入了解 JPA - 3

热度:84   发布时间:2024-01-04 02:35:18.0

前言:

   经过了前两篇的详细介绍,终于迎来了 JPA 的终结篇,楼主认为如果仅仅了解了 JPA 的话,大家可能感觉与 Hibernate 几乎差不多,没有什么亮点,但是等大家了解了 SpringData 后,JPA 与 SpringData 相结合,便会发挥出它巨大的优势,极大的简化了我们操作数据库的步骤,使我们的代码具有很强的可维护性,楼主随后的博客也将继续介绍。

六、JPA 的二级缓存

1. 大家对一级缓存比较熟悉,即若查询一条同样的记录,因为一级缓存的存在只发送一条 SQL 语句。那么 JPA 的二级缓存又体现在哪呢?楼主给大家解释为:查询一条同样的记录,在第一次查询后关闭 EntityManager、提交事务后,再重新获取 EntityManager 并开启事务再查询同样的记录,因为有二级缓存的存在也会只发送一条记录。如下:

//测试 JPA 的二级缓存@Testpublic 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);}

大家可以看到,4 行和 13 行的查询语句一样,6 行,7 行 提交了事务关闭了 EntityManager。若不进行二级缓存的配置,这样的操作会发送两次一模一样的 SQL 语句,结果就不贴上了,大家可以试一试。若配置了二级缓存,同样的操作便只会发送一条 SQL ,这样可以减小服务器的压力,减少访问数据库的次数。那么如何来配置二级缓存呢?

2. 如何配置二级缓存:

1)persistence.xml 文件中配置二级缓存相关

<?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,则可以不配置该节点 --><provider>org.hibernate.ejb.HibernatePersistence</provider><!-- 添加持久化类 --><class>com.software.jpa.helloworld.Customer</class><class>com.software.jpa.helloworld.Order</class><class>com.software.jpa.helloworld.Manager</class><class>com.software.jpa.helloworld.Department</class><class>com.software.jpa.helloworld.Category</class><class>com.software.jpa.helloworld.Item</class><!-- 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 --><!-- 在 Connection 选项中配置后会自动生成如下信息 --><property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" /><property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/jpa" /><property name="javax.persistence.jdbc.user" value="root" /><property name="javax.persistence.jdbc.password" value="qiqingqing" /><!-- 配置 JPA 实现产品的基本属性,即配置 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>

2)导入 ehcache 的 jar 包和配置文件 ehcache.xml 

jar 包:

配置文件:对二级缓存参数的配置

<ehcache><!-- Sets the path to the directory where cache .data files are created.If the path is a Java System Property it is replaced byits value in the running VM.The following properties are translated:user.home - User's home directoryuser.dir - User's current working directoryjava.io.tmpdir - Default temp file path --><diskStore path="java.io.tmpdir"/><!--Default Cache configuration. These will applied to caches programmatically created throughthe CacheManager.The following attributes are required for defaultCache:maxInMemory       - Sets the maximum number of objects that will be created in memoryeternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the elementis never expired.timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only usedif the element is not eternal. Idle time is now - last accessed timetimeToLiveSeconds - Sets the time to live for an element before it expires. Is only usedif the element is not eternal. TTL is now - creation timeoverflowToDisk    - Sets whether elements can overflow to disk when the in-memory cachehas reached the maxInMemory limit.--><defaultCachemaxElementsInMemory="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 theCacheManager startsThe 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 memoryeternal           - Sets whether elements are eternal. If eternal,  timeouts are ignored and the elementis never expired.timeToIdleSeconds - Sets the time to idle for an element before it expires. Is only usedif the element is not eternal. Idle time is now - last accessed timetimeToLiveSeconds - Sets the time to live for an element before it expires. Is only usedif the element is not eternal. TTL is now - creation timeoverflowToDisk    - Sets whether elements can overflow to disk when the in-memory cachehas reached the maxInMemory limit.--><!-- Sample cache named sampleCache1This cache contains a maximum in memory of 10000 elements, and will expirean element if it is idle for more than 5 minutes and lives for more than
minutes.If there are more than 10000 elements it will overflow to thedisk cache, which in this configuration will go to wherever java.io.tmp isdefined 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 sampleCache2This 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>

3)给需要缓存的类添加 @Cacheable(true) 注解,有前面的代码可知,楼主获取的是 Customer 对象

二级缓存就给大家介绍到这里。

七、JPQL

1.什么是 JPQL:JPQL语言,即 Java Persistence Query Language 的简称。

2.然后来看一个 JPQL 的 Helloworld:

//JPQL 的 HelloWorld@Testpublic void testHelloJPQL() {String jpql = "FROM Customer c WHERE c.age > ?";Query query = entityManager.createQuery(jpql);//占位符的索引是从 1 开始query.setParameter(1, 21);List<Customer> lists = query.getResultList();System.out.println(lists.size());}

乍一看,大家可能感觉 JPQL 像极了 Hibernate 的 HQL 查询,没错,这两种查询的相似度极高。需要注意的是,使用 Query 的 setParameter() 的方法填占位符是,索引是从 1

开始的。

3. 查询部分属性:

@Testpublic void testPartlyProperties() {String jpql = "SELECT new Customer(c.lastName, c.age) FROM Customer c WHERE c.id > ?";Query query = entityManager.createQuery(jpql);query.setParameter(1, 1);List lists = query.getResultList();System.out.println(lists);}

默认情况下若只查询部分属性,则将返回 Object[] 类型的结果或 Object[] 类型的 List,可以在实体类中创建对应的构造器,然后在 jpql 中利用对应的构造器返回实体类对应的对象,这样得到的结果可以很令人满意,也很方便我们来操作。

 4.命名查询 NamedQuery:

1)在需要查询的对象类上添加 @NamedQuery 注解:

2)创建测试方法:

5. 本地 SQL 查询使用 EntityManager 的 createNativeQuery() 方法:

//本地 SQL 查询@Testpublic void testNativeQuery() {String sql = "SELECT age FROM jpa_customer WHERE id = ?";Query query = entityManager.createNativeQuery(sql).setParameter(1, 1);Object result = query.getSingleResult();System.out.println(result);}

6. 可以使用 Order By 字句:

// jpql 中的 Order By 子句@Testpublic void testOrderBy() {String jpql = "FROM Customer c WHERE c.age > ? ORDER BY c.age DESC";Query query = entityManager.createQuery(jpql);//占位符的索引是从 1 开始query.setParameter(1, 21);List<Customer> lists = query.getResultList();System.out.println(lists.size());}

 7.还可以使用 Group By 子句:

//查询 order 数量大于 2 的那些 Customer@Testpublic void testGroupBy() {String jpql = "SELECT o.customer FROM Order o GROUP BY o.customer HAVING count(o.id) >= 2";List<Customer> lists = entityManager.createQuery(jpql).getResultList();System.out.println(lists);}

8.也可以使用子查询

//子查询@Testpublic void testSubQuery() {//查询所有 Customer 的 lastName 为 YY 的 OrderString jpql = "SELECT o FROM Order o"+ " WHERE o.customer = (SELECT c FROM Customer c WHERE c.lastName = ?)";List<Order> orders = entityManager.createQuery(jpql).setParameter(1, "YY").getResultList();System.out.println(orders.size());}

八、Spring 整合 JPA

JPA 的一些 API 也可以放到 Spring 的 IOC 容器中,交由 Spring 容器管理,那么如何用 Spring 来整合 JPA 呢?

1.新建 JPA 工程,导入所需的 jar包(Hibernate、JPA、c3p0、Spring、MySQL 驱动)

2.类路径下创建 db.properties 数据库配置文件,配置数据库的链接信息(楼主在这只配置了必须属性)

1 jdbc.user=root
2 jdbc.password=qiqingqing
3 jdbc.driverClass=com.mysql.jdbc.Driver
4 jdbc.jdbcUrl=jdbc:mysql://localhost:3306/jpa

3.类路径下创建 Spring 的配置文件 applicationContext.xml,配置自动扫描的包,将 db.propertiest 文件导入,并配置 c3p0 数据源

<!-- 配置自动扫描的包 --><context:component-scan base-package="com.software.jpa"></context:component-scan><!-- 配置数据源 --><context:property-placeholder location="classpath:db.properties"/><bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"><property name="user" value="${jdbc.user}"></property><property name="password" value="${jdbc.password}"></property><property name="driverClass" value="${jdbc.driverClass}"></property><property name="jdbcUrl" value="${jdbc.jdbcUrl}"></property></bean>

4.在 applicationContext.xml 中配置 JPA 的 EntityManagerFactory

<!-- 配置 EntityManagerFactory --><bean id="entityManagerFactory"class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"><!-- 配置数据源 --><property name="dataSource" ref="dataSource"></property><!-- 配置 JPA 提供商的适配器,可以通过内部 bean 的方式来配置 --><property name="jpaVendorAdapter"><bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter"></bean></property><!-- 配置实体类所在的包 --><property name="packagesToScan" value="com.software.jpa.spring.entities"></property><!-- 配置 JPA 的基本属性,比如,JPA 实现产品的属性 --><property name="jpaProperties"><props><prop key="hibernate.show_sql">true</prop><prop key="hibernate.format_sql">true</prop><prop key="hibernate.hbm2ddl.auto">update</prop></props></property></bean>

5.配置 JPA 使用的事务管理器及配置支持基于注解的事务配置

<!-- 配置  JPA 使用的事务管理器 --><bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager"><property name="entityManagerFactory" ref="entityManagerFactory"></property></bean><!-- 配置支持基于注解的事务配置 --><tx:annotation-driven transaction-manager="transactionManager"/>

6.为了测试创建实体类 Person,添加相应的 JPA 注解,生成对应的数据表

package com.software.jpa.spring.entities;import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;@Table(name="JPA_PERSONS")
@Entity
public class Person {private Integer id;private String lastName;private String email;private Integer age;@GeneratedValue@Idpublic Integer getId() {return id;}public void setId(Integer id) {this.id = id;}@Column(name="LAST_NAME")public String getLastName() {return lastName;}public void setLastName(String lastName) {this.lastName = lastName;}public String getEmail() {return email;}public void setEmail(String email) {this.email = email;}public Integer getAge() {return age;}public void setAge(Integer age) {this.age = age;}}

7.创建 PersonDao 使用 @PersistenceContext 获取和当前事务关联的 EntityManager 对象

package com.software.jpa.dao;import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;import org.springframework.stereotype.Repository;import com.software.jpa.spring.entities.Person;@Repository
public class PersonDao {//使用 @PersistenceContext 获取和当前事务关联的 EntityManager 对象@PersistenceContextprivate EntityManager entityManager;public void save(Person p) {entityManager.persist(p);}}

8.创建 PersonService ,模拟事务操作,20 行楼主设计了一个算数异常,若整合成功,因为添加了事务操作,所以 18 行和 22 行的两条记录都没有插入进数据库。

package com.software.jpa.service;import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;import com.software.jpa.dao.PersonDao;
import com.software.jpa.spring.entities.Person;@Service
public class PersonService {@Autowiredprivate PersonDao dao;@Transactionalpublic void save(Person p1, Person p2) {dao.save(p1);int i = 10/0;dao.save(p2);}}

9.创建测试方法,并执行

package com.software.jpa.spring;import javax.sql.DataSource;import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;import com.software.jpa.service.PersonService;
import com.software.jpa.spring.entities.Person;public class JPATest {private ApplicationContext ctx = null;private PersonService personService = null;{ctx = new ClassPathXmlApplicationContext("applicationContext.xml");personService = ctx.getBean(PersonService.class);}@Testpublic void testSave() {Person p1 = new Person();p1.setAge(11);p1.setEmail("aa@163.com");p1.setLastName("AA");Person p2 = new Person();p2.setAge(12);p2.setEmail("bb@163.com");p2.setLastName("BB");System.out.println(personService.getClass().getName());personService.save(p1, p2);}@Testpublic void testDataSourct() throws Exception {DataSource dataSource = ctx.getBean(DataSource.class);System.out.println(dataSource.getConnection());}}