在上一节中，我们简单的讲述了jpa的查询语法和使用教程，而这一节咱们来看看Spring Boot中对jpa是如何进行自动配置的。
在Spring Boot自动配置的时候，一旦引入spring-boot-starter-data-jpa，就会完成JpaRepositoriesAutoConfiguration的自动配置。
JpaRepositoriesAutoConfiguration
@Configuration
@ConditionalOnBean(DataSource.class)
@ConditionalOnClass(JpaRepository.class)
@ConditionalOnMissingBean({ JpaRepositoryFactoryBean.class,
JpaRepositoryConfigExtension.class })
@ConditionalOnProperty(prefix = "spring.data.jpa.repositories", name = "enabled", havingValue = "true", matchIfMissing = true)
@Import(JpaRepositoriesAutoConfigureRegistrar.class)
@AutoConfigureAfter(HibernateJpaAutoConfiguration.class)
public class JpaRepositoriesAutoConfiguration {}spring-data-jpa底层使用的是Hibernate作为实现，所以jpa的自动配置操作在Hibernate的自动配置之后。
@AutoConfigureAfter：表示在指定类完成后再进行自动配置，所以来看HibernateJpaAutoConfiguration源码。
咱们继续：
HibernateJpaAutoConfiguration
@Configuration
@ConditionalOnClass({ LocalContainerEntityManagerFactoryBean.class, EntityManager.class })
@Conditional(HibernateEntityManagerCondition.class)
@AutoConfigureAfter({ DataSourceAutoConfiguration.class })
public class HibernateJpaAutoConfiguration extends JpaBaseConfiguration {
//other...
public HibernateJpaAutoConfiguration(DataSource dataSource,
JpaProperties jpaProperties,
ObjectProvider<JtaTransactionManager> jtaTransactionManager,
ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
//调用父类的构造方法
super(dataSource, jpaProperties, jtaTransactionManager,
transactionManagerCustomizers);
}
@Override
protected AbstractJpaVendorAdapter createJpaVendorAdapter() {
return new HibernateJpaVendorAdapter();
}
@Override
protected Map<String, Object> getVendorProperties() {
Map<String, Object> vendorProperties = new LinkedHashMap<String, Object>();
vendorProperties.putAll(getProperties().getHibernateProperties(getDataSource()));
return vendorProperties;
}
//other...
}
同样在HibernateJpaAutoConfiguration的源码中表示该自动配置需要在DataSourceAutoConfiguration完成后再进行，之前在分析JdbcTemplateAutoConfiguration的源码的时候已经分析过DataSourceAutoConfiguration，此处就不再讲述，一笔带过。
回想之前在使用Spring和JPA集成的时候，会配置一个jpaVendorAdapter属性，一般使用HibernateJpaVendorAdapter作为JPA持久化实现厂商类。如下是spring和jpa集成时的部分配置：
<property name="jpaVendorAdapter">
<bean id="hibernateJpaVendorAdapter" class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
<!-- 该bean的属性配置 -->
</bean>
</property>在JpaBaseConfiguration类中有个createJpaVendorAdapter()抽象方法，而在HibernateJpaAutoConfiguration类中进行了重载，创建使用HibernateJpaVendorAdapter作为JPA底层持久化实现厂商。
咱们来看父类的源码：
父类JpaBaseConfiguration
@EnableConfigurationProperties(JpaProperties.class)
@Import(DataSourceInitializedPublisher.Registrar.class)
public abstract class JpaBaseConfiguration implements BeanFactoryAware {
private final DataSource dataSource;
private final JpaProperties properties;
private final JtaTransactionManager jtaTransactionManager;
private final TransactionManagerCustomizers transactionManagerCustomizers;
private ConfigurableListableBeanFactory beanFactory;
protected JpaBaseConfiguration(DataSource dataSource, JpaProperties properties,
ObjectProvider<JtaTransactionManager> jtaTransactionManager,
ObjectProvider<TransactionManagerCustomizers> transactionManagerCustomizers) {
this.dataSource = dataSource;
this.properties = properties;
this.jtaTransactionManager = jtaTransactionManager.getIfAvailable();
this.transactionManagerCustomizers = transactionManagerCustomizers
.getIfAvailable();
}
//创建了TransactionManager的Bean
@Bean
@ConditionalOnMissingBean(PlatformTransactionManager.class)
public PlatformTransactionManager transactionManager() {
JpaTransactionManager transactionManager = new JpaTransactionManager();
if (this.transactionManagerCustomizers != null) {
this.transactionManagerCustomizers.customize(transactionManager);
}
return transactionManager;
}
//创建了jpaVendorAdapter适配器，并填充相应属性，最后返回
@Bean
@ConditionalOnMissingBean
public JpaVendorAdapter jpaVendorAdapter() {
//通过HibernateJpaAutoConfiguration子类创建HibernateJpaVendorAdapter适配器
AbstractJpaVendorAdapter adapter = createJpaVendorAdapter();
adapter.setShowSql(this.properties.isShowSql());
adapter.setDatabase(this.properties.determineDatabase(this.dataSource));
adapter.setDatabasePlatform(this.properties.getDatabasePlatform());
adapter.setGenerateDdl(this.properties.isGenerateDdl());
return adapter;
}
//通过jpaVendorAdapter与其他配置信息创建Builder构建器
@Bean
@ConditionalOnMissingBean
public EntityManagerFactoryBuilder entityManagerFactoryBuilder(
JpaVendorAdapter jpaVendorAdapter,
ObjectProvider<PersistenceUnitManager> persistenceUnitManager) {
EntityManagerFactoryBuilder builder = new EntityManagerFactoryBuilder(
jpaVendorAdapter, this.properties.getProperties(),
persistenceUnitManager.getIfAvailable());
builder.setCallback(getVendorCallback());
return builder;
}
//创建LocalContainerEntityManagerFactoryBean的Bean，用于JPA的容器管理EntityManagerFactory
@Bean
@Primary
@ConditionalOnMissingBean({ LocalContainerEntityManagerFactoryBean.class,
EntityManagerFactory.class })
public LocalContainerEntityManagerFactoryBean entityManagerFactory(
EntityManagerFactoryBuilder factoryBuilder) {
Map<String, Object> vendorProperties = getVendorProperties();
customizeVendorProperties(vendorProperties);
return factoryBuilder.dataSource(this.dataSource).packages(getPackagesToScan())
.properties(vendorProperties).jta(isJta()).build();
}
//other...
}在父类JpaBaseConfiguration中创建了几个重要的Bean，这样创建Bean的过程类似之前spring-jpa集成时使用的xml配置文件：
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
<property name="driverClass" value="${jdbc.driverClass}"></property>
<property name="jdbcUrl" value="${jdbc.jdbcUrl}"></property>
<property name="user" value="${jdbc.user}"></property>
<property name="password" value="${jdbc.password}"></property>
<property name="initialPoolSize" value="${jdbc.initialPoolSize}"></property>
<property name="maxPoolSize" value="${jdbc.maxPoolSize}"></property>
</bean>
<!-- 配置jpa的EntityManagerFactory -->
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
<property name="dataSource" ref="dataSource"></property>
<!-- 配置jpa生产商的适配器 -->
<property name="jpaVendorAdapter">
<bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter"></bean>
</property>
<!-- 配置jpa的基本属性 -->
<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>
<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
<property name="entityManagerFactory" ref="entityManagerFactory"></property>
</bean>
<!-- 配置SpringData -->
<jpa:repositories base-package="xxxxx包名" entity-manager-factory-ref="entityManagerFactory" transaction-manager-ref="transactionManager"></jpa:repositories>通过这几步java配置，基本上就完成了之前spring-jpa集成所需的所有配置信息，当然Spring Boot内部做了很多工作，这里就不再描述了。
本章节讲述原理，比较抽象，如有讲述不清晰或者不理解的，还请各位担待，有问题可以加我，咱们一起讨论，谢谢啦！
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