Spring Data之EntityManagerFactory创建及源码分析

背景

在Spring Data之JPA开篇中可以看到Spring Boot的启动日志，先是创建了HikariDataSource，然后紧接着构建了EntityManagerFactory

2018-10-25 09:32:20.645  INFO 37469 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Starting...
2018-10-25 09:32:20.791  INFO 37469 --- [           main] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Start completed.
2018-10-25 09:32:20.846  INFO 37469 --- [           main] j.LocalContainerEntityManagerFactoryBean : Building JPA container EntityManagerFactory for persistence unit 'default'

其中HikariDataSource的创建在Spring Data之DataSource创建及源码分析中进行了介绍，这篇文章我们就来探寻一下EntityManagerFactory是如何自动创建出来的。

JPA规范

首先有必要了解一下JPA规范（JSR 338）以便更好的理解EntityManagerFactory的创建过程。

JSR 338主要定义了如何通过普通的Java domain进行关系数据库的操作及映射，其中主要概念包含如下：

• Entity
  • 必须是顶级类
  • @Entity注解的类
  • 必须有一个无参的public 或 protected的构造方法
  • 不能是final类，且不能有final方法或final变量
  • 一个Entity类通常与数据库的一张表进行对应
  • 一个Entity实例表现为数据库的一条数据
  • 对Entity的操作即对数据库的操作
  • 生命周期包含初始、托管、释放、消亡
• EntityManager
  • 对Entity持久化操作的主要对象
  • 通过EntityManagerFactory获取实例
  • 一个实例代表一个数据库连接
  • 每个线程拥有自己的EntityManager实例
  • 主要方法有persist、remove、merge、createQuery、find
  • 可使用@PersistenceContext注入
• EntityManagerFactory
  • 创建EntityManager的工厂
  • EntityManagerFactory的创建成本很高，对于给定的数据库，系统应该只创建一个与之关联的Factory
  • 可使用@PersistenceUnit注入
• EntityTransaction
  • 表示数据库事务，在增、删、改时使用
  • 可通过EntityManager.getTransaction()获取
• Persistence Context
  • 维护一组托管状态的Entity实例
  • 与EntityManager是相关联的
• Persistence Unit
  • 一组Entity及相关设置的逻辑单元
  • 定义创建EntityManagerFactory所需要的参数
  • 通过persistence.xml定义或者通过一个PersistenceUnitInfo对象

总结一下，通过Persistence Unit创建EntityManagerFactory，再通过EntityManagerFactory获取EntityManager。下面我们就按照这个方向进行源码分析。

自动配置

同DataSource一样，EntityManagerFactory的创建也是通过一些列@Conditional的注解最终找到Hibernate的实现。我们先看下自动配置的入口类HibernateJpaAutoConfiguration

@Configuration
//先校验是否存在LocalContainerEntityManagerFactoryBean
//该类是spring-orm包中的类，从名称上看就知道是跟EntityManagerFactory的创建有关的
//要使用JPA，EntityManager自然也是要存在的
@ConditionalOnClass({ LocalContainerEntityManagerFactoryBean.class, EntityManager.class })
//自定义的校验类，校验是否存在CLASS_NAMES中的类
@Conditional(HibernateEntityManagerCondition.class)
//在application.properties中配置的spring.jpa开头的设置，会注入到JpaProperties中
//换句话说在spring.jpa能设置什么，要看JpaProperties有什么属性
@EnableConfigurationProperties(JpaProperties.class)
//这个设置很关键，意思是要在DataSource之后再创建，因为JPA会用到DataSource
@AutoConfigureAfter({ DataSourceAutoConfiguration.class })
//上面的校验都满足后，则进入HibernateJpaConfiguration
@Import(HibernateJpaConfiguration.class)
public class HibernateJpaAutoConfiguration {

	@Order(Ordered.HIGHEST_PRECEDENCE + 20)
	static class HibernateEntityManagerCondition extends SpringBootCondition {

		private static final String[] CLASS_NAMES = {
				"org.hibernate.ejb.HibernateEntityManager",
				"org.hibernate.jpa.HibernateEntityManager" };

		/**
		 * 通过ClassUtils.isPresent检查是否存在CLASS_NAMES中指定的类
		 */
		@Override
		public ConditionOutcome getMatchOutcome(ConditionContext context,
				AnnotatedTypeMetadata metadata) {
			ConditionMessage.Builder message = ConditionMessage
					.forCondition("HibernateEntityManager");
			for (String className : CLASS_NAMES) {
				if (ClassUtils.isPresent(className, context.getClassLoader())) {
					return ConditionOutcome
							.match(message.found("class").items(Style.QUOTE, className));
				}
			}
			return ConditionOutcome.noMatch(message.didNotFind("class", "classes")
					.items(Style.QUOTE, Arrays.asList(CLASS_NAMES)));
		}
	}
}

主要逻辑已在代码中添加注释说明，接下来就看下@Import的HibernateJpaConfiguration

@Configuration
@ConditionalOnSingleCandidate(DataSource.class)
class HibernateJpaConfiguration extends JpaBaseConfiguration {
	//省略。。。
	
	@Override
	protected AbstractJpaVendorAdapter createJpaVendorAdapter() {
		return new HibernateJpaVendorAdapter();
	}
	
	//省略。。。
}

省略了一些不重要的代码，只留下createJpaVendorAdapter方法。再接着看父类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();
	}

	@Bean
	@ConditionalOnMissingBean
	public PlatformTransactionManager transactionManager() {
		JpaTransactionManager transactionManager = new JpaTransactionManager();
		if (this.transactionManagerCustomizers != null) {
			this.transactionManagerCustomizers.customize(transactionManager);
		}
		return transactionManager;
	}

	@Bean
	@ConditionalOnMissingBean
	public JpaVendorAdapter jpaVendorAdapter() {
		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;
	}

	@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;
	}

	@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).mappingResources(getMappingResources())
				.jta(isJta()).build();
	}

	protected abstract AbstractJpaVendorAdapter createJpaVendorAdapter();

}

可以看到该类有4个@Bean注解的方法，咱们按顺序分析

1. transactionManager
   创建JPA事务的管理器

2. jpaVendorAdapter
   JpaVendorAdapter是一个接口，里面有一个方法是getPersistenceProvider，而PersistenceProvider是JPA规范中定义的创建EntityManagerFactory的接口方法。
   createJpaVendorAdapter的实现是在HibernateJpaConfiguration中返回的是 HibernateJpaVendorAdapter.

   public class HibernateJpaVendorAdapter extends AbstractJpaVendorAdapter{
      private final HibernateJpaDialect jpaDialect = new HibernateJpaDialect();
   
      private final PersistenceProvider persistenceProvider;
   
      private final Class<? extends EntityManagerFactory> entityManagerFactoryInterface;
   
      private final Class<? extends EntityManager> entityManagerInterface;
   
     /**
     	* 明确指明了persistenceProvider的实现是SpringHibernateJpaPersistenceProvider
     	*/
      @SuppressWarnings("deprecation")
      public HibernateJpaVendorAdapter() {
          this.persistenceProvider = new SpringHibernateJpaPersistenceProvider();
          this.entityManagerFactoryInterface = org.hibernate.jpa.HibernateEntityManagerFactory.class;
          this.entityManagerInterface = org.hibernate.jpa.HibernateEntityManager.class;
      }
   }
   
   

   /**
    * 继承了Hibernate的PersistenceProvider
    */
   class SpringHibernateJpaPersistenceProvider extends HibernatePersistenceProvider{
   
      /**
       * 重要方法，调用了Hibernate的EntityManagerFactoryBuilderImpl，通过build创建出了EntityManagerFactory
       */
      public EntityManagerFactory createContainerEntityManagerFactory(PersistenceUnitInfo info, Map properties) {
      	final List<String> mergedClassesAndPackages = new ArrayList<>(info.getManagedClassNames());
      	if (info instanceof SmartPersistenceUnitInfo) {
      		mergedClassesAndPackages.addAll(((SmartPersistenceUnitInfo) info).getManagedPackages());
      	}
      	return new EntityManagerFactoryBuilderImpl(
      			new PersistenceUnitInfoDescriptor(info) {
      				@Override
      				public List<String> getManagedClassNames() {
      					return mergedClassesAndPackages;
      				}
      			}, properties).build();
      }
   }
   

3. entityManagerFactoryBuilder
   建造者模式，将需要的原料都准备好

4. entityManagerFactory
   LocalContainerEntityManagerFactoryBean就是启动日志中打印出*Building JPA container EntityManagerFactory for persistence unit ‘default’*的类。
   该类及其父类AbstractEntityManagerFactoryBean包含了创建EntityManagerFactory的所有元素，前面的步骤都是在为LocalContainerEntityManagerFactoryBean的属性准备数据赋值

   public class LocalContainerEntityManagerFactoryBean extends AbstractEntityManagerFactoryBean
      implements ResourceLoaderAware, LoadTimeWeaverAware {
       /**
      	*根据provider（前面提到的SpringHibernateJpaPersistenceProvider）创建EntityManagerFactory
      	*/
       protected EntityManagerFactory createNativeEntityManagerFactory() throws PersistenceException {
      	Assert.state(this.persistenceUnitInfo != null, "PersistenceUnitInfo not initialized");
   
      	PersistenceProvider provider = getPersistenceProvider();
      	if (provider == null) {
      		String providerClassName = this.persistenceUnitInfo.getPersistenceProviderClassName();
      		if (providerClassName == null) {
      			throw new IllegalArgumentException(
      					"No PersistenceProvider specified in EntityManagerFactory configuration, " +
      					"and chosen PersistenceUnitInfo does not specify a provider class name either");
      		}
      		Class<?> providerClass = ClassUtils.resolveClassName(providerClassName, getBeanClassLoader());
      		provider = (PersistenceProvider) BeanUtils.instantiateClass(providerClass);
      	}
   
      	if (logger.isInfoEnabled()) {
      		logger.info("Building JPA container EntityManagerFactory for persistence unit '" +
      				this.persistenceUnitInfo.getPersistenceUnitName() + "'");
      	}
      	EntityManagerFactory emf =
      			provider.createContainerEntityManagerFactory(this.persistenceUnitInfo, getJpaPropertyMap());
      	postProcessEntityManagerFactory(emf, this.persistenceUnitInfo);
   
      	return emf;
      }
   }
   

需要注意的是AbstractEntityManagerFactoryBean有两个EntityManagerFactory类型的属性，一个是通过PersistenceProvider获取的原生对象，另一个是Spring创建的代理对象ManagedEntityManagerFactoryInvocationHandler，它的目的是为了拦截EntityManager的创建，至少为什么要代理，下篇文档会进行分析。

结束语

到这里EntityManagerFactory就创建出来了，总的来说整个过程是遵循JPA规范定义的接口的，例如SpringHibernateJpaPersistenceProvider实现的createContainerEntityManagerFactory方法。只不过Spring作为骨干框架，使用了很多中间对象和过程。
最后用一张类图，再串一下创建的过程

1. 首先以左下角的HibernateJpaAutoConfiguration为入口，经过@Conditional的判断，@Import进入HibernateJpaConfiguration

2. 在HibernateJpaConfiguration中依赖类HibernateJpaVendorAdapter，而HibernateJpaVendorAdapter又依赖了SpringHibernateJpaPersistenceProvider，PersistenceProvider的具体实现就确定了

3. 在JpaBaseConfiguration中通过EntityManagerFactoryBuilder创建出了LocalContainerEntityManagerFactoryBean，该类是整个结构的纽带。可以看到它关联了PersistenceProvider和PersistenceUnitManager，而PersistenceUnitManager是产生PersistenceUnitInfo的，PersistenceProvider和PersistenceUnitInfo是createEntityFactory的两个必要对象

4. 最后在LocalContainerEntityManagerFactoryBean中调用SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory创建出了EntityManagerFactory

那么EntityManagerFactory创建出来后有什么用呢，当然是用来获取EntityManager，下篇文章我们继续分析EntityManager的创建过程。