JDBC 是通过Class.forName("")来实现加载具体数据库实现的,而JPA是如何实现加载具体JPA实现的呢?Persistence.createEntityManagerFactory("mysqljpa")打开该方法的实现:
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public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName, Map properties) { EntityManagerFactory emf = null; List<PersistenceProvider> providers = getProviders(); for ( PersistenceProvider provider : providers ) { emf = provider.createEntityManagerFactory( persistenceUnitName, properties ); if ( emf != null ) { break; } } if ( emf == null ) { throw new PersistenceException( "No Persistence provider for EntityManager named " + persistenceUnitName ); } return emf; }
它的内部调用了getProviders()方法,该方法用于获取JPA的具体实现:
private static List<PersistenceProvider> getProviders() { return PersistenceProviderResolverHolder .getPersistenceProviderResolver() .getPersistenceProviders(); }
该方法又调用loadResolverClasses();方法,
该方法内部调用ClassLoader.getResources( "META-INF/services/" + PersistenceProvider.class.getName() );及通过类路径下META-INF/services/查找具体实现,查看hibernate-entitymanager-4.3.9.Final.jar存在该javax,persistence.spi.PersistenceProvider文件,打开该文件内容为:
org.hibernate.jpa.HibernatePersistenceProvider # The deprecated provider, logs warnings when used. org.hibernate.ejb.HibernatePersistence
该文件中定义了JPA的具体实现。