JPA是如何加载各种JPA实现的？（二）

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：

1. 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。