spring 的@PersistenceUnit和@PersistenceContext

前提：spring注入了PersistenceAnnotationBeanPostProcessor，或者自定义一个继承PersistenceAnnotationBeanPostProcessor的子类，然后注入到spring中，但只能存在一个。

1.当field上使用了@PersistenceUnit和@PersistenceContext，使用PersistenceAnnotationBeanPostProcessor进行解析。

2.在PersistenceAnnotationBeanPostProcessor有内部类PersistenceElement。spring会构造一个PersistenceElement实例。

public PersistenceElement(Member member, PropertyDescriptor pd) {
			super(member, pd);
			AnnotatedElement ae = (AnnotatedElement) member;
			PersistenceContext pc = ae.getAnnotation(PersistenceContext.class);
			PersistenceUnit pu = ae.getAnnotation(PersistenceUnit.class);
			Class<?> resourceType = EntityManager.class;
			if (pc != null) {
				if (pu != null) {
					throw new IllegalStateException("Member may only be annotated with either " +
							"@PersistenceContext or @PersistenceUnit, not both: " + member);
				}
				Properties properties = null;
				PersistenceProperty[] pps = pc.properties();
				if (!ObjectUtils.isEmpty(pps)) {
					properties = new Properties();
					for (PersistenceProperty pp : pps) {
						properties.setProperty(pp.name(), pp.value());
					}
				}
				this.unitName = pc.unitName();
				this.type = pc.type();
				this.properties = properties;
			}
			else {
				resourceType = EntityManagerFactory.class;
				this.unitName = pu.unitName();
			}
			checkResourceType(resourceType);
		}

 3.PersistenceElement实例生成后。调用PersistenceElement的getResourceToInject方法，返回相应的对象。

protected Object getResourceToInject(Object target, String requestingBeanName) {
			// Resolves to EntityManagerFactory or EntityManager.
			if (this.type != null) {
				return (this.type == PersistenceContextType.EXTENDED ?
						resolveExtendedEntityManager(target, requestingBeanName) :
						resolveEntityManager(requestingBeanName));
			}
			else {
				// OK, so we need an EntityManagerFactory...
				return resolveEntityManagerFactory(requestingBeanName);
			}
		}

如果this.type为空返回EntityManagerFactory，否则，根据type类型返回不同的EntityManager，如果你使用的@PersistenceContext注解，则type必有值，默认是TRANSACTION。

4.至于如何生成这个EntityManagerFactory或者EntityManager，请参阅PersistenceAnnotationBeanPostProcessor中相应方法的源码，例如resolveExtendedEntityManager。