SPRING IN ACTION 第4版笔记-第十一章Persisting data with object-relational mapping-003编写JPA-based repository( @PersistenceUnit、 @PersistenceContext、PersistenceAnnotationBeanPostProcessor)

一、注入EntityManagerFactory的方式

 package com.habuma.spittr.persistence;
 import java.util.List;
 import javax.persistence.EntityManagerFactory;
 import javax.persistence.PersistenceUnit;
 import org.springframework.dao.DataAccessException;
 import org.springframework.stereotype.Repository;
 import org.springframework.transaction.annotation.Transactional;
 import com.habuma.spittr.domain.Spitter;
 import com.habuma.spittr.domain.Spittle;
 @Repository
 @Transactional
 public class JpaSpitterRepository implements SpitterRepository {
     @PersistenceUnit
     private EntityManagerFactory emf;
     public void addSpitter(Spitter spitter) {
         emf.createEntityManager().persist(spitter);
     }
     public Spitter getSpitterById(long id) {
         return emf.createEntityManager().find(Spitter.class, id);
     }
     public void saveSpitter(Spitter spitter) {
         emf.createEntityManager().merge(spitter);
     }
         ...
 }

这种方式有一个问题，Aside from presenting a troubling code-duplication situation, it also means a new EntityManager is created every time one ofthe repository methods is called. This complicates matters concerning transactions.

二、注入EntityManager的方式(其实并注入代理)

若用注入EntityManager的方式，也有一个问题，The problem is that an EntityManager isn’t thread-safe and generally shouldn’t be injected into a shared singleton bean like your repository. But that doesn’t mean you can’t ask for an EntityManager anyway.

 package com.habuma.spittr.persistence;
 import java.util.List;
 import javax.persistence.EntityManager;
 import javax.persistence.PersistenceContext;
 import org.springframework.dao.DataAccessException;
 import org.springframework.stereotype.Repository;
 import org.springframework.transaction.annotation.Transactional;
 import com.habuma.spittr.domain.Spitter;
 import com.habuma.spittr.domain.Spittle;
 @Repository
 @Transactional
 public class JpaSpitterRepository implements SpitterRepository {
     @PersistenceContext
     private EntityManager em;
     public void addSpitter(Spitter spitter) {
         em.persist(spitter);
     }
     public Spitter getSpitterById(long id) {
         return em.find(Spitter.class, id);
     }
     public void saveSpitter(Spitter spitter) {
             em.merge(spitter);
         }
         ...
 }

关于线程的安全问题，The truth is that @PersistenceContext doesn’t inject an EntityManager —at least,not exactly. Instead of giving the repository a real EntityManager , it gives a proxy to a real EntityManager . That real EntityManager either is one associated with the current transaction or, if one doesn’t exist, creates a new one. Thus, you know that you’re

always working with an entity manager in a thread-safe way.

@PersistenceUnit and  @PersistenceContext是JPA的标准，要让Spring明白这两个标签的注入方式，则要配置PersistenceAnnotationBeanPostProcessor，但若是使用 <context:annotation-config> or  <context:component-scan> ，则默认是已经配置了的

 @Bean
 public PersistenceAnnotationBeanPostProcessor paPostProcessor() {
     return new PersistenceAnnotationBeanPostProcessor();
 }

@Transactional indicates that the persistence methods in this repository are involved in a transactional context.