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.