无需交易的JPA [英] JPA without transaction

问题描述

我是JPA的新手.我正在开发一个使用JPA(Hibernate实现)和Spring的应用程序.我已经在我的persistence.xml中声明了一个持久性单元，并在我的Spring配置文件中声明了关于EntityManagerFactory的配置.像这样:

I´m new to JPA. I´m developing an application which uses JPA (Hibernate implementation) and Spring. I´ve declared a persistence unit in my persistence.xml and configuration about EntityManagerFactory in my Spring config files. Something like this:

<bean id="myEmf"   class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
  <property name="dataSource" ref="dataSource" />
  <property name="packagesToScan" value="my.package" />
  <property name="jpaVendorAdapter">
     <bean class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter" />
  </property>
  <property name="jpaProperties">
     <props>
        <prop key="hibernate.hbm2ddl.auto">create-drop</prop>
        <prop key="hibernate.dialect">org.hibernate.dialect.MySQL5Dialect</prop>
     </props>
  </property>

然后我有一些DAO，其中我在实体管理器中注入了@PersistenceContext批注:

Then I have some DAOs where I inject the entityManager with the @PersistenceContext annotation:

public MyDaoImpl implements MyDao{
   private EntityManager entityManager;

   @PersistenceContext
   private void setEntityManager(EntityManager em){
        this.entityManager = em;
    }
 } 

最后，我有一些服务在其中注入了DAO(通过@Autowired Spring的注释):

And finally, I have some services where DAOs are injected (by @Autowired Spring's annotation):

public MyServiceImpl implements MyService{
  @Autowired
  private MyDao myDao;

  public List<MyEntity> readOperation(){
   //
   return myDAo.searchAll();
 }
}

由于它是只读操作，所以我认为它不需要@Transactional批注，但是如果没有它，则会有一个例外:

As its a read only operation I thought it wasn´t needed the @Transactional annotation, but without it, there is an exception:

java.lang.IllegalStateException: No transactional EntityManager available
    at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(SharedEntityManagerCreator.java:223)
    at $Proxy121.unwrap(Unknown Source) 

我还阅读了其他类似的帖子: java .lang.IllegalStateException:没有可用的事务性EntityManager

I´ve read some other posts like this: java.lang.IllegalStateException: No transactional EntityManager available

所有所说的是事务注释是必需的.它确实可以使用它，但是我想知道(以及为什么)所有方法(甚至是只读操作)是否都必须是事务性的.

And all is said is that the transactional annotation is needed. It´s true that it works with it, but I´d like to know (and why) if all methods (even read only operations) must be transactional.

推荐答案

所有方法都需要进行JPA事务处理-本质上来说，事务处理是打开Hibernate会话的原因，而您需要一个开放的会话才能与其进行交互.

A JPA Transaction is needed for all your methods - essentially a transaction is what opens a Hibernate session, and you need an open session to interact with it.

您可以将事务注释为只读或可读写，也可以在类级别进行注释，以节省对每个方法的注释.例如:

You can annotate the transactions as readonly or readwrite, and you can also annotate at the class level to save you annotating each method. For example:

@Transactional(readOnly = true)
public MyDaoImpl implements MyDao{
    private EntityManager entityManager;

    @PersistenceContext
    private void setEntityManager(EntityManager em){
        this.entityManager = em;
    }

    @Transactional(readOnly = false)
    public void saveItem(MyEntity entity) {
    }

    public List<MyEntity> searchAll() {
    }
} 

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