Spring mvc 与 hibernate 和 jackson-hibernate-mapper 的集成测试：非法尝试将集合与两个会话相关联

Question

“非法尝试将集合与两个会话相关联”的异常是众所周知的，当在两个不同的会话中引用同一实体时会发生这种情况。我有一个 spring mvc 集成测试，它发生在哪里，需要一个建议如何使它正常工作。

另外值得注意的是，我使用的是https://github.com/FasterXML/jackson-datatype-hibernate，这有助于在将域对象转换为 json 时避免 LazyInitializationException。我的猜测是它在内部打开 Hibernate 会话以获取惰性关系。

这是一个测试：

@RunWith(SpringJunit4ClassRunner.class)
@WebAppConfiguration
@ContextHierarchy({
    @ContextConfiguration(name = "root", locations = "classpath:application.xml"),
    @ContextConfiguration(name = "servlet", locations = "classpath:servlet.xml")
})
@TransactionalConfiguration
public class IntegrationTest {
    private MockMvc mockMvc;

    //some initialization and autowiring

    @Test
    @Transactional // It is important to rollback all the db changes after the test. So, it's a common pattern to make all tests transactional.
    // So here we have HibernateSession#1
    public void testController() {
        // repository is spring-data-jpa repository
        // but you may think of it as a DAO which returns persisted 
        // hibernate entity
        ChildEntity child = new ChildEntity();
        child = childRepository.save();

        MyEntity entity = new MyEntity();
        entity.setChildEntities(Collections.singletoneList(child));
        entity = repository.save(entity);
        // entity and its child are still preserved in the HibernateSession#1

        mockMvc.perform(get("/entity/by_child_id/" + child.getId()).andExpect(status().is("200"));
    }

}

@RestController
@RequestMapping("/entity")
public class MyController {

    @RequestMapping("by_child_id/{id}")
    // My guess: Jackson hibernate mapper will open another session
    // and try to fetch children  
    // So here we have HibernateSession#2
    public MyEntity getByChildId(@PathVariable("id") childId) {
         return repository.findByChildrenId(childId);
    }

}

@Entity 
public class MyEntity {
    @OneToMany(fetch = FetchType.EAGER)
    private List<ChildEntity> children;
}

//and finally here is a piece of servlet.xml
<mvc:annotation-driven>
    <mvc:message-converters>
        <!-- Use the HibernateAware mapper instead of the default -->
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="objectMapper">
                <bean class="path.to.your.HibernateAwareObjectMapper" />
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven> 

我的例外是：

Caused by org.hibernate.HibernateException: Illegal attempt to associate a collection with two open sessions: [MyEntity.children#1]
    at com.fasterxml.jackson.datatype.hibernate4.PersistentCollectionSerializer.inistializeCollection(PersistentCollectionSerializer.java:195)
    at com.fasterxml.jackson.datatype.hibernate4.PersistentCollectionSerializer.findLazyValue(PersistentCollectionSerializer.java:160)
    at com.fasterxml.jackson.datatype.hibernate4.PersistentCollectionSerializer.serialize(PersistentCollectionSerializer.java:115)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:505)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase:639)

jackson-databind-2.4.2.jar

那么，除了避免使用 jackson-hibernate-mapper 进行测试外，有什么方法可以避免这个问题？使用 DTO 和 Dozer 代替域对象不被视为解决方案（将来会这样做，但不是现在）。

提前致谢

Answer 1

感谢 M. Deinum，我决定采用最简单的解决方案并创建了一个 test-servlet-context.xml：

<beans>

   <import resource="classpath:servlet.xml">   

   <bean id="jsonConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"/>

</beans>

我还更改了servlet.xml中转换器的定义：

<mvc:annotation-driven>
    <mvc:message-converters>
        <!-- Use the HibernateAware mapper instead of the default -->
        <ref bean="jsonConverter"/>
    </mvc:message-converters>
</mvc:annotation-driven> 

<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
    <property name="objectMapper">
        <bean class="path.to.your.HibernateAwareObjectMapper" />
    </property>
</bean>

现在我的测试没有使用任何杰克逊休眠魔法。它没有用，因为所有测试本身都是事务性的。

缺陷报告在这里：https://github.com/FasterXML/jackson-datatype-hibernate/issues/74