Spring JPA 延迟加载 - 无法初始化代理答案

【问题标题】：Spring JPA Lazy Loading - Could Not Initialize ProxySpring JPA 延迟加载 - 无法初始化代理
【发布时间】：2015-11-01 17:24:21
【问题描述】：

下面是一些背景代码：

InitiativeProfileQuestion.java：

@Entity
@Table
public class InitiativeProfileQuestion implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column(nullable = false)
    private String question;

    @Column
    private String description;

    @Column
    private int sortOrder;

    @OneToMany(mappedBy = "initiativeProfileQuestion", fetch = FetchType.LAZY)
    private List<InitiativeProfileAnswer> answers;

    public List<InitiativeProfileAnswer> getAnswers() {
        return answers;
    }

    public void setAnswers(List<InitiativeProfileAnswer> answers) {
        this.answers = answers;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getQuestion() {
        return question;
    }

    public void setQuestion(String question) {
        this.question = question;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public int getSortOrder() {
        return sortOrder;
    }

    public void setSortOrder(int sortOrder) {
        this.sortOrder = sortOrder;
    }

}

InitiativeProfileAnswer.java：

@Entity
@Table
public class InitiativeProfileAnswer {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column
    private String answer;

    @Column
    private int sortOrder;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "initiativeProfileQuestionId")
    @JsonIgnore
    private InitiativeProfileQuestion initiativeProfileQuestion;

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getAnswer() {
        return answer;
    }

    public void setAnswer(String answer) {
        this.answer = answer;
    }

    public int getSortOrder() {
        return sortOrder;
    }

    public void setSortOrder(int sortOrder) {
        this.sortOrder = sortOrder;
    }

    public InitiativeProfileQuestion getInitiativeProfileQuestion() {
        return initiativeProfileQuestion;
    }

    public void setInitiativeProfileQuestion(InitiativeProfileQuestion initiativeProfileQuestion) {
        this.initiativeProfileQuestion = initiativeProfileQuestion;
    }
}

InitiativeProfileQuestionRepository.java：

public interface InitiativeProfileQuestionRepository extends JpaRepository<InitiativeProfileQuestion, Long> {

    @Query("select ipa from InitiativeProfileQuestion ipa join fetch ipa.answers")
    public List<InitiativeProfileQuestion> getAllQuestions();
}

InitiativeProfileService.java：

@Service
public class InitiativeProfileService {

    @Autowired
    private InitiativeProfileQuestionRepository initiativeProfileQuestionRepository;

    public List<InitiativeProfileQuestion> getAllQuestions() {
        return initiativeProfileQuestionRepository.findAll();
    }

    public List<InitiativeProfileQuestion> getAllQuestionsFetch() {
        return initiativeProfileQuestionRepository.getAllQuestions();
    }

}

BaseController.java：

@RestController
@RequestMapping("/api")
public class BaseController {

    @Autowired
    InitiativeProfileService initiativeProfileService;

    @RequestMapping("/question")
    public List<InitiativeProfileQuestion> getQuestions() {
        return initiativeProfileService.getAllQuestions();
    }

    @RequestMapping("/questionFetch")
    public List<InitiativeProfileQuestion> getQuestionsFetch() {
        return initiativeProfileService.getAllQuestionsFetch();
    }
}

在我的 BaseController 中调用 getQuestions() 会返回“无法初始化代理 - 无会话”错误。但是，在我的 BaseController 中调用 getQuestionsFetch() 加载就好了。

我希望它以某种方式工作，如果我调用 getQuestions()，对象将返回没有答案（因为不会在任何地方调用延迟加载的对象）。但是，它只是给了我一个错误。如果我正在使用连接提取进行查询，它也会通过显示答案来工作（预期行为）。

我做错了什么？我在不同的地方尝试了@Transactional，但没有运气。我也没有 .xml 文件——到目前为止，一切都是使用注释完成的。

我得到的错误是：

exception

org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: failed to lazily initialize a collection of role: com.testApp.domain.InitiativeProfileQuestion.answers, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.testApp.domain.InitiativeProfileQuestion["answers"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.testApp.domain.InitiativeProfileQuestion.answers, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.testApp.domain.InitiativeProfileQuestion["answers"])
    org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:238)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:208)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:161)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
    org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:185)
    org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:126)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:776)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:705)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:858)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:85)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
root cause

com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.testApp.domain.InitiativeProfileQuestion.answers, could not initialize proxy - no Session (through reference chain: java.util.ArrayList[0]->com.testApp.domain.InitiativeProfileQuestion["answers"])
    com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:210)
    com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:177)
    com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:187)
    com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:647)
    com.fasterxml.jackson.databind.ser.std.BeanSerializerBase._serializeWithObjectId(BeanSerializerBase.java:558)
    com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:145)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:100)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:21)
    com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:183)
    com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:128)
    com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:1902)
    org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.writeInternal(AbstractJackson2HttpMessageConverter.java:231)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:208)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:161)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:101)
    org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.handleReturnValue(RequestResponseBodyMethodProcessor.java:185)
    org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:71)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:126)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:776)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:705)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:959)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:893)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:858)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:843)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
    org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:85)
    org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)

【问题讨论】：

"它只是给我一个错误" ...什么错误？特别是，堆栈跟踪会告诉您谁正在访问“不会在任何地方调用”的延迟加载对象。
无法写入内容：无法延迟初始化角色集合：com.testApp.domain.InitiativeProfileQuestion.answers，无法初始化代理 - 无会话（通过引用链：java.util.ArrayList[ 0]->com.testApp.domain.InitiativeProfileQuestion["answers"])‌；嵌套异常是 com.fasterxml.jackson.databind.JsonMappingException：未能延迟初始化角色集合：com.testApp.domain.InitiativeProfileQuestion.answers，无法初始化代理 - 无会话（通过引用链：java.util.ArrayList[ 0]->com.testApp.domain.InitiativeProfileQuestion
使用问题下方的“编辑”链接将此信息添加到您的答案中（人们通常不会阅读整个评论链，没有长度限制，您可以更好地格式化它） .另外，我要求提供 stacktrace，而不仅仅是异常消息。
以上更新。谢谢
几个小时后，我从这篇文章中弄清楚了：stackoverflow.com/questions/21708339/…

标签： java jpa lazy-loading spring-data-jpa lazy-initialization

【解决方案1】：

对于延迟加载的例外情况，您必须尝试在会话之外获取关联的对象。要么将延迟加载更改为渴望，要么将获取关联对象的代码放入会话中。例如：

    public void test() {

            Session session = HibernateUtil.currentSession();
            session.beginTransaction();

            Vehicle  vehicle= (Vehicle) session.get(Vehicle.class, 2);

            System.out.println(vehicle.getVehicleName());

            session.getTransaction().commit();
            session.close();
            System.out.println(vehicle.getVehicleName()); //No exception here
            System.out.println(vehicle.getUser().getUserName()); 
// Exception here change the loading to EAGER or put this line of code within the session above. Put it before session.close() or  before session.getTransaction().commit();
        }

【讨论】：

如果我没记错的话，OP 想避免急切加载。
2 个选项是他们的。使用 Eager 或获取会话中的关联对象。
user3360241 是正确的。我想避免急于加载。有时我想在没有答案的情况下检索 InitiativeProfileQuestions 对象，因此我认为延迟加载是最好的方法。
据我所知，答案只显示了急切的选择。最后一行的注释表示避免异常急切地获取用户。
我提供了一个示例，说明您的代码中最可能的原因以及修复它的 2 个选项。其他选项是：如果使用条件，您可以使用查询来获取或使用 FetchMode.EAGER。

【解决方案2】：

从堆栈跟踪中可以看出，当 Jackson 尝试访问延迟加载的对象以将其序列化为 JSON 时，就会发生错误。此时spring事务已经完成，Hibernate会话关闭，这就是为什么Hibernate不能再加载那个对象了。

如果您不打算序列化该字段，您可能希望使用@JsonIgnore 或Spring's Jackson Serialization View Support。

【讨论】：