java - @OneToMany 列返回 LazyInitializationException - Spring

Question

我的用户 DTO 中有以下映射来存储用户属性的Map:

@Entity(name = "User")
public class User{

    @Id
    @Column(name = "Id")
    private long userId;

    @OneToMany
    @JoinColumn(name = "properties")
    private Map<Long, Property> properties= new LinkedHashMap<Long, LkpAIT>();

}

@Entity(name = "Property")
public class Property{

    @Id
    @Column(name = "Id")
    private long propertyId;

    @Column(name = "Name")
    private String propertyName;

} 

以及下面的@Controller方法

@RequestMapping(value = "/getUser", produces = "application/json", method = RequestMethod.GET)
public @ResponseBody Page<LkpFeed> getLookupFeeds(Model model) {
    page = 1;
    Page<User> users = userRepo.getUsers(new PageRequest(page, 100)); //returns 100 users
    return feeds;
}

@Controller 方法执行正常(找到所有用户)，但是当执行 return 语句时，出现以下错误:

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.myapp.dto.Properties.properties, could not initialize proxy - no Session
    org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:575)
    org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:214)
    org.hibernate.collection.internal.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:155)
    org.hibernate.collection.internal.PersistentMap.isEmpty(PersistentMap.java:145)
    com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:399)
    com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:27)
    com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:505)
    com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:639)
    com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:152)
    com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:100)

根据异常，我的属性字段是导致错误的原因。我已经尝试过 fetchTypes 和 PersistenceContexts ，但目前不知所措。有什么想法吗？

Answer 1

你可以做几件事

1. 在@OneToMany中添加fetch Eager

   @Entity(name = "User")
   public class User{
   
       @Id
       @Column(name = "Id")
       private long userId;
   
       @OneToMany(fetch=FetchType.EAGER)
       @JoinColumn(name = "properties")
       private Map<Long, Property> properties= new LinkedHashMap<Long, LkpAIT>();
   
   }
   

2. 或者，当您只想列出用户时，如果您不想在返回的 JSON 中包含属性。您可以为 properties 添加 @JsonIgnore

   @Entity(name = "User")
   public class User{
   
       @Id
       @Column(name = "Id")
       private long userId;
   
       @JsonIgnore
       @OneToMany
       @JoinColumn(name = "properties")
       private Map<Long, Property> properties= new LinkedHashMap<Long, LkpAIT>();
   
   }