java - 使用外键从 JSON 反序列化

Question

我有一个多对一的关系:A *<-->1 B，我想从具有 B 的 JSON 反序列化 A> 的主键(B 存在于具有该主键的数据库中):

{
    "b": 1
}

我尝试过以下方法:

@Entity
@Table(name = "table_a")
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class A implements Serializable {

    @JsonIgnore
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name = "b", unique = true, nullable = false)
    private B b;
}

和

@Entity
@Table(name = "table_b")
public class B implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;

    @OneToMany(mappedBy = "b")
    private List<A> a = new ArrayList<>();
}

但是对象A是使用b = null创建的。如何使用从数据库正确实例化的 b 属性反序列化 A ？

注意:我使用的是 Jackson 版本 2.6.1。

Answer 1

您有多种选择，这里是similar question :

1. B 类中的@JsonCreator 工厂 ( More info )

2. 自定义解串器

3. 为@JsonIdentityInfo自定义ObjectIdResolver，例如

   private class MyObjectIdResolver implements ObjectIdResolver {
       private Map<ObjectIdGenerator.IdKey,Object> _items  = new HashMap<>();
   
       @Override
       public void bindItem(ObjectIdGenerator.IdKey id, Object pojo) {
           if (!_items.containsKey(id)) _items.put(id, pojo); 
       }
   
       @Override
       public Object resolveId(ObjectIdGenerator.IdKey id) {
           Object object = _items.get(id);
           return object == null ? getById(id) : object;
       }
   
       protected Object getById(ObjectIdGenerator.IdKey id){
           Object object = null;
           try {
               //can resolve object from db here
               //objectRepository.getById((Integer)idKey.key, idKey.scope)
               object = id.scope.getConstructor().newInstance();
               id.scope.getMethod("setId", int.class).invoke(object, id.key);
           } catch (Exception e) {
               e.printStackTrace();
           }
           return object;
       }
   
       @Override
       public ObjectIdResolver newForDeserialization(Object context) {
           return new MyObjectIdResolver();
       }
   
       @Override
       public boolean canUseFor(ObjectIdResolver resolverType) {
           return resolverType.getClass() == getClass();
       }
   }
   

   并像这样使用它:

   @JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class,
           resolver = MyObjectIdResolver.class, 
           property = "id", scope = B.class)
   public class B  {
      // ...
   }
   

这是您的案例相关gist demo更广泛github project一些序列化的想法