Jackson：如何让@JsonBackReference 和自定义反序列化器同时工作？

Question

我有一个 JSON 结构，其中包含字符串到对象的映射：

{

    "domains": {

        "ldap": {

            "users": {

                "fwalther": {

                    "firstName": "Fritz",

                    "lastName": "Walther"

                },

                // ...

            }

        }

    },

    // ...

}       

我想将此结构反序列化为使用 Jackson 的对象，Domain并且User我希望每个用户都有一个对其映射键（即用户 ID）和Domain容器的反向引用。这两种方法都有效，但如果我尝试同时获取两个反向引用，它就会失败。

@JsonManagedReference带有和的 Java 类@JsonBackReference：

public class Domain {

    @JsonManagedReference

    @JsonDeserialize(contentUsing = UserDeserializer.class)

    private Map<String, User> users;

    public Map<String, User> getUsers() {

        return users;

    }

}

public class User {

    @JsonBackReference

    private Domain domain;

    String userId;

    private String firstName;

    private String lastName;

    // ... getters

}

自定义反序列化器获取映射键：

public class UserDeserializer extends JsonDeserializer<User> {

    @Override

    public User deserialize(JsonParser p, DeserializationContext ctxt)

            throws IOException, JsonProcessingException {

        String key = p.getCurrentName();

        User result = p.readValueAs(User.class);

        result.userId = key;

        return result;

    }

}

如果我只激活其中一个，这两种机制，即@JsonManagedReference/@JsonBackReference对和自定义反序列化器都可以工作。@JsonDeserialize但是如果我结合这些机制（如上面的代码所示），在解析 JSON 时会出现以下异常：

查看引发异常的代码，我发现我需要findBackReference在我的自定义反序列化器中实现，但我不知道如何，我也找不到这方面的信息。有任何想法吗？

还是有其他方法可以同时获取映射键和对包含对象的反向引用？

Answer 1

在这个答案的帮助下，我找到了解决方案：自定义反序列化器需要基于默认反序列化器，它正确实现了反向引用机制。这比仅仅从正确的基类继承要复杂一些。相反，您需要通过自定义获取（完全配置的）默认反序列化器实例BeanDeserializerModifier，然后将此实例传递给您的子类DelegatingDeserializer：public ObjectMapper getMapperWithCustomDeserializer() {&nbsp; &nbsp; ObjectMapper objectMapper = new ObjectMapper();&nbsp; &nbsp; SimpleModule module = new SimpleModule();&nbsp; &nbsp; module.setDeserializerModifier(new BeanDeserializerModifier() {&nbsp; &nbsp; &nbsp; &nbsp; @Override&nbsp; &nbsp; &nbsp; &nbsp; public JsonDeserializer<?> modifyDeserializer(DeserializationConfig config,&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; BeanDescription beanDesc, JsonDeserializer<?> defaultDeserializer) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (beanDesc.getBeanClass() == User.class) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return new UserDeserializer(defaultDeserializer);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; } else {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return defaultDeserializer;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; });&nbsp; &nbsp; objectMapper.registerModule(module);&nbsp; &nbsp; return objectMapper;}然后自定义反序列化器需要如下所示：public class UserDeserializer extends DelegatingDeserializer {&nbsp; &nbsp; public UserDeserializer(JsonDeserializer<?> delegate) {&nbsp; &nbsp; &nbsp; &nbsp; super(delegate);&nbsp; &nbsp; }&nbsp; &nbsp; @Override&nbsp; &nbsp; protected JsonDeserializer<?> newDelegatingInstance(JsonDeserializer<?> newDelegate) {&nbsp; &nbsp; &nbsp; &nbsp; return new UserDeserializer(newDelegate);&nbsp; &nbsp; }&nbsp; &nbsp; @Override&nbsp; &nbsp; public User deserialize(JsonParser p, DeserializationContext ctxt)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; throws IOException {&nbsp; &nbsp; &nbsp; &nbsp; String key = p.getCurrentName();&nbsp; &nbsp; &nbsp; &nbsp; User result = (User) super.deserialize(p, ctxt);&nbsp; &nbsp; &nbsp; &nbsp; result.userId = key;&nbsp; &nbsp; &nbsp; &nbsp; return result;&nbsp; &nbsp; }}最后，您需要删除@JsonDeserialize注释。然后，自定义反序列化器和@JsonBackReference应该工作。