如何(De)使用Jackson基于注释从对象序列化字段？

Question

我需要以特定方式配置Jackson,我将在下面介绍.

要求

1. 带注释的字段仅使用其ID进行序列化:
   • 如果该字段是普通对象,则序列化它 id
   • 如果该字段是对象的集合,则序列化一个数组 id
2. 带注释的字段以不同方式序列化其属性名称:
   • 如果该字段是普通对象,请将"_id"后缀添加到属性名称
   • 如果该字段是对象的集合,请将"_ids"后缀添加到属性名称
3. 对于注释,我想这样的自定义@JsonId,最好有一个可选的值来覆盖名字就像@JsonProperty做
4. id属性应由用户定义,使用:
   • 已经存在的杰克逊 @JsonIdentityInfo
   • 或者通过创建另一个类或字段注释
   • 或者通过决定检查id属性可发现性的注释(例如,对JPA场景有用)
5. 应使用包装的根值序列化对象
6. Camel case命名应该用下划线转换为小写
7. 所有这些都应该是可反序列化的(通过构造一个只设置了id的实例)

一个例子

考虑到这些POJO:

//Inform Jackson which property is the id
@JsonIdentityInfo(
    generator = ObjectIdGenerators.PropertyGenerator.class,
    property = "id"
)
public abstract class BaseResource{
    protected Long id;

    //getters and setters
}

public class Resource extends BaseResource{
    private String name;
    @JsonId
    private SubResource subResource;
    @JsonId
    private List<SubResource> subResources;

    //getters and setters
}

public class SubResource extends BaseResource{
    private String value;

    //getters and setters
}

Resource实例的可能序列化可以是:

{
    "resource":{
        "id": 1,
        "name": "bla",
        "sub_resource_id": 2,
        "sub_resource_ids": [
            1,
            2,
            3
        ]
    }
}

至今...

• 要求#5可以通过ObjectMapper以下方式配置来完成:

  objectMapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
  objectMapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
  

  然后@JsonRootName("example_root_name_here")在我的POJO中使用.

• 要求#6可以通过ObjectMapper以下方式配置来完成:

  objectMapper.setPropertyNamingStrategy(
      PropertyNamingStrategy.CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES);
  

如您所见,仍有许多要求需要实现.对于那些想知道为什么我需要这样的配置的人来说,这是因为我正在为ember.js(更具体地说是Ember Data)开发一个REST web服务.如果您可以帮助满足任何要求,您将非常感激.

谢谢!

Answer 1

大多数(全部？)您的要求可以通过使用上下文序列化器来完成.从ContextualDeserializer获得一个答案,用于将JSON映射到Jackson和Jackson的wiki(http://wiki.fasterxml.com/JacksonFeatureContextualHandlers)的不同类型的地图,我能够得到以下内容.

您需要从@JsonId注释开始,这是指示属性只需要使用Id属性的键.

import com.fasterxml.jackson.annotation.*;
import java.lang.annotation.*;

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@JacksonAnnotation // important so that it will get included!
public @interface JsonId {
}

接下来是实际的ContextualSerializer,它可以完成繁重的工作.

import com.fasterxml.jackson.databind.ser.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.core.*;
import java.io.*;

public class ContextualJsonIdSerializer
    extends JsonSerializer<BaseResource>
    implements ContextualSerializer/*<BaseResource>*/
{
    private ObjectMapper mapper;
    private boolean useJsonId;

    public ContextualJsonIdSerializer(ObjectMapper mapper) { this(mapper, false); }
    public ContextualJsonIdSerializer(ObjectMapper mapper, boolean useJsonId) {
        this.mapper = mapper;
        this.useJsonId = useJsonId;
    }

    @Override
    public void serialize(BaseResource br, JsonGenerator jgen, SerializerProvider provider) throws IOException
    {
        if ( useJsonId ) {
            jgen.writeString(br.getId().toString());
        } else {
            mapper.writeValue(jgen, br);
        }
    }

    @Override
    public JsonSerializer<BaseResource> createContextual(SerializerProvider config, BeanProperty property)
            throws JsonMappingException
    {
        // First find annotation used for getter or field:
        System.out.println("Finding annotations for "+property);

        if ( null == property ) {
            return new ContextualJsonIdSerializer(mapper, false);
        }

        JsonId ann = property.getAnnotation(JsonId.class);
        if (ann == null) { // but if missing, default one from class
            ann = property.getContextAnnotation(JsonId.class);
        }
        if (ann == null ) {//|| ann.length() == 0) {
            return this;//new ContextualJsonIdSerializer(false);
        }
        return new ContextualJsonIdSerializer(mapper, true);
    }
}

此类查看BaseResource属性并检查它们以查看是否@JsonId存在注释.如果是,则仅使用Id属性,否则传入ObjectMapper用于序列化该值.这很重要,因为如果你尝试使用(基本上)在上下文中的映射器,ContextualSerializer你将得到一个堆栈溢出,因为它最终将反复调用这些方法.

您的资源应该类似于以下内容.我使用了@JsonProperty注释而不是将功能包装在中,ContextualSerializer因为重新发明轮子似乎很愚蠢.

import java.util.*;
import com.fasterxml.jackson.annotation.*;

public class Resource extends BaseResource{
    private String name;

    @JsonProperty("sub_resource_id")
    @JsonId
    private SubResource subResource;

    @JsonProperty("sub_resource_ids")
    @JsonId
    private List<SubResource> subResources;

    //getters and setters
    public String getName() {return name;}
    public void setName(String name) {this.name = name;}

    public SubResource getSubResource() {return subResource;}
    public void setSubResource(SubResource subResource) {this.subResource = subResource;}

    public List<SubResource> getSubResources() {return subResources;}
    public void setSubResources(List<SubResource> subResources) {this.subResources = subResources;}
}

最后,执行序列化的方法只会创建一个额外的ObjectMapper并在原始模块中注册一个模块ObjectMapper.

// Create the original ObjectMapper
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
objectMapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
objectMapper.setPropertyNamingStrategy(PropertyNamingStrategy.CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES);

// Create a clone of the original ObjectMapper
ObjectMapper objectMapper2 = new ObjectMapper();
objectMapper2.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
objectMapper2.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
objectMapper2.setPropertyNamingStrategy(PropertyNamingStrategy.CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES);

// Create a module that references the Contextual Serializer
SimpleModule module = new SimpleModule("JsonId", new Version(1, 0, 0, null));
// All references to SubResource should be run through this serializer
module.addSerializer(SubResource.class, new ContextualJsonIdSerializer(objectMapper2));
objectMapper.registerModule(module);

// Now just use the original objectMapper to serialize