如何使用Jackson在JSON序列化中重命名根密钥

Question

I am using Jackson for JSON serialization of a list of objects.

Here is what I get:

{"ArrayList":[{"id":1,"name":"test name"}]}

But I want this :

{"rootname":[{"id":1,"name":"test name"}]} // ie showing the string I want as the root name.

Below is my approach to this:

Interface:

public interface MyInterface {
    public long getId();
    public String getName();
}

Implementation class:

@JsonRootName(value = "rootname")
public class MyImpl implements MyInterface {
    private final long id;
    private String name;

    public MyImpl(final long id,final name) {
        this.id = id;
        this.name = name;
    }

   // getters     
}

JSon serialization:

public class MySerializer {
    public static String serializeList(final List<MyInterface> lists) {
        //check for null value.Throw Exception
        final ObjectMapper mapper = new ObjectMapper();
        mapper.configure(SerializationConfig.Feature.WRAP_ROOT_VALUE, true);
        return mapper.writeValueAsString(lists);
    }
}

Test:

final List<MyInterface> list = new ArrayList<MyImpl>();
MyImpl item = new MyImpl(1L,"test name");
list.add(item);
final String json = MySerializer.serializeList(list);
System.out.println(json);

Here is what I get:

{"ArrayList":[{"id":1,"name":"test name"}]}

But I want this :

{"rootname":[{"id":1,"name":"test name"}]} // ie showing the string I want as the root     name.

I have tried all suggested solutions I could find but failed to achieve my goal. I have looked at:

• Jackson : custom collection serialization to JSON
• How do I rename the root key of a JSON with Java Jackson?
• Jackson : custom collection serialization to JSON

Or am I missing something? I am using jackson 1.9.12 for this. Any help in this regard is welcome.

Answer 1

Well, by default Jackson uses one of two annotations when trying to determine the root name to be displayed for wrapped values - @XmlRootElement or @JsonRootName . It expects this annotation to be on the type being serialized, else it will use the simple name of the type as the root name.

In your case, you are serializing a list, which is why the root name is 'ArrayList' (simple name of the type being serialized). Each element in the list may be of a type annotated with @JsonRootName, but the list itself is not .

When the root value you are trying to wrap is a collection then you need some way of defining the wrap name:

Holder/Wrapper Class

You can create a wrapper class to hold the list, with an annotation to define the desired property name ( you only need to use this method when you do not have direct control of the ObjectMapper /JSON transformation process ):

class MyInterfaceList {
    @JsonProperty("rootname")
    private List<MyInterface> list;

    public List<MyInterface> getList() {
        return list;
    }

    public void setList(List<MyInterface> list) {
        this.list = list;
    }
}

final List<MyInterface> lists = new ArrayList<MyInterface>(4);
lists.add(new MyImpl(1L, "test name"));
MyInterfaceList listHolder = new MyInterfaceList();
listHolder.setList(lists);
final String json = mapper.writeValueAsString(listHolder);

Object Writer

This is the preferable option. Use a configured ObjectWriter instance to generate the JSON. In particular, we are interested in the withRootName method:

final List<MyInterface> lists = new ArrayList<MyInterface>(4);
lists.add(new MyImpl(1L, "test name"));
final ObjectWriter writer = mapper.writer().withRootName("rootName");
final String json = writer.writeValueAsString(lists);

Answer 2

I know, I am late , but I have better approach which don't require Holder/Wrapper Class. It picks root key from annotation.

package com.test;

import com.fasterxml.jackson.annotation.JsonRootName;


@JsonRootName("Products")
public class ProductDTO {
    private String name;
    private String description;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

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

Here is test class:-

package com.test;

import java.io.IOException;
import java.util.ArrayList;

import org.junit.Test;

import com.fasterxml.jackson.annotation.JsonRootName;
import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;


public class ProductDTOTestCase {
    @Test
    public void testPersistAndFindById() throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper mapper = new ObjectMapper();
        ProductDTO productDTO = new ProductDTO();
        productDTO.setDescription("Product 4 - Test");

        ArrayList<ProductDTO> arrayList = new ArrayList<ProductDTO>();
        arrayList.add(productDTO);

        String rootName = ProductDTO.class.getAnnotation(JsonRootName.class).value();
        System.out.println(mapper.writer().withRootName(rootName).writeValueAsString(arrayList));

    }


}

It will give following output

{"Products":[{"name":null,"description":"Product 4 - Test"}]}

Answer 3

@JsonTypeName("usuarios")
@JsonTypeInfo(include= JsonTypeInfo.As.WRAPPER_OBJECT,use= JsonTypeInfo.Id.NAME)
public class UsuarioDT extends ArrayList<Usuario> {

    @JsonProperty("rowsAffected")
    private Integer afectados;

    public Integer getAfectados() {
        return afectados;
    }

    public void setAfectados(Integer afectados) {
        this.afectados = afectados;
    }
}

Answer 4

您需要在类的顶部使用此批注

@JsonTypeName("rootname")