Jackson序列化：将字段值设置为XML元素名称 [英] Jackson Serialization: Setting field value as XML element name

问题描述

我们正在使用Jackson jax-rs XML内容提供程序来处理基于jax-rs的REST API项目中的XML内容类型。
在序列化POJO列表时，我们需要从POJO中的字段动态设置xml元素名称。

We are using Jackson jax-rs XML content providers for handling XML content type in our jax-rs based REST API project. In the serializing a List of POJOs, we need to set the xml element name dynamically from a field in the POJO.

public class ResponsePOJO {
     @JacksonXmlProperty
     @JacksonXmlElementWrapper(useWrapping = false)
     private List<Message> message = new ArrayList<Message>();
}

public class Message {
     private String type; // "Error" or "Warning"
     private String msg; // The actual message
}

默认Jackson序列化XML：

Default Jackson serialized XML:

<ResponsePOJO>
    <message>
        <type>Error</type>
        <msg>Some random error message</msg>
    </message>
    <message>
        <type>Warning</type>
        <msg>Some random warning message</msg>
    </message>
</ResponsePOJO>

我们的要求，即将类型设置为XML元素名称。

Our requirement, ie., set type as the XML element name.

<ResponsePOJO>
    <Error>
        <msg>Some random error message</msg>
    </Error>
    <Warning>
        <msg>Some random warning message</msg>
    </Warning>
</ResponsePOJO>

为了实现这一目标，我们以下列方式编写了一个自定义XML序列化程序：

In order to achieve this, we wrote a custom XML serializer in the following manner:

public class MessageListSerializer extends
        JsonSerializer<List<Message>> {
    @Override
    public void serialize(List<Message> value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {

        for(Message me : value){
            jgen.writeObjectField(me.getType(), me);
        }
    }
}

并使用注释添加了序列化程序：

And added the serializer using annotation:

@JacksonXmlProperty
@JacksonXmlElementWrapper(useWrapping = false)
@JsonSerialize(using=MessageListSerializer.class)
private List<Message> message = new ArrayList<Message>();

但是在使用Jackson XMLMapper序列化ResponsePOJO时，我们得到以下异常......

But while serializing the ResponsePOJO using Jackson XMLMapper, we are getting the following exception...

Exception in thread "main" com.fasterxml.jackson.databind.JsonMappingException: Array index out of range: -2
    at com.fasterxml.jackson.dataformat.xml.ser.XmlSerializerProvider.serializeValue(XmlSerializerProvider.java:100)
    at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:2866)
    at com.fasterxml.jackson.databind.ObjectMapper.writeValue(ObjectMapper.java:2289)
Caused by: java.lang.ArrayIndexOutOfBoundsException: Array index out of range: -2
    at com.ctc.wstx.sw.BufferingXmlWriter.writeRaw(BufferingXmlWriter.java:241)
    at com.ctc.wstx.sw.BaseStreamWriter.writeRaw(BaseStreamWriter.java:1113)
    at com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator.writeRaw(ToXmlGenerator.java:592)
    at com.fasterxml.jackson.dataformat.xml.util.DefaultXmlPrettyPrinter$Lf2SpacesIndenter.writeIndentation(DefaultXmlPrettyPrinter.java:517)
    at com.fasterxml.jackson.dataformat.xml.util.DefaultXmlPrettyPrinter.writeEndObject(DefaultXmlPrettyPrinter.java:223)
    at com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator.writeEndObject(ToXmlGenerator.java:422)
    at com.fasterxml.jackson.dataformat.xml.ser.XmlBeanSerializer.serialize(XmlBeanSerializer.java:119)
    at com.fasterxml.jackson.dataformat.xml.ser.XmlSerializerProvider.serializeValue(XmlSerializerProvider.java:92)
    ... 3 more

你能否帮我解决这个问题...

Could you please help me resolve this issue...

推荐答案

编辑上一个解决方案：
你几乎就在那里，只需要将 @JsonIgnore 添加到私有字符串类型; //错误或警告

Edit to previous solution: You're nearly there, just need to add @JsonIgnore to private String type; // "Error" or "Warning"

<ResponsePOJO>
    <Error>
        <msg>error message</msg>
    </Error>
    <Warning>
        <msg>warning message</msg>
    </Warning>
</ResponsePOJO>

以下将输出上述xml：

The following will output the above xml:

import com.fasterxml.jackson.annotation.JsonIgnore;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.annotation.JsonSerialize;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlElementWrapper;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlProperty;

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        Main demo = new Main();
        demo.run();
    }


    public void run(){

        ObjectMapper xmlMapper = new XmlMapper();

        ResponsePOJO responsePOJO = new ResponsePOJO();

        Message message = new Message();
        message.setType("Error");
        message.setMsg("error message");
        Message message2 = new Message();
        message2.setType("Warning");
        message2.setMsg("warning message");

        responsePOJO.getMessage().add(message);
        responsePOJO.getMessage().add(message2);
        try {
            String xml = xmlMapper.writeValueAsString(responsePOJO);
            System.out.println(xml);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }

    }


    public class ResponsePOJO {
        @JacksonXmlProperty
        @JacksonXmlElementWrapper(useWrapping = false)
        @JsonSerialize(using=MessageListSerializer.class)
        private List<Message> message = new ArrayList<Message>();

        public List<Message> getMessage() {
            return message;
        }

    }


    public class Message {
        @JsonIgnore
        private String type; // "Error" or "Warning"
        private String msg; // The actual message

        public String getType() {
            return type;
        }

        public void setType(String type) {
            this.type = type;
        }

        public String getMsg() {
            return msg;
        }

        public void setMsg(String msg) {
            this.msg = msg;
        }
    }


}

与班级一起

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;

import java.io.IOException;
import java.util.List;

/**
 * Created by Pand on 08/04/2015.
 */
public class MessageListSerializer extends
        JsonSerializer<List<Main.Message>> {


    @Override
    public void serialize(List<Main.Message> value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {

        for(Main.Message me : value){
            jgen.writeObjectField(me.getType(), me);
        }
    }


}

with dependencies

with dependencies

    <dependencies>
        <dependency>
            <groupId>com.fasterxml.jackson.dataformat</groupId>
            <artifactId>jackson-dataformat-xml</artifactId>
            <version>2.4.0</version>
        </dependency>
        <dependency>
            <groupId>org.codehaus.woodstox</groupId>
            <artifactId>woodstox-core-asl</artifactId>
            <version>4.1.4</version>
        </dependency>
    </dependencies>

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