Java 操作 XML(14)--jackson-dataformat-xml 使用

Jackson 除了可以处理 JSON，还可以用来处理 XML(jackson-dataformat-xml  模块)，可以轻松完成 Java 对象和 XML 文档的互转；本文主要介绍使用 Jackson 来处理 XML，文中所使用到的软件版本：Java 1.8.0_321、Jackson 2.13.3。

1、简介

jackson-dataformat-xml 模拟 JAXB "代码优先" 的数据绑定方式，提供低级以及高级的方法来完成数据绑定工作。值得注意的是，jackson-dataformat-xml 的目标不是完整的 JAXB 克隆，或者成为一个通用的 XML 工具包。具体来说：

• While XML serialization should ideally be similar to JAXB output, deviations are not automatically considered flaws (there are reasons for some differences)
• What should be guaranteed is that any XML written using this module must be readable using module as well ("read what I wrote"): that is, we do aim for full round-trip support
• From above: there are XML constructs that module will not be able to handle; including some cases JAXB (and other Java XML libraries) supports
• This module also support constructs and use cases JAXB does not handle: specifically, rich type and object id support of Jackson are supported.

详情可参考官网文档：https://github.com/FasterXML/jackson-dataformat-xml.

2、Jackson 配置

处理 XML 的 XMLMapper 对象继承自处理 JSON 的 ObjectMapper 对象，因此他们的配置是类似的，具体可参考：Java 操作 JSON 数据(4)--Jackson 操作 JSON 数据。这里列出仅针对 XML 处理的注解：

注解	作用域	说明	实现时机
@JacksonXmlRootElement	类上	指定 XML 根标签的名字	序列化时
@JacksonXmlProperty	属性或getter/setter方法上	指定属性对应 XML 映射的名称	序列化和反序列化时
@JacksonXmlText	属性或getter/setter方法上	将属性直接作为未被标签包裹的普通文本	序列化和反序列化时
@JacksonXmlCData	属性或getter/setter方法上	将属性值包裹在 CDATA 标签中	序列化时
@JacksonXmlElementWrapper	属性或getter/setter方法上	类中有集合的属性时，是否生成包裹的标签	序列化时

3、具体使用

3.1、引入依赖

<dependency>
    <groupId>com.fasterxml.jackson.dataformat</groupId>
    <artifactId>jackson-dataformat-xml</artifactId>
    <version>2.13.3</version>
</dependency>

3.2、定义实体类

3.2.1、Grade

package com.abc.demo.xml;


import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlElementWrapper;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlProperty;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlRootElement;
import lombok.Data;
import lombok.ToString;

import java.util.List;

@Data
@ToString
@JacksonXmlRootElement(namespace = "http://www.w3.org/TR/html4/school/", localName = "school:grade")
public class Grade {
    @JacksonXmlElementWrapper(useWrapping = false)
    @JacksonXmlProperty(localName = "student", namespace = "http://www.w3.org/TR/html4/school/")
    List<Student> students;
}

3.2.2、Student

package com.abc.demo.xml;

import com.fasterxml.jackson.dataformat.xml.annotation.*;
import lombok.Data;
import lombok.ToString;

@Data
@ToString
public class Student {
    @JacksonXmlProperty(isAttribute = true)
    private int rollno;

    @JacksonXmlProperty(isAttribute = true)
    private int age;

    @JacksonXmlProperty(namespace = "http://www.w3.org/TR/html4/school/")
    private String firstname;

    private String lastname;

    private String nickname;

    private String marks;

    public Student() {}

    public Student(int rollno, int age, String firstname, String lastname, String nickname, String marks) {
        this.rollno = rollno;
        this.age = age;
        this.firstname = firstname;
        this.lastname = lastname;
        this.nickname = nickname;
        this.marks = marks;
    }
}

3.3、定义待解析 XML(student.xml)

<?xml version="1.0" encoding="utf-8" ?>
<school:grade xmlns:school="http://www.w3.org/TR/html4/school/">
    <school:student rollno="1" school:age="10">
        <school:firstname>cxx1</school:firstname>
        <lastname>Bob1</lastname>
        <nickname>stars1</nickname>
        <marks>85</marks>
    </school:student>
    <student rollno="2">
        <firstname>cxx2</firstname>
        <lastname>Bob2</lastname>
        <nickname>stars2</nickname>
        <marks>85</marks>
    </student>
    <student rollno="3">
        <firstname>cxx3</firstname>
        <lastname>Bob3</lastname>
        <nickname>stars3</nickname>
        <marks>85</marks>
    </student>
</school:grade>

3.4、序列化及反序列化

package com.abc.demo.xml;

import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import org.junit.Test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

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

public class JacksonCase {
    private static final Logger logger = LoggerFactory.getLogger(JacksonCase.class);

    /**
     * 序列化
     */
    @Test
    public void serialize() throws IOException {
        List<Student> students = new ArrayList<>();
        Student student1 = new Student(1, 11,"cxx1", "Bob1", "stars1", "85");
        Student student2 = new Student(2, 12, "cxx2", "Bob2", "stars2", "85");
        Student student3 = new Student(3, 13, "cxx3", "Bob3", "stars3", "85");
        students.add(student1);
        students.add(student2);
        students.add(student3);
        Grade grade = new Grade();
        grade.setStudents(students);
        XmlMapper xmlMapper = new XmlMapper();
        //美化输出
        xmlMapper.enable(SerializationFeature.INDENT_OUTPUT);
        logger.info(xmlMapper.writeValueAsString(grade));
    }

    /**
     * 反序列化
     */
    @Test
    public void deserialize() throws IOException {
        XmlMapper xmlMapper = new XmlMapper();

        InputStream inputStream = JacksonCase.class.getResourceAsStream("student.xml");
        //反序列化为实体对象
        Grade grade = xmlMapper.readValue(inputStream, Grade.class);
        logger.info(grade.toString());

        inputStream = JacksonCase.class.getResourceAsStream("student.xml");
        TypeReference<List<Student>> typeReference = new TypeReference<List<Student>>(){};
        //反序列化为集合
        List<Student> students = xmlMapper.readValue(inputStream, typeReference);
        logger.info(students.toString());

        inputStream = JacksonCase.class.getResourceAsStream("student.xml");
        //使用树模型 API 解析 XML
        JsonNode root = xmlMapper.readTree(inputStream);
        JsonNode jsonNode = root.get("student").get(1);
        logger.info("第一条数据,{}",jsonNode.toString());
        logger.info("第一条数据,firstname={}", jsonNode.get("firstname").asText());
        logger.info("第一条数据,marks={}", jsonNode.get("marks").asInt());
    }
}