Jackson XmlMapper 高级配置：确保XML输出包含标准声明头

import com.fasterxml.jackson.dataformat.xml.XmlMapper;
import com.fasterxml.jackson.dataformat.xml.ser.ToXmlGenerator;
import com.fasterxml.jackson.annotation.JsonPropertyOrder;

public class JacksonXmlDeclarationTutorial {

    // 定义一个简单的Java对象用于序列化
    // @JsonPropertyOrder 注解用于控制XML元素的顺序
    @JsonPropertyOrder({ "name", "id" })
    static class Source {
        public String name;
        public int id;

        public Source(String name, int id) {
            this.name = name;
            this.id = id;
        }

        // Jackson需要无参构造函数进行反序列化，但序列化时不是必须
        public Source() {}
    }

    public static void main(String[] args) throws Exception {
        Source mySource = new Source("ExampleSource", 123);

        // --- 1. 默认行为：不包含XML声明头 ---
        XmlMapper defaultXmlMapper = new XmlMapper();
        // 默认情况下，WRITE_XML_DECLARATION 为 false
        String defaultXmlString = defaultXmlMapper.writeValueAsString(mySource);
        System.out.println("--- 生成的XML（不含声明头，默认行为）：---");
        System.out.println(defaultXmlString);
        System.out.println("-----------------------------------------\n");

        // --- 2. 配置XmlMapper：包含XML声明头 ---
        XmlMapper configuredXmlMapper = new XmlMapper();
        // 启用 WRITE_XML_DECLARATION 特性以添加 XML 声明头
        configuredXmlMapper.configure(ToXmlGenerator.Feature.WRITE_XML_DECLARATION, true);

        // 将对象序列化为XML字符串
        String xmlStringWithDeclaration = configuredXmlMapper.writeValueAsString(mySource);

        System.out.println("--- 生成的XML（包含声明头）：---");
        System.out.println(xmlStringWithDeclaration);
        System.out.println("-----------------------------------------\n");
    }
}

--- 生成的XML（不含声明头，默认行为）：---
<Source><name>ExampleSource</name><id>123</id></Source>
-----------------------------------------

--- 生成的XML（包含声明头）：---
<?xml version='1.0' encoding='UTF-8'?>
<Source><name>ExampleSource</name><id>123</id></Source>
-----------------------------------------