要保留XML结构，可以使用以下代码示例：

1. 使用DOM解析器解析XML文件，然后将其保存为字符串：

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import org.w3c.dom.Document;
import org.xml.sax.SAXException;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;

public class XmlParser {
    public static void main(String[] args) {
        try {
            // 创建DOM解析器工厂
            DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
            // 创建DOM解析器
            DocumentBuilder builder = factory.newDocumentBuilder();
            // 解析XML文件
            Document document = builder.parse(new File("input.xml"));
            
            // 将Document对象保存为字符串
            String xmlString = documentToString(document);
            
            System.out.println(xmlString);
        } catch (ParserConfigurationException | SAXException | IOException e) {
            e.printStackTrace();
        }
    }
    
    // 将Document对象保存为字符串
    private static String documentToString(Document document) {
        try {
            ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
            javax.xml.transform.TransformerFactory.newInstance().newTransformer().transform(
                    new javax.xml.transform.dom.DOMSource(document),
                    new javax.xml.transform.stream.StreamResult(outputStream)
            );
            return outputStream.toString(StandardCharsets.UTF_8.name());
        } catch (Exception e) {
            e.printStackTrace();
        }
        return null;
    }
}

2. 使用JAXB将XML文件转换为Java对象，然后将对象转换回XML字符串：

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;
import java.io.StringReader;
import java.io.StringWriter;

public class XmlConverter {
    public static void main(String[] args) {
        try {
            // 创建JAXB上下文
            JAXBContext context = JAXBContext.newInstance(Root.class);
            
            // 创建Unmarshaller
            Unmarshaller unmarshaller = context.createUnmarshaller();
            
            // 将XML字符串转换为Java对象
            String xmlString = "Value 1Value 2";
            StringReader reader = new StringReader(xmlString);
            Root root = (Root) unmarshaller.unmarshal(reader);
            
            // 将Java对象转换为XML字符串
            Marshaller marshaller = context.createMarshaller();
            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            StringWriter writer = new StringWriter();
            marshaller.marshal(root, writer);
            
            System.out.println(writer.toString());
        } catch (JAXBException e) {
            e.printStackTrace();
        }
    }
}

import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement(name = "root")
class Root {
    @XmlElement
    private String element1;
    
    @XmlElement
    private String element2;
    
    public String getElement1() {
        return element1;
    }
    
    public void setElement1(String element1) {
        this.element1 = element1;
    }
    
    public String getElement2() {
        return element2;
    }
    
    public void setElement2(String element2) {
        this.element2 = element2;
    }
}

这两种方法都可以保留XML结构，你可以根据自己的需求选择适合的方法。