如何从Java打印XML？

我有一个包含XML的Java字符串，没有换行或缩进。 我想把它变成一个带有格式良好的XML的字符串。 我该怎么做呢？

String unformattedXml = "<tag><nested>hello</nested></tag>";
String formattedXml = new [UnknownClass]().format(unformattedXml);

注意：我的输入是一个字符串 。 我的输出是一个字符串 。

---

Transformer transformer = TransformerFactory.newInstance().newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
//initialize StreamResult with File object to save to file
StreamResult result = new StreamResult(new StringWriter());
DOMSource source = new DOMSource(doc);
transformer.transform(source, result);
String xmlString = result.getWriter().toString();
System.out.println(xmlString);

注意：结果可能因Java版本而异。 搜索特定于您的平台的解决方法。

---

这是我自己问题的答案。 我将来自各种结果的答案结合起来，编写出可以打印XML的类。

无法保证它如何响应无效的XML或大型文档。

package ecb.sdw.pretty;

import org.apache.xml.serialize.OutputFormat;
import org.apache.xml.serialize.XMLSerializer;
import org.w3c.dom.Document;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import java.io.IOException;
import java.io.StringReader;
import java.io.StringWriter;
import java.io.Writer;

/**
 * Pretty-prints xml, supplied as a string.
 * <p/>
 * eg.
 * <code>
 * String formattedXml = new XmlFormatter().format("<tag><nested>hello</nested></tag>");
 * </code>
 */
public class XmlFormatter {

    public XmlFormatter() {
    }

    public String format(String unformattedXml) {
        try {
            final Document document = parseXmlFile(unformattedXml);

            OutputFormat format = new OutputFormat(document);
            format.setLineWidth(65);
            format.setIndenting(true);
            format.setIndent(2);
            Writer out = new StringWriter();
            XMLSerializer serializer = new XMLSerializer(out, format);
            serializer.serialize(document);

            return out.toString();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    private Document parseXmlFile(String in) {
        try {
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            DocumentBuilder db = dbf.newDocumentBuilder();
            InputSource is = new InputSource(new StringReader(in));
            return db.parse(is);
        } catch (ParserConfigurationException e) {
            throw new RuntimeException(e);
        } catch (SAXException e) {
            throw new RuntimeException(e);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(String[] args) {
        String unformattedXml =
                "<?xml version="1.0" encoding="UTF-8"?><QueryMessagen" +
                        "        xmlns="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message"n" +
                        "        xmlns:query="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/query">n" +
                        "    <Query>n" +
                        "        <query:CategorySchemeWhere>n" +
                        "   ttttt         <query:AgencyID>ECBnnnn</query:AgencyID>n" +
                        "        </query:CategorySchemeWhere>n" +
                        "    </Query>nnnnn" +
                        "</QueryMessage>";

        System.out.println(new XmlFormatter().format(unformattedXml));
    }

}

---

基于这个答案的更简单的解决方案：

public static String prettyFormat(String input, int indent) {
    try {
        Source xmlInput = new StreamSource(new StringReader(input));
        StringWriter stringWriter = new StringWriter();
        StreamResult xmlOutput = new StreamResult(stringWriter);
        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        transformerFactory.setAttribute("indent-number", indent);
        Transformer transformer = transformerFactory.newTransformer(); 
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.transform(xmlInput, xmlOutput);
        return xmlOutput.getWriter().toString();
    } catch (Exception e) {
        throw new RuntimeException(e); // simple exception handling, please review it
    }
}

public static String prettyFormat(String input) {
    return prettyFormat(input, 2);
}

测试用例：

prettyFormat("<root><child>aaa</child><child/></root>");

收益：

<?xml version="1.0" encoding="UTF-8"?>
<root>
  <child>aaa</child>
  <child/>
</root>

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