下面的代码工作得很好

import javax.xml.transform.OutputKeys;
import javax.xml.transform.Source;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.stream.StreamSource;

String formattedXml1 = prettyFormat("<root><child>aaa</child><child/></root>");

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

public static String prettyFormat(String input, String indent) {
    Source xmlInput = new StreamSource(new StringReader(input));
    StringWriter stringWriter = new StringWriter();
    try {
        TransformerFactory transformerFactory = TransformerFactory.newInstance();
        Transformer transformer = transformerFactory.newTransformer();
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", indent);
        transformer.transform(xmlInput, new StreamResult(stringWriter));

        String pretty = stringWriter.toString();
        pretty = pretty.replace("\r\n", "\n");
        return pretty;              
    } catch (Exception e) {
        throw new RuntimeException(e);
    }
}

下面是一种使用dom4j的方法:

进口:

import org.dom4j.Document;  
import org.dom4j.DocumentHelper;  
import org.dom4j.io.OutputFormat;  
import org.dom4j.io.XMLWriter;

代码:

String xml = "<your xml='here'/>";  
Document doc = DocumentHelper.parseText(xml);  
StringWriter sw = new StringWriter();  
OutputFormat format = OutputFormat.createPrettyPrint();  
XMLWriter xw = new XMLWriter(sw, format);  
xw.write(doc);  
String result = sw.toString();

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:根据Java版本的不同，结果可能有所不同。搜索特定于您的平台的解决方案。

我总是使用下面的函数:

public static String prettyPrintXml(String xmlStringToBeFormatted) {
    String formattedXmlString = null;
    try {
        DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();
        documentBuilderFactory.setValidating(true);
        DocumentBuilder documentBuilder = documentBuilderFactory.newDocumentBuilder();
        InputSource inputSource = new InputSource(new StringReader(xmlStringToBeFormatted));
        Document document = documentBuilder.parse(inputSource);

        Transformer transformer = TransformerFactory.newInstance().newTransformer();
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");

        StreamResult streamResult = new StreamResult(new StringWriter());
        DOMSource dOMSource = new DOMSource(document);
        transformer.transform(dOMSource, streamResult);
        formattedXmlString = streamResult.getWriter().toString().trim();
    } catch (Exception ex) {
        StringWriter sw = new StringWriter();
        ex.printStackTrace(new PrintWriter(sw));
        System.err.println(sw.toString());
    }
    return formattedXmlString;
}

除了max、codeskrap、David Easley和milosmns给出的答案外，还可以看看我的轻量级、高性能漂亮打印机库:xml-formatter

// construct lightweight, threadsafe, instance
PrettyPrinter prettyPrinter = PrettyPrinterBuilder.newPrettyPrinter().build();

StringBuilder buffer = new StringBuilder();
String xml = ..; // also works with char[] or Reader

if(prettyPrinter.process(xml, buffer)) {
     // valid XML, print buffer
} else {
     // invalid XML, print xml
}

有时，就像直接从文件运行模拟SOAP服务时，有一个漂亮的打印机也能处理已经打印好的XML是很好的:

PrettyPrinter prettyPrinter = PrettyPrinterBuilder.newPrettyPrinter().ignoreWhitespace().build();

正如一些人评论的那样，漂亮打印只是一种以更适合人类阅读的形式表示XML的方法——严格来说，XML数据中不应该有空格。

该库用于日志记录的漂亮打印，还包括用于过滤(子树移除/匿名化)和漂亮打印CDATA和Text节点中的XML的函数。

稍微改进了milosmns的版本…

public static String getPrettyXml(String xml) {
    if (xml == null || xml.trim().length() == 0) return "";

    int stack = 0;
    StringBuilder pretty = new StringBuilder();
    String[] rows = xml.trim().replaceAll(">", ">\n").replaceAll("<", "\n<").split("\n");

    for (int i = 0; i < rows.length; i++) {
        if (rows[i] == null || rows[i].trim().length() == 0) continue;

        String row = rows[i].trim();
        if (row.startsWith("<?")) {
            pretty.append(row + "\n");
        } else if (row.startsWith("</")) {
            String indent = repeatString(--stack);
            pretty.append(indent + row + "\n");
        } else if (row.startsWith("<") && row.endsWith("/>") == false) {
            String indent = repeatString(stack++);
            pretty.append(indent + row + "\n");
            if (row.endsWith("]]>")) stack--;
        } else {
            String indent = repeatString(stack);
            pretty.append(indent + row + "\n");
        }
    }

    return pretty.toString().trim();
}

private static String repeatString(int stack) {
     StringBuilder indent = new StringBuilder();
     for (int i = 0; i < stack; i++) {
        indent.append(" ");
     }
     return indent.toString();
}