如果您确信您有一个有效的XML，那么这个很简单，并且避免了XML DOM树。可能有一些错误，如果你看到任何错误，请评论

public String prettyPrint(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("<?")) {
                            // xml version tag
                            pretty.append(row + "\n");
                    } else if (row.startsWith("</")) {
                            // closing tag
                            String indent = repeatString("    ", --stack);
                            pretty.append(indent + row + "\n");
                    } else if (row.startsWith("<")) {
                            // starting tag
                            String indent = repeatString("    ", stack++);
                            pretty.append(indent + row + "\n");
                    } else {
                            // tag data
                            String indent = repeatString("    ", stack);
                            pretty.append(indent + row + "\n");
                    }
            }

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

基于这个答案的一个更简单的解决方案:

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);
        transformerFactory.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
        transformerFactory.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
        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);
}

testcase:

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

返回:

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

//忽略:原始编辑只需要在代码中的类名中缺少s。为了在SO上获得超过6个字符的验证，添加了多余的6个字符

使用jdom2: http://www.jdom.org/

import java.io.StringReader;
import org.jdom2.input.SAXBuilder;
import org.jdom2.output.Format;
import org.jdom2.output.XMLOutputter;

String prettyXml = new XMLOutputter(Format.getPrettyFormat()).
                         outputString(new SAXBuilder().build(new StringReader(uglyXml)));

有一个非常好的命令行XML实用程序叫做xmlstarlet(http://xmlstar.sourceforge.net/)，它可以做很多事情，很多人都在使用它。

您可以使用Runtime以编程方式执行此程序。然后读入格式化的输出文件。它具有比几行Java代码所能提供的更多选项和更好的错误报告。

下载xmlstarlet: http://sourceforge.net/project/showfiles.php?group_id=66612&package_id=64589

我把它们混合在一起，写了一个小程序。它从xml文件中读取并打印出来。而不是xzy给出你的文件路径。

    public static void main(String[] args) throws Exception {
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    dbf.setValidating(false);
    DocumentBuilder db = dbf.newDocumentBuilder();
    Document doc = db.parse(new FileInputStream(new File("C:/Users/xyz.xml")));
    prettyPrint(doc);

}

private static String prettyPrint(Document document)
        throws TransformerException {
    TransformerFactory transformerFactory = TransformerFactory
            .newInstance();
    Transformer transformer = transformerFactory.newTransformer();
    transformer.setOutputProperty(OutputKeys.INDENT, "yes");
    transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
    transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
    transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");
    DOMSource source = new DOMSource(document);
    StringWriter strWriter = new StringWriter();
    StreamResult result = new StreamResult(strWriter);transformer.transform(source, result);
    System.out.println(strWriter.getBuffer().toString());

    return strWriter.getBuffer().toString();

}

我也有同样的问题，我用JTidy (http://jtidy.sourceforge.net/index.html)取得了巨大的成功。

例子:

Tidy t = new Tidy();
t.setIndentContent(true);
Document d = t.parseDOM(
    new ByteArrayInputStream("HTML goes here", null);

OutputStream out = new ByteArrayOutputStream();
t.pprint(d, out);
String html = out.toString();