现在已经是2012年了，Java可以比以前的XML做更多的事情，我想在我公认的答案之外添加一个替代方案。这在Java 6之外没有依赖关系。

import org.w3c.dom.Node;
import org.w3c.dom.bootstrap.DOMImplementationRegistry;
import org.w3c.dom.ls.DOMImplementationLS;
import org.w3c.dom.ls.LSSerializer;
import org.xml.sax.InputSource;

import javax.xml.parsers.DocumentBuilderFactory;
import java.io.StringReader;

/**
 * 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 String format(String xml) {

        try {
            final InputSource src = new InputSource(new StringReader(xml));
            final Node document = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(src).getDocumentElement();
            final Boolean keepDeclaration = Boolean.valueOf(xml.startsWith("<?xml"));

        //May need this: System.setProperty(DOMImplementationRegistry.PROPERTY,"com.sun.org.apache.xerces.internal.dom.DOMImplementationSourceImpl");


            final DOMImplementationRegistry registry = DOMImplementationRegistry.newInstance();
            final DOMImplementationLS impl = (DOMImplementationLS) registry.getDOMImplementation("LS");
            final LSSerializer writer = impl.createLSSerializer();

            writer.getDomConfig().setParameter("format-pretty-print", Boolean.TRUE); // Set this to true if the output needs to be beautified.
            writer.getDomConfig().setParameter("xml-declaration", keepDeclaration); // Set this to true if the declaration is needed to be outputted.

            return writer.writeToString(document);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    public static void main(String[] args) {
        String unformattedXml =
                "<?xml version=\"1.0\" encoding=\"UTF-8\"?><QueryMessage\n" +
                        "        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" +
                        "   \t\t\t\t\t         <query:AgencyID>ECB\n\n\n\n</query:AgencyID>\n" +
                        "        </query:CategorySchemeWhere>\n" +
                        "    </Query>\n\n\n\n\n" +
                        "</QueryMessage>";

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

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();

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)));

如果使用第三方XML库是可行的，那么您可以使用一些比目前票数最高的答案所建议的要简单得多的方法。

它声明输入和输出都应该是字符串，所以这里有一个实用程序方法，用XOM库实现:

import nu.xom.*;
import java.io.*;

[...]

public static String format(String xml) throws ParsingException, IOException {
    ByteArrayOutputStream out = new ByteArrayOutputStream();
    Serializer serializer = new Serializer(out);
    serializer.setIndent(4);  // or whatever you like
    serializer.write(new Builder().build(xml, ""));
    return out.toString("UTF-8");
}

我对它进行了测试，结果不依赖于JRE版本或类似的东西。要了解如何根据自己的喜好定制输出格式，请查看Serializer API。

这实际上比我想象的要长——需要一些额外的行，因为Serializer想要写入一个OutputStream。但是请注意，这里很少有用于实际XML处理的代码。

(这个答案是我对XOM的评估的一部分，在我关于替代dom4j的最佳Java XML库的问题中，XOM被建议作为一个选项。在dom4j中，您可以使用XMLWriter和OutputFormat轻松实现这一点。编辑:…正如mlo55的答案所示。)

为了将来的参考，这里有一个对我有用的解决方案(感谢@George Hawkins在其中一个答案中发表的评论):

DOMImplementationRegistry registry = DOMImplementationRegistry.newInstance();
DOMImplementationLS impl = (DOMImplementationLS) registry.getDOMImplementation("LS");
LSSerializer writer = impl.createLSSerializer();
writer.getDomConfig().setParameter("format-pretty-print", Boolean.TRUE);
LSOutput output = impl.createLSOutput();
ByteArrayOutputStream out = new ByteArrayOutputStream();
output.setByteStream(out);
writer.write(document, output);
String xmlStr = new String(out.toByteArray());

I have found that in Java 1.6.0_32 the normal method to pretty print an XML string (using a Transformer with a null or identity xslt) does not behave as I would like if tags are merely separated by whitespace, as opposed to having no separating text. I tried using <xsl:strip-space elements="*"/> in my template to no avail. The simplest solution I found was to strip the space the way I wanted using a SAXSource and XML filter. Since my solution was for logging I also extended this to work with incomplete XML fragments. Note the normal method seems to work fine if you use a DOMSource but I did not want to use this because of the incompleteness and memory overhead.

public static class WhitespaceIgnoreFilter extends XMLFilterImpl
{

    @Override
    public void ignorableWhitespace(char[] arg0,
                                    int arg1,
                                    int arg2) throws SAXException
    {
        //Ignore it then...
    }

    @Override
    public void characters( char[] ch,
                            int start,
                            int length) throws SAXException
    {
        if (!new String(ch, start, length).trim().equals("")) 
               super.characters(ch, start, length); 
    }
}

public static String prettyXML(String logMsg, boolean allowBadlyFormedFragments) throws SAXException, IOException, TransformerException
    {
        TransformerFactory transFactory = TransformerFactory.newInstance();
        transFactory.setAttribute("indent-number", new Integer(2));
        Transformer transformer = transFactory.newTransformer();
        transformer.setOutputProperty(OutputKeys.INDENT, "yes");
        transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");
        StringWriter out = new StringWriter();
        XMLReader masterParser = SAXHelper.getSAXParser(true);
        XMLFilter parser = new WhitespaceIgnoreFilter();
        parser.setParent(masterParser);

        if(allowBadlyFormedFragments)
        {
            transformer.setErrorListener(new ErrorListener()
            {
                @Override
                public void warning(TransformerException exception) throws TransformerException
                {
                }

                @Override
                public void fatalError(TransformerException exception) throws TransformerException
                {
                }

                @Override
                public void error(TransformerException exception) throws TransformerException
                {
                }
            });
        }

        try
        {
            transformer.transform(new SAXSource(parser, new InputSource(new StringReader(logMsg))), new StreamResult(out));
        }
        catch (TransformerException e)
        {
            if(e.getCause() != null && e.getCause() instanceof SAXParseException)
            {
                if(!allowBadlyFormedFragments || !"XML document structures must start and end within the same entity.".equals(e.getCause().getMessage()))
                {
                    throw e;
                }
            }
            else
            {
                throw e;
            }
        }
        out.flush();
        return out.toString();
    }