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());

请注意，排名靠前的答案需要使用xerces。

如果您不想添加这个外部依赖，那么您可以简单地使用标准jdk库(实际上是在内部使用xerces构建的)。

注意:jdk 1.5版本有一个bug，请参阅http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6296446，但现在已经解决了。

(注意，如果发生错误，将返回原始文本)

package com.test;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;

import javax.xml.transform.OutputKeys;
import javax.xml.transform.Source;
import javax.xml.transform.Transformer;
import javax.xml.transform.sax.SAXSource;
import javax.xml.transform.sax.SAXTransformerFactory;
import javax.xml.transform.stream.StreamResult;

import org.xml.sax.InputSource;

public class XmlTest {
    public static void main(String[] args) {
        XmlTest t = new XmlTest();
        System.out.println(t.formatXml("<a><b><c/><d>text D</d><e value='0'/></b></a>"));
    }

    public String formatXml(String xml){
        try{
            Transformer serializer= SAXTransformerFactory.newInstance().newTransformer();
            serializer.setOutputProperty(OutputKeys.INDENT, "yes");
            //serializer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
            serializer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
            //serializer.setOutputProperty("{http://xml.customer.org/xslt}indent-amount", "2");
            Source xmlSource=new SAXSource(new InputSource(new ByteArrayInputStream(xml.getBytes())));
            StreamResult res =  new StreamResult(new ByteArrayOutputStream());            
            serializer.transform(xmlSource, res);
            return new String(((ByteArrayOutputStream)res.getOutputStream()).toByteArray());
        }catch(Exception e){
            //TODO log error
            return xml;
        }
    }

}

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);
    }
}

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());

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scala:

import xml._
val xml = XML.loadString("<tag><nested>hello</nested></tag>")
val formatted = new PrettyPrinter(150, 2).format(xml)
println(formatted)

如果你依赖scala-library.jar，你也可以在Java中这样做。它是这样的:

import scala.xml.*;

public class FormatXML {
    public static void main(String[] args) {
        String unformattedXml = "<tag><nested>hello</nested></tag>";
        PrettyPrinter pp = new PrettyPrinter(150, 3);
        String formatted = pp.format(XML.loadString(unformattedXml), TopScope$.MODULE$);
        System.out.println(formatted);
    }
}

PrettyPrinter对象是用两个整数构造的，第一个是最大行长，第二个是缩进步骤。