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();

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();

}

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;
        }
    }

}

只是另一种适合我们的解决方法

import java.io.StringWriter;
import org.dom4j.DocumentHelper;
import org.dom4j.io.OutputFormat;
import org.dom4j.io.XMLWriter;

**
 * Pretty Print XML String
 * 
 * @param inputXmlString
 * @return
 */
public static String prettyPrintXml(String xml) {

    final StringWriter sw;

    try {
        final OutputFormat format = OutputFormat.createPrettyPrint();
        final org.dom4j.Document document = DocumentHelper.parseText(xml);
        sw = new StringWriter();
        final XMLWriter writer = new XMLWriter(sw, format);
        writer.write(document);
    }
    catch (Exception e) {
        throw new RuntimeException("Error pretty printing xml:\n" + xml, e);
    }
    return sw.toString();
}

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));
    }
}

我过去使用org.dom4j.io.OutputFormat.createPrettyPrint()方法打印过

public String prettyPrint(final String xml){  

    if (StringUtils.isBlank(xml)) {
        throw new RuntimeException("xml was null or blank in prettyPrint()");
    }

    final StringWriter sw;

    try {
        final OutputFormat format = OutputFormat.createPrettyPrint();
        final org.dom4j.Document document = DocumentHelper.parseText(xml);
        sw = new StringWriter();
        final XMLWriter writer = new XMLWriter(sw, format);
        writer.write(document);
    }
    catch (Exception e) {
        throw new RuntimeException("Error pretty printing xml:\n" + xml, e);
    }
    return sw.toString();
}