急！急!!如何验证xml文件的有效性？

如何验证xml文件的有效性?
请求帮助，谢谢

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Java code:

////XSDTest.java:
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.DocumentBuilder;
import org.xml.sax.ErrorHandler;
import org.xml.sax.SAXParseException;
import org.w3c.dom.Document;

public class XSDTest implements ErrorHandler
{
public static void main(String[] args)
{
DocumentBuilderFactory docBuilderFactory = null;
DocumentBuilder docBuilder = null;

try
{
docBuilderFactory = DocumentBuilderFactory.newInstance();
docBuilderFactory.setValidating(true);
docBuilderFactory.setNamespaceAware(true);
docBuilder = docBuilderFactory.newDocumentBuilder();
docBuilder.setErrorHandler(new XSDTest());
Document doc = docBuilder.parse("WellFormed.xml");
}
catch(Exception se)
{
se.printStackTrace();
}
}

public void error(SAXParseException se)
{
System.out.println("Caught an error:" + se.getMessage());
//se.printStackTrace();
}

public void fatalError(SAXParseException se)
{
System.out.println("Caught an fatal error.");
se.printStackTrace();
}