Error: java.lang.NoClassDefFoundError: org/jaxen/JaxenException

Error when tomcat is running:

java.lang.NoClassDefFoundError: org/jaxen/JaxenException
    at org.dom4j.DocumentFactory.createXPath(DocumentFactory.java:230)
    at org.dom4j.tree.AbstractNode.createXPath(AbstractNode.java:207)
    at org.dom4j.tree.AbstractNode.selectSingleNode(AbstractNode.java:183)

the code is

Element element=(Element) document.selectSingleNode("//user[@username='"+username+"' and @password='"+password+"']");

The main reason is:

When parsing XML with dom4j, the following two packages are required:
dom4j-1.6.1.jar
jaxen-1.1.2.jar

I only imported dom4j-1.6.1.jar

Later re-import jaxen-1.1.2.jar

the problem is gone