This parser does not support specification "null" version "null"

If you start Tomcat 7 and got the error:

java.lang.UnsupportedOperationException: This parser does not support specification "null" version "null"

Just put the parameter below and restart Tomcat:
-Djavax.xml.parsers.SAXParserFactory=com.sun.org.apache.xerces.internal.jaxp.SAXParserFactoryImpl

This happens because you have two parsers in your classpath, and the Tomcat is getting a error loading the wrong version.

*This happened to me when I use Paypal SDK in Tomcat 7 and MyFaces.