Null Pointer Exception - Servlet

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>Jena</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  
  <servlet>
  	<servlet-name>srvGrafo</servlet-name>
	<servlet-class>servlets.CtrlGrafo</servlet-class>
  </servlet>
	<servlet-mapping>
		<servlet-name>srvGrafo</servlet-name>
		<url-pattern>/chamaGrafo</url-pattern>
	</servlet-mapping>
		
	
</web-app>

public class CtrlGrafo extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		int no;
		String pNo,predicado, literal;
		pNo=request.getParameter("no");
		no=Integer.parseInt(pNo);
		predicado=request.getParameter("predicado");
		literal=request.getParameter("literal");
		Grafo gr = new Grafo();
		gr.setLiteral(literal);
		gr.setNo(no);
		gr.setPredicado(predicado);
		ModelaGrafo model = new ModelaGrafo();
		model.adicionar(gr);
		
	}

}

java.lang.ClassNotFoundException: servlets.CtrlGrafo
	at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1645)
	at org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1491)
	at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1095)
	at org.apache.catalina.core.StandardWrapper.allocate(StandardWrapper.java:809)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:129)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
	at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
	at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
	at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
	at java.lang.Thread.run(Unknown Source)

<form onsubmit="(mensagem())" action="chamaGrafo" method="post">