Why getRequestDispatcher method cannot accept full URL?

Question

Examples*
This code works well:
public void doPost(HttpServletRequest request, HttpServletResponse response)
               throws ServletException, IOException {
          String url = "/jsp/pageStart.jsp";
                /* requestDispatcher is NOT null */
          RequestDispatcher requestDispatcher = getServletContext()
                    .getRequestDispatcher(url);
          requestDispatcher.forward(request, response);
     }This code doesn't work, get NullPointerException, requestDispatcher is null:
public void doPost(HttpServletRequest request, HttpServletResponse response)
               throws ServletException, IOException {
          String url = "http://localhost:8080/mymvc/jsp/pageStart.jsp";
                /* requestDispatcher is null */
          RequestDispatcher requestDispatcher = getServletContext()
                    .getRequestDispatcher(url)
          requestDispatcher.forward(request, response);
Notes*
- JSP path is \src\main\webapp\jsp\pageStart.jsp
- I am sure, I can manually open 'http://localhost:8080/mymvc/jsp/pageStart.jsp', just copy and paste this url into address of new browser window.
- Base url is http://localhost:8080/mymvc/servlet/ControllerServlet
- I use Servlet mapping:
<servlet-mapping>
     <servlet-name>ControllerServlet</servlet-name>
     <url-pattern>/servlet/*</url-pattern>
</servlet-mapping>
<servlet>
     <servlet-name>ControllerServlet</servlet-name>
     <servlet-class>controller.ControllerServlet</servlet-class>
</servlet>
Questions*
1. Why getRequestDispatcher method cannot accept full URL?
2. Could you please explain to me the reason why ? and please provide me the better resolutions.
Thanks u in advance!

Answer 1

As per Java API documentation:
ServletContext#getRequestDispatcher(String path)
The pathname must begin with a "/" and is interpreted as relative to the current context root.
For details: [http://java.sun.com/javaee/5/docs/api/javax/servlet/ServletContext.html#getRequestDispatcher(java.lang.String)|http://java.sun.com/javaee/5/docs/api/javax/servlet/ServletContext.html#getRequestDispatcher(java.lang.String)]
ServletRequest.html#getRequestDispatcher(java.lang.String)
To allow RequestDispatcher objects to be obtained using relative paths that are relative to the path of the current request (not relative to the root of the ServletContext), the getRequestDispatcher method is provided in the ServletRequest interface. The pathname specified may be relative, although it cannot extend outside the current servlet context.
For details : [http://java.sun.com/javaee/5/docs/api/javax/servlet/ServletRequest.html#getRequestDispatcher(java.lang.String)|http://java.sun.com/javaee/5/docs/api/javax/servlet/ServletRequest.html#getRequestDispatcher(java.lang.String)]