WebContainer

Tomcat Jar

API	Jar
J2EE specifications	servlet-api.jar
Implementation	catalina.jar

Inheritance hierarchy Request Object

• ServletRequest(Interface)

  • HttpServletRequest(Interface)

    • HttpServletRequestWrapper(Class)

Similarly, for Response Object, i.e., replace Request with Response.

How web Container calls service method?

• Web container interacts with public service(rq, rs) which internally calls the protected service(rq,rs) of the HttpServlet class.

  /* No need to override this */
  public void service(ServletRequest req,ServletResponse res) throws ServletException, IOException

  /* No need to override this */
  protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException

  /*
    Internally, it calls protected do*** method of 
    abstract HttpServlet Class
  */

Servlet - JDBC Integration

• Layers involved :

     
      Servlet ↴
        DAO (Uses DBUtils) ↴
                         POJO ↴
                              Database

Note: JDBC Jar can be placed either in <WEB-INF>/lib folder in IDE created project structure OR <tomcat>/lib folder under tomcat installation folder.

ServletException

  
  /*Checked Exception*/
  javax.servlet.ServletException
  /*Preferred exception constructor*/
  public ServletException(String message, Throwable rootCause);

• Defines a general exception a servlet can throw when it encounters difficulty.

• During Initialization by init() style method always throw Serlvet Exception to let Web Container know some Exception(anamoly) has occured.

• Similarly, in destroy() style method always catch checked exception and convert it into unchecked Exception to let the web container know that some exception occured during destruction of Web Servlet.

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