Java Web

Java Web

1. web application

   • deployable unit of content and configuration

2. deployment

   • 2 ways
     • copying files
     • deploy war

3. Servlet

   • must implement servlet interface
     • protocol agnostic (can use others tha HTTP)
   • Generic servlet class
     • HttpServlet class
       • contains
         • a request obj
         • a response obj
         • service method
         • 4 http methods
           • get response object as param
             • response can redirect
             • return string
             • return component
           • get request as param
             • get headers, parameter, body for POST etc
         • init method
   • servlet mapping(rounting)
     • notation @WebServlet("/home")
       • also can use @WebServlet(urlPatterns = {"", "*.something"})
     • mapping info held in web.xml
       • define 2 sections
         • servlet
           • creates a servlet and associated to a class
         • servlet mapping
           • bind an URL or URL patter to an existing servlet
     • try not to mix both approches
   • Initialization
     • override init method from generic servlet
       • use annotation
         • initParams = {@WebInitParam (name="", value="")}
       • override in web.xml
         • define values in xml, and bind in init()
   • ServletContext
     • container context app wide scope
       • all servlet in container have access
       • in order to used
         • define in web.xml a "context-param" tag
   • RequestDispatcher
     • a servlet catch request
       • then use request dispatcher to send it into other Servlet or JSP
     • Accessible from Servlet Context
       • RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/index.jsp")
         • use 1 of 3 methods available to pass beans
           • dispatchet.forward(request, response)

4. Java Server Pages

   • contain multiple lenguages
     • HTML
       • to define component
     • java
       • to define behavior
       • wrap it inside <% ... %>
   • is compiled
     • into a servlet(index_jsp.java)
       • extend HttpJspBase which
         • extends HttpServlet
       • here, HTML is replaced by its java counterpart
     • page translation time
       • when JSP is translated into java
         • and java is compiled into class file
   • life cycle (same as servlet)
     • init()
     • Destroy
     • Service
   • directives
     • include
       • <%@include file="" %>
         • used as a reference to add a splitted html fragment
     • <%@page..."%>
       • to modify page behavior
         • <%@page autoFlush="" "%>
         • <%@page errorPage="" "%>
         • <%@page buffer="8kb" "%>
         • <%@page isErrorPage="true" "%>
           • give us access to exception element
             • <%= exception.getMessage() %> and more error info avai.
     • define behavior on run time of the page
   • handling exception
     • define in web.xml an <error-page> specifing the JSP path
       • throw new exception
   • implicit objects
     • request
     • response
     • objects created by the web container for each jsp page

5. MVC

   • Model = POJO or Beans
     • May contain logic or just data
     • should be stored in a context to access it
       • globally so any JSP can access it
         • getServletContext().setAttribute(name, object);
       • inside a session so only an specific user has access
         • request.getSession.setAttribute("user", user)
       • passed through Request Dispatcher
         • request.setAttribute("user", user)
     • properties accessibility
       • dot object.property
       • [] : object["property"]
   • Controller = Servlet or Listeners
   • View = JSP

6. expression language

   • language simpler than java tag
     • used inside html tag
       • ${user.name}
     • could be used to replace java code tags
   • not need to specify scope
     • it look in all scopes for the object
       • can use this technique to assing dinamically styles
     • has access to implicit objects
   • operators
     • mathematical
     • logical
     • empty
       • validate the value of a var

7. Java Standard Tag Library

   • provide encapsulation of UI logic
   • set of standard tags used in JSP
     • individual tags are java classes
     • tag metadata held in Tag Library Descriptor file
     • wrapped inside a JAR
   • in order to use it, need to import the tag core
     • <% taglib uri="http://java.sun.com/jstl/core" prefix="c" %>
       • then can use it throug: <c::tag_name value="" >
   • core tag
     • conditional tags
     • formatting tags
     • SQL tags
   • can use Exp language to validate conditions

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