Directories and DDs Jan 13 2019 Web apps A web application is basically a web site that A web application can consist of multiple pieces Knows who you are it doesn t just give you static pages it interacts with you Can permanently change data such as in a database Static web pages possibly containing forms Servlets JSP Tomcat organizes all these parts into a single directory structure for each web application but you have to help with the organization 2 Directories To create servlets you really should have two directory structures Tomcat requires a particular set of directories for your web application A development directory in which you can write and partially debug your code A deployment directory in which you put live code It is extremely picky about having everything in the right place Since your web application must typically co exist with other web applications you should use packages to avoid name conflicts This further complicates the Tomcat directory structure 3 Packages A package statement in Java must be the very first line of code in the file Example package com example model import javax servlet import javax servlet http import java io public class MyServlet extends HttpServlet This implies that This program is in a file named MyServlet java which is in a directory named model which is in a directory named example which is in a directory named com That is the file is something com example model MyServlet java 4 Separation of concerns Business logic is the part of your application that does the actual computational work It is a good idea to separate the business logic of any application from the part that communicates with the outside world This allows you to use the same business logic as an applet a servlet an application or an API used by another program In the following examples we will assume The business logic is in the package com example model The servlet logic is in the package com example web 5 Tomcat directory structure jakarta tomcat 5 5 9 your Tomcat home directory whatever it s named webapps myApplicationDirectory your name for your application directory myWebForm html static HTML pages go in your application directory myJspResult jsp JSP pages also go in your application directory WEB INF must have this directory named exactly like this web xml this is the deployment descriptor it must have this name lib you can use this optional directory for external jar files classes must have this directory named exactly like this com The com example model package directory example web myServlet class in package com example web model myModel class in package com example model 6 My files myWebForm html This is the web page with a form that sends data to the servlet com example web myServlet class This is the servlet I intend to use it will use the myModel class but to do this it needs an import statement com example model myModel class This does the business logic it is good form to keep it separate myJspPage jsp import com example model myModel The optional JSP page to create the HTML output could be done directly by myServlet web xml A file required by Tomcat to tell it what class to start with and how to refer to that class 7 myWebForm html html body form method POST action NameSeenByUser do various form elements form body html 8 How Tomcat finds the servlet Tomcat reads XML files that tell it about the servlets Each web application must have a Deployment Descriptor DD file WEB INF web xml file The root element of the DD file is web app This element has a bunch of boilerplate attributes You don t have to know what any of it means The rest of the XML file gives names for the servlet provides parameters etc We ll cover only the most essential parts 9 Three names Every servlet has three names The real name given to it by the programmer An internal name used only within the web xml file The name that the user client knows it by The servlet element associates the fully qualified class name with the internal name The servlet mapping element associates the internal name with the name known to the client The reason for all this is to increase security by hiding the real name from the user 10 web xml xml version 1 0 encoding ISO 8859 1 web app xmlns http java sun com xml ns j2ee xmlns xsi http www w3 org 2001 XMLSchemainstance xsi schemaLocation http java sun com xml ns j2ee web app 2 4 xsd version 2 4 servlet servlet name Some internal name servlet name servlet class com example web MyServlet servletclass servlet servlet mapping servlet name Some internal name servlet name url pattern NameSeenByUser do url pattern servlet mapping 11 The Servlet public class MyServlet extends HttpServlet public void doPost HttpServletRequest request HttpServletResponse response throws IOException ServletException response setContentType text html PrintWriter out response getWriter String value request getParameter name out println html body I got name 12 Flow The user submits an HTML form Tomcat finds the servlet based on the URL and the deployment descriptor web xml and passes the request to the servlet The servlet computes a response Either Or The servlet writes an HTML page containing the response The servlet forwards the response to the JSP The JSP embeds the response in an HTML page Tomcat returns the HTML page to the user 13 The End 14
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