Advanced Internet Technology Lab # 4 Servlets

Transcription

1 Faculty of Engineering Computer Engineering Department Islamic University of Gaza 2011 Advanced Internet Technology Lab # 4 Servlets Eng. Doaa Abu Jabal

2 Advanced Internet Technology Lab # 4 Servlets Objective: To be familiar with Servlet Concepts. Topics Eclipse installation and integration with Tomcat. Servlets basics. Life cycle of a servlet Eclipse Definition: Eclipse is an IDE for java basically is a project from IBM Co. It give a support to J2EE. Eclipse Installation: 1. Download eclipse Ganymade for J2EE developers from 2. Decompress.zip file 3. Run eclipse.exe 2

3 Integrate Eclipse and Tomcat: 1- Choose new from file menu then choose other from new submenu. 2- Choose server from server category 3

4 3- Choose Tomcat v6.0server from apache category 4- Determine location of Tomcat server 5- Finally press finish button 4

5 Create new Servlet: 1- Go to new menu another time and choose Dynamic web project. 2- Next then finish 5

6 3- Right click on created project 4- Then write your Servlet name and package and press finish 6

7 5- Congratulations it is your first Servlet Servlets Basics Basic Servlet Structure: import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class ServletTemplate extends HttpServlet { public void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException { // Use "request" to read incoming HTTP headers // (e.g., cookies) and query data from HTML forms. // Use "response" to specify the HTTP response status // code and headers (e.g., the content type, cookies). 7

8 PrintWriter out = response.getwriter(); // Use "out" to send content to browser. The previous code listing outlines a basic Servlet that handles GET requests. GET requests are the usual type of browser requests for Web pages. How to create Servlet? 1- Servlets must extending HttpServlet 2- Servlet must have public no argument constructor. 3- Override doxxx depending on whether the data is being sent by GET, POST doxxx take two arguments: 1- HttpServletRequest. 2- HttpServletResponse. HttpServletRequest lets you get at all of the incoming data HttpServletResponse lets you specify outgoing information and most importantly, HttpServletResponse lets you obtain a PrintWriter that you use to send document content back to the client. Since doxxx throw two exceptions: 1- ServletException 2- IOException You must including them in the method declaration. Finally, you must import the following classes: java.io (for PrintWriter, IOException etc.) javax.servlet (for ServletException) javax.servlet.http (for HttpServlet, HttpServletRequest and htttpservletresponse) Servlets Example 1: A servlet that generates plan text:- package servlet; import java.io.ioexception; import java.io.printwriter; import javax.servlet.servletexception; 8

9 import javax.servlet.http.httpservlet; import javax.servlet.http.httpservletrequest; import javax.servlet.http.httpservletresponse; /** * Servlet implementation class HelloServlet */ public class HelloServlet extends HttpServlet { /** HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */ protected void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub //response.setcontenttype("text/html"); //OutputStream os=response.getoutputstream(); PrintWriter writer=response.getwriter(); writer.print("hello World"); 9

10 Servlets Example 2: A Servlet that generates HTML:- package servlet; import java.io.ioexception; import java.io.printwriter; import javax.servlet.servletexception; import javax.servlet.http.httpservlet; import javax.servlet.http.httpservletrequest; import javax.servlet.http.httpservletresponse; /** * Servlet implementation class HelloHTMLServlet */ public class HelloHTMLServlet extends HttpServlet { /** HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */ protected void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter writer=response.getwriter(); writer.print("<html>"); writer.print("<body><br>"); writer.print("hello"+"<br>"); writer.print("<body>"); writer.print("</html>"); 10

11 Life Cycle of a Servlet Loaded: The Servlet is normally created when a user first invokes a URL corresponding to the Servlet. Servlet container create Servlets by using reflection API method Class.forName(ClassName).newInstance(). Initialized: When the Servlet is created, its init method will be invoked, so init is where you put one-time setup code. Servicing: After this, each user request results in a thread that calls the service method of the previously created instance. Multiple concurrent requests normally result in multiple threads calling service simultaneously. The service method then calls doget, dopost, or another doxxx method, depending on the type of HTTP request it received. Destroyed: Finally, if the server decides to unload a Servlet, it first calls the Servlet s destroy method Unloaded: after calling destroy method Servlet container remove all referencing to the Servlet then garbage collector will collect Servlet object and remove it from the memory. 11

12 Init method The init method is called when the Servlet is first requested by a browser request. It is not called again for each request. Used for initialization. Example 1: when Servlet was born: package lab4; import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class Birth extends HttpServlet { Date birthdate; // Init is called first public void init() throws ServletException { birthdate = new Date(); // Handle an HTTP GET Request public void doget(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { response.setcontenttype("text/plain"); PrintWriter out = response.getwriter(); out.println ("I was born on: "+birthdate); out.close(); 12

13 Service Method Only one instance of the Servlet is instantiated. This one instance handles all browser requests. Each time the server receives a request for a Servlet; the server spawns a new thread and calls the servlet s service method. By default the service method checks the HTTP Header. Based on the header, service calls either dopost or doget. dopost and doget is where you put the majority of your code. If your Servlets needs to handle both get and post identically, have your dopost method call doget or vice versa. It s recommended to only override the doxxx methods not service method. Example 2: counter Servlet The Counter Servlet keeps track of the number of times it has been accessed. This example maintains a single instance variable, called count. Each time the Servlet is called, the count variable is incremented. If the Server created a new instance of the Servlet for each request, count would always be 0! package lab4; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Counter extends HttpServlet { // Create an instance variable 13

14 int count = 0; // Handle an HTTP GET Request public void doget(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { response.setcontenttype("text/plain"); PrintWriter out = response.getwriter(); count++; out.println ("Since loading, this servlet has " out.close(); + "been accessed "+ count + " times."); Destroy Method Before a server shuts down or Servlet unloaded, it will call the servlet s destroy method. You can handle any Servlet clean up here. For example: Updating log files, Closing database connections, and Closing any socket connections. Example3- I am alive While alive, the Servlet will say I am alive! When the server is stopped, the destroy method is called, and the Servlet records its time of death in a log.txt text file. package lab4; import java.io.*; import java.util.*; 14

15 import javax.servlet.*; import javax.servlet.http.*; public class Death extends HttpServlet { // Handle an HTTP GET Request public void doget(httpservletrequest request, HttpServletResponse response) throws IOException, ServletException { response.setcontenttype("text/plain"); PrintWriter out = response.getwriter(); out.println ("I am alive!"); out.close(); // This method is called when one stops // the Java Web Server public void destroy() { try { FileWriter filewriter = new FileWriter ("c:\\rip.txt"); Date now = new Date(); String rip = "I was destroyed at: "+now.tostring(); filewriter.write (rip); filewriter.close(); catch (IOException e) { e.printstacktrace(); 15