Servlets Servlets are modules that extend the functionality of a “java-enabled” web-server They normally generate HTML code and web content dynamically. This is sent to the browser which displays it. For example, they send a query to a database based on parameters sent by the browser and send the results to the browser in html format

Development Environments There are many good development environments which help to write and test the servlets They include an editor and a java-enabled sever They also include all the necessary jar files an import statements Some of them are Eclipse (need to download plugins) and Netbeans (which also has full j2ee support

Anatomy of a Servlet A new servlet can be written by extending the HttpServlet class which has the following pre-defined methods init() is called when the servlet is “uploaded” the first time (this can vary depending on the server) doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException is called every time the servlet is contacted by a GET request (which is the default way) doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException is called when the client contacted the servlet with a POST request

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class MyServlet extends HttpServlet { public void init() { //Sobreescribir para que haga lo que queramos } public void doGet ( HttpServletRequestrequest, HttpServletResponse response) throws ServletException, IOException { //Sobreescribir para que haga lo que queramos } public void doPost( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //Sobreescribir para que haga lo que queramos }

The HttpServletRequest Parameter HttpServletRequest is the class of the first parameter the server uses to calls doGet and doPost. It gives access to: –Information about the client, for example, parameters passed, protocol used, client’s host, etc. –The input stream, ServletInputStream is used by the servlet to receive data from the client when the method POST or PUT has been used.

The HttpServletResponse parameter HttpServletResponse is the class of the second argument. Provides methods for : –Declaring the MIME type of the answer that will be sent to the client –Getting the output stream ServletOutputStream and a Writer through which the servlet can send dinamically generated html code to the browser. –Sending other information to the browser (cookies, refreshment time, etc…)

Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequestrequest, HttpServletResponse response) throws ServletException, IOException { // set content type response.setContentType("text/html"); // open print writer to browser PrintWriter out = response.getWriter(); //send data out.println(" ") out.println(" Mi Primer Servlet "); out.println(" Fecha y hora: "+(new Date())+" "); out.println(" "); out.close(); }

Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequestrequest, HttpServletResponse response) throws ServletException, IOException { // set content type response.setContentType("text/html"); // open print writer to browser PrintWriter out = response.getWriter(); //send data out.println(" ") out.println(" Mi Primer Servlet "); out.println(" Fecha y hora: "+(new Date())+" "); out.println(" "); out.close(); } Imports necessary classes This is for the Date class

Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequestrequest, HttpServletResponse response) throws ServletException, IOException { // set content type response.setContentType("text/html"); // open print writer to browser PrintWriter out = response.getWriter(); //send data out.println(" ") out.println(" Mi Primer Servlet "); out.println(" Fecha y hora: "+(new Date())+" "); out.println(" "); out.close(); } Every servlet extends HttpServlet Overwrites doGet method

Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequestrequest, HttpServletResponse response) throws ServletException, IOException { // set content type response.setContentType("text/html"); // open print writer to browser PrintWriter out = response.getWriter(); //send data out.println(" ") out.println(" Mi Primer Servlet "); out.println(" Fecha y hora: "+(new Date())+" "); out.println(" "); out.close(); } Tells the browser the content type of the answer Gets writer to browser from response parameter

Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequestrequest, HttpServletResponse response) throws ServletException, IOException { // set content type response.setContentType("text/html"); // open print writer to browser PrintWriter out = response.getWriter(); //send data out.println(" ") out.println(" Mi Primer Servlet "); out.println(" Fecha y hora: "+(new Date())+" "); out.println(" "); out.close(); } Get date and time from system Print data to browser Close connection to browser

Running the first example Writing a servlet with Netbeans is very easy Also the deployment is done automatically –Open netbeans –Create a web project (this will create a lot of directories for putting the different kind of files) –Create a servlet –Copy the code of SimpleServlet.java –Run the file

A second example Implementing a web counter It will count how many times an object of this class has been creates It will show the Address of the computer that contacted the servlet It will show a

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Count extends HttpServlet { int count = 0; // a counter starts in 0 public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter out = res.getWriter(); res.setContentType("text/html"); out.println(" A web page counter "); out.println(" "); out.println("This servlet was accessed "+count+" time(s)"); out.println(" "); out.println("Your computer is "+req.getRemoteHost()); out.println(" "); out.close(); }

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Count extends HttpServlet { int count = 0; // a counter starts in 0 public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter out = res.getWriter(); res.setContentType("text/html"); out.println(" A web page counter "); out.println(" "); out.println("This servlet was accessed "+count+" time(s)"); out.println(" "); out.println("Your computer is "+req.getRemoteHost()); out.println(" "); out.close(); }

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Count extends HttpServlet { int count = 0; // a counter starts in 0 public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter out = res.getWriter(); res.setContentType("text/html"); out.println(" A web page counter "); out.println(" "); out.println("This servlet was accessed "+count+" time(s)"); out.println(" "); out.println("Your computer is "+req.getRemoteHost()); out.println(" "); out.close(); } Increments counter every time doGet is called by the web server

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Count extends HttpServlet { int count = 0; // a counter starts in 0 public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter out = res.getWriter(); res.setContentType("text/html"); out.println(" A web page counter "); out.println(" "); out.println("This servlet was accessed "+count+" time(s)"); out.println(" "); out.println("Your computer is "+req.getRemoteHost()); out.println(" "); out.close(); } Sine Qua Non

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class Count extends HttpServlet { int count = 0; // a counter starts in 0 public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter out = res.getWriter(); res.setContentType("text/html"); out.println(" A web page counter "); out.println(" "); out.println("This servlet was accessed "+count+" time(s)"); out.println(" "); out.println("Your computer is "+req.getRemoteHost()); out.println(" "); out.close(); } Print data to browser

What happens if the server crashes and starts again ? The counter will start from 0 again To “remember” the value of the counter in cast of an unexpected crash, we will write the value of the variable in a file every time it changes. At the beginning, the servlet reads the initial value from a file, if it exists, or creates the file with the initial value = 0

public class Count extends HttpServlet { int count = 0; // a counter for the object public void init() { try { BufferedReader in = new BufferedReader( newFileReader(„count.txt“)); String l = in.readLine(); count = Integer.parseInt(l); } catch (FileNotFoundException e) { //no need to do anything here } public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter outFile = new PrintWriter( new Filereader(„count.txt“)); outFile.println(count); outFile.close(); PrintWriter outBrowser = res.getWriter(); res.setContentType("text/html"); outBrowser.println(" A web page counter "); outBrowser.println(" ");..... }

public class Count extends HttpServlet { int count = 0; // a counter for the object public void init() { try { BufferedReader in = new BufferedReader( newFileReader(„count.txt“)); String l = in.readLine(); count = Integer.parseInt(l); } catch (FileNotFoundException e) { //no need to do anything here } public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter outFile = new PrintWriter( new Filereader(„count.txt“)); outFile.println(count); outFile.close(); PrintWriter outBrowser = res.getWriter(); res.setContentType("text/html"); outBrowser.println(" A web page counter "); outBrowser.println(" ");..... } Try to open the file when the servlet is called the first time

public class Count extends HttpServlet { int count = 0; // a counter for the object public void init() { try { BufferedReader in = new BufferedReader( newFileReader(„count.txt“)); String l = in.readLine(); count = Integer.parseInt(l); } catch (FileNotFoundException e) { //no need to do anything here } public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter outFile = new PrintWriter( new Filereader(„count.txt“)); outFile.println(count); outFile.close(); PrintWriter outBrowser = res.getWriter(); res.setContentType("text/html"); outBrowser.println(" A web page counter "); outBrowser.println(" ");..... } Read the line and convert the content to its integer value

public class Count extends HttpServlet { int count = 0; // a counter for the object public void init() { try { BufferedReader in = new BufferedReader( newFileReader(„count.txt“)); String l = in.readLine(); count = Integer.parseInt(l); } catch (FileNotFoundException e) { //no need to do anything here } public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter outFile = new PrintWriter( new Filereader(„count.txt“)); outFile.println(count); outFile.close(); PrintWriter outBrowser = res.getWriter(); res.setContentType("text/html"); outBrowser.println(" A web page counter "); outBrowser.println(" ");..... }

public class Count extends HttpServlet { int count = 0; // a counter for the object public void init() { try { BufferedReader in = new BufferedReader( newFileReader(„count.txt“)); String l = in.readLine(); count = Integer.parseInt(l); } catch (FileNotFoundException e) { //no need to do anything here } public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter outFile = new PrintWriter( new Filereader(„count.txt“)); outFile.println(count); outFile.close(); PrintWriter outBrowser = res.getWriter(); res.setContentType("text/html"); outBrowser.println(" A web page counter "); outBrowser.println(" ");..... } After count is incremented, open the file to write (overwrite), write the new number and close file

public class Count extends HttpServlet { int count = 0; // a counter for the object public void init() { try { BufferedReader in = new BufferedReader( newFileReader(„count.txt“)); String l = in.readLine(); count = Integer.parseInt(l); } catch (FileNotFoundException e) { //no need to do anything here } public void doGet ( HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { count++; PrintWriter outFile = new PrintWriter( new Filereader(„count.txt“)); outFile.println(count); outFile.close(); PrintWriter outBrowser = res.getWriter(); res.setContentType("text/html"); outBrowser.println(" A web page counter "); outBrowser.println(" ");..... } The rest is the same

Exercise Escriba un servlet que genere un número aleatorio entre 1 y 100 cada vez que es contactado y lo muestra al cliente junto con la IP del computador del cliente Además muestra el número más grande generado hasta ahora y la IP del computador para el cual lo generó La dirección del computador del cliente puede ser obtenida String s = request.getRemoteHost(); –returns the IP number (like String s = request.getRemoteAddress(); –returns the name if it can be retrieved (like Un número aleatorio entre 1 y 100 se genera con la siguiente instrucción en Java int numero = (int)(1 + Math.random()*100);

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class NewServlet extends HttpServlet { int maxNumber = 0; String maxIP = ""; public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { int aleatorio = (int)(1 + Math.random()*100); String ip = request.getRemoteAddr(); if (aleatorio > maxNumber) { maxNumber = aleatorio; maxIP = ip; } response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(" Bienvenido "); out.println(" Su dirección IP es "+ ip+ " "); out.println("Su número es "+aleatorio+" "); out.println("El numero mayor generado es "+maxNumber); out.println(" Fue generado para "+maxIP); out.close(); }

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class NewServlet extends HttpServlet { int maxNumber = 0; String maxIP = ""; public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { int aleatorio = (int)(1 + Math.random()*100); String ip = request.getRemoteAddr(); if (aleatorio > maxNumber) { maxNumber = aleatorio; maxIP = ip; } response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(" Bienvenido "); out.println(" Su dirección IP es "+ ip+ " "); out.println("Su número es "+aleatorio+" "); out.println("El numero mayor generado es "+maxNumber); out.println(" Fue generado para "+maxIP); out.close(); } Inicializacion variables para guardar La IP y el valor del máximo

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class NewServlet extends HttpServlet { int maxNumber = 0; String maxIP = ""; public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { int aleatorio = (int)(1 + Math.random()*100); String ip = request.getRemoteAddr(); if (aleatorio > maxNumber) { maxNumber = aleatorio; maxIP = ip; } response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(" Bienvenido "); out.println(" Su dirección IP es "+ ip+ " "); out.println("Su número es "+aleatorio+" "); out.println("El numero mayor generado es "+maxNumber); out.println(" Fue generado para "+maxIP); out.close(); } Generación número aleatorio Obtencion ip del cliente

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class NewServlet extends HttpServlet { int maxNumber = 0; String maxIP = ""; public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { int aleatorio = (int)(1 + Math.random()*100); String ip = request.getRemoteAddr(); if (aleatorio > maxNumber) { maxNumber = aleatorio; maxIP = ip; } response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(" Bienvenido "); out.println(" Su dirección IP es "+ ip+ " "); out.println("Su número es "+aleatorio+" "); out.println("El numero mayor generado es "+maxNumber); out.println(" Fue generado para "+maxIP); out.close(); } Actualización de los datos del máximo generado

import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class NewServlet extends HttpServlet { int maxNumber = 0; String maxIP = ""; public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { int aleatorio = (int)(1 + Math.random()*100); String ip = request.getRemoteAddr(); if (aleatorio > maxNumber) { maxNumber = aleatorio; maxIP = ip; } response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(" Bienvenido "); out.println(" Su dirección IP es "+ ip+ " "); out.println("Su número es "+aleatorio+" "); out.println("El numero mayor generado es "+maxNumber); out.println(" Fue generado para "+maxIP); out.close(); } Enviar info al cliente