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Servlets

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.

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Servlets

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  1. 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

  2. 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

  3. 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

  4. import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class MyServlet extends HttpServlet { public void init() { //Overwrite } public void doGet ( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Overwrite } public void doPost( HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //Overwrite } }

  5. 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.

  6. 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…)

  7. Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequest request, 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("<HTML>") out.println("<H1> Mi Primer Servlet </H1>"); out.println("<BR> <H2>Fecha y hora: "+(new Date())+"<H2>"); out.println("</HTML>"); out.close(); } }

  8. Example 1 Imports necessary classes This is for the Date class import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequest request, 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("<HTML>") out.println("<H1> Mi Primer Servlet </H1>"); out.println("<BR> <H2>Fecha y hora: "+(new Date())+"<H2>"); out.println("</HTML>"); out.close(); } }

  9. Example 1 Every servlet extends HttpServlet import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequest request, 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("<HTML>") out.println("<H1> Mi Primer Servlet </H1>"); out.println("<BR> <H2>Fecha y hora: "+(new Date())+"<H2>"); out.println("</HTML>"); out.close(); } } Overwrites doGet method

  10. Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequest request, 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("<HTML>") out.println("<H1> Mi Primer Servlet </H1>"); out.println("<BR> <H2>Fecha y hora: "+(new Date())+"<H2>"); out.println("</HTML>"); out.close(); } } Tells the browser the content type of the answer Gets writer to browser from response parameter

  11. Example 1 import java.io.*; import javax.servlet.*; import javax.servlet.http.*; import java.util.Date; public class SimpleServlet extends HttpServlet { public void doGet ( HttpServletRequest request, 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("<HTML>") out.println("<H1> Mi Primer Servlet </H1>"); out.println("<BR> <H2>Fecha y hora: "+(new Date())+"<H2>"); out.println("</HTML>"); out.close(); } } Print data to browser Get date and time from system Close connection to browser

  12. 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

  13. 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

  14. 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("<H1> A web page counter </H1>"); out.println("<HR>"); out.println("This servlet was accessed "+count+" time(s)"); out.println("<HR>"); out.println("Your computer is "+req.getRemoteHost()); out.println("<HR>"); out.close(); } }

  15. 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("<H1> A web page counter </H1>"); out.println("<HR>"); out.println("This servlet was accessed "+count+" time(s)"); out.println("<HR>"); out.println("Your computer is "+req.getRemoteHost()); out.println("<HR>"); out.close(); } }

  16. 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("<H1> A web page counter </H1>"); out.println("<HR>"); out.println("This servlet was accessed "+count+" time(s)"); out.println("<HR>"); out.println("Your computer is "+req.getRemoteHost()); out.println("<HR>"); out.close(); } } Increments counter every time doGet is called by the web server

  17. 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("<H1> A web page counter </H1>"); out.println("<HR>"); out.println("This servlet was accessed "+count+" time(s)"); out.println("<HR>"); out.println("Your computer is "+req.getRemoteHost()); out.println("<HR>"); out.close(); } } Sine Qua Non

  18. 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("<H1> A web page counter </H1>"); out.println("<HR>"); out.println("This servlet was accessed "+count+" time(s)"); out.println("<HR>"); out.println("Your computer is "+req.getRemoteHost()); out.println("<HR>"); out.close(); } } Print data to browser

  19. 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

  20. 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("<H1> A web page counter </H1>"); outBrowser.println("<HR>"); ..... ..... } }

  21. 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("<H1> A web page counter </H1>"); outBrowser.println("<HR>"); ..... ..... } } Try to open the file when the servlet is called the first time

  22. 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("<H1> A web page counter </H1>"); outBrowser.println("<HR>"); ..... ..... } } Read the line and convert the content to its integer value

  23. 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("<H1> A web page counter </H1>"); outBrowser.println("<HR>"); ..... ..... } }

  24. 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("<H1> A web page counter </H1>"); outBrowser.println("<HR>"); ..... ..... } } After count is incremented, open the file to write (overwrite), write the new number and close file

  25. 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("<H1> A web page counter </H1>"); outBrowser.println("<HR>"); ..... ..... } } The rest is the same

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