Java Servlets

1
Java Servlets
2
Basics

• What is Java Servlets?
• Javas solution to CGI programs.
• Wrong way to use Java as CGI
• Write stand-alone Java program
• Invoke it via new CGI shell process.
• Right way to use Java as CGI
• Use servlets and a servlet-compatible server.

3
How it works?

• Browser makes a servlet request to webserver.
• Webserver recognizes the URL as a servlet URL.
  (e.g., http//www.obvious.com/servlets/blah)
• Webserver loads servlet if not already loaded.
• Servlet executes and delivers output to webserver
  (HTML output).
• Webserver delivers output to Browser.

4
What you need to write servlets?

• Either JDK 1.1 and JSDK (Java Servlet Development
  Kit).
• JDK 1.2
• A servlet-compatible webserver.

5
GET POST

• When Form data is sent by the browser, it can be
  sent in one of two ways (1) using the GET method
  and (2) using the POST method.
• In the GET method, the form data (parameters) is
  appended to the URL, as in
• http//www.yahoo.com/search?music
• Here, the text field contains music.
• In the POST method, the browser simply sends the
  form data directly.
• When you create an HTML form, you decide whether
  you want to use GET or POST.
• When you use GET, the doGet() method of your
  servlet is called, otherwise the doPost() method
  is called.
• The standard practice is to use POST, unless you
  need to use GET.
• You need to use GET when you want a link to
  invoke a CGI program (servlet).

6
Your first servlet -- Hello World

• import javax.servlet.
• import javax.servlet.http.
• import java.io.
• public class helloworld extends HttpServlet
• public void doGet (HttpServletRequest req,
  HttpServletResponse resp) throws
  ServletException, IOException
• // Set the content type of the response.
• resp.setContentType ("text/html")
• // Create a PrintWriter to write the response.
• java.io.PrintWriter out new PrintWriter
  (resp.getOutputStream())
• // The first part of the response.
• out.println ("") out.println ("") out.println
  ("")
• // The greeting.
• out.println ("Yo, Hello World!")
• // Last part. out.println ("") out.println
  ("") out.close()
• // Screen I/O
• System.out.println ("Inside servlet ... servlet
  complete")
• public void doPost (HttpServletRequest req,
  HttpServletResponse resp) throws
  ServletException, IOException doGet (req,
  resp)

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Hello World Notes

• The servlet class libraries need to be imported,
  along with java.io.PrintWriter.
• There are two methods, one for each type of
  request GET or POST.
• You can decide not to "really" implement one
  method and instead have it call the other.
• There are two object parameters to each method,
  the "request" and the "respponse".
• The HttpServletResponse instance has an
  OutputStream that is used to write directly to
  the requesting browser.
• To make writing easier, we wrap a PrintWriter
  around the OutputStream instance
• // Create a PrintWriter to write the
  response.
• java.io.PrintWriter out new
  PrintWriter (resp.getOutputStream())
• We set the content-type (as required by the HTTP
  protocol)
• // Set the content type of the
  response.
• resp.setContentType ("text/html")
• We write HTML to the output, e.g.,
• out.println ("")
• out.println ("")
• Don't forget to close the output stream
• out.close()
• Optionally, for debugging, we can also write to
  the local screen where the webserver is running
• System.out.println ("Inside servlet ...
  servlet complete")

8
Extracting Parameters in a Servlet

• Idea -- create an HTML Form, and have a servlet
  pick up the Form data entered by the user.
• HTML Form
• lthtmlgt
• ltheadgtlttitlegtTest Postlt/titlegtlt/headgt
• ltbodygt
• ltform action"http//localhost8502/servlets/Tes
  tform" method"post"gt Enter a string ltinput
  type"text" name"param1"gt And then press "Go"
  ltinput type"submit" value"Go"gt
• lt/formgt
• lt/bodygt
• lt/htmlgt

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Extracting Parameters in a Servlet

• Next the Java Servlet
• import javax.servlet.
• import javax.servlet.http.
• import java.io.
• import java.util.
• public class TestForm extends HttpServlet
• public void doPost (HttpServletRequest req,
  HttpServletResponse resp) throws
  ServletException, IOException
• // Set the content type of the response.
• resp.setContentType ("text/html")
• // Create a PrintWriter to write the response.
• java.io.PrintWriter out new PrintWriter
  (resp.getOutputStream()) // The first part of
  the response.
• out.println ("") out.println ("") out.println
  ("")
• // Now get the parameters and output them back.
  
• out.println ("Request parameters ")
• Enumeration e req.getParameterNames()
• while (e.hasMoreElements())
• String name (String) e.nextElement()
• String value req.getParameter (name)
• if (value ! null) out.println (" name"
  name " value" value "")

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Notes for Extracting Parameters

• We need to import java.util.Enumeration. Whatever
  parameters were provided are all in the
  HttpServletRequest instance. We can list these
  parameters by getting an Enumeration instance
  from the request by calling getParameterNames()
• Enumeration e req.getParameterNames()
  
• The "name" of a parameter is really the string in
  the name attribute of the tag for the particular
  Form element. For example, the name string is
  param1 below.
• Enter a string ltinput type"text"
  name"param1"gt
• Now, if we want to retrieve the actual string
  typed in by the user, we use that name in
  getParameter()
• String whatTheUserTyped
  req.getParameter
• ("param1")