Servlets

Servlets and CGI
Anatomy of a servlet
Example
Form parameters
Cookies
Maintaining sessions

Servlets and CGI

Increased efficiency
- Each CGI request is handled by spawning off a separate process
- Servlet requests are directed to a servlet engine, which executes the servlet
Servlets are only instantiated once, on the first request
- Every other request is handled in a thread using the same servlet instance

Only one instance serving multiple requests

Instance variables (not local to a method) of a servlet are shared by all requests
Access to shared variables needs therefore to be synchronized

public class MyServlet extends HttpServlet {
    int globalVar;	// shared by all threads
            		// executing the servlet
    public void myMethod() {
        int localVar;
    }
}

Anatomy of a Servlet

Servlets extend the HttpServlet class in the javax.servlet package
Servlets define an init() method, which is only executed when the first request to servlet is made
When the servlet engine is shut down, the destroy() method is called
Requests are processed in the doGet() and doPost() methods (depending on the request method)
Servlet engine passes two objects to these methods
- HttpServletRequest (containing request parameters and input stream)
- HttpServletResponse (containing output stream)

Servlet Lifecycle

Example - HelloWorldServlet

Here is an example of a "Hello world!" servlet

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorldServlet extends HttpServlet {
    public void doPost(HttpServletRequest req,
        HttpServletResponse res) throws ServletException, IOException {
        PrintWriter out;
        
        res.setContentType("text/html");
        out = res.getWriter();
        out.println("<html><head></head>");
        out.println("<body bgcolor=\"white\">");
        out.println("<b>Hello World!</b>");
        out.println("</body></html>");
        out.close();
    }
}

We invoke this servlet from the following web page

<html><head></head>
<body>
<form action="http://localhost:8080/examples/servlet/HelloWorldServlet"
    method="POST">
<input type="submit" value="HelloWorld">
</form>
</body></html>

Example - HelloWorldServlet

To use this example
- Move helloWorldServlet.html to the webpages directory
- Move HelloWorldServlet.class to the examples/WEB_INF/servlets directory
In the doPost() method we set the content type of the response
```
res.setContentType("text/html");
```
We then get a handle to the output stream
```
out = res.getWriter();
```
Everything we write to this stream will be sent back to the browser

Form Parameters

Access form parameters through methods on the request object
- getParameter(String name)
- getParameterValues(String name)
- getParameterNames()
getParameter(String name)
- Returns the lone value of a parameter
getParameterValues(String name)
- Returns a list of the values of a parameter (e.g. a radio button)
getParameterNames()
- Returns an enumeration on the parameter names

Example - SurveyServlet

When invoked through GET
- Servlet returns a survey form, asking the user to choose her favorite browser

When invoked through POST

Servlet returns the result of the survey including the answer given via the form

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class SurveyServlet extends HttpServlet {
    private String choices[] = 
        {"Netscape Communicator", "Internet Explorer", 
         "Opera", "None of the above"};
}

Example - SurveyServlet - GET

    public void doGet(HttpServletRequest req,
        HttpServletResponse res)
        throws ServletException, IOException {
        res.setContentType("text/html");
        PrintWriter out = res.getWriter();
        out.println("<html><head>");
        out.println("<title>Survey</title>");
        out.println("</head><body>");
        out.println("<form method=\"POST\">");
        out.println("What is your favorite browser?");
        out.println("<p>");
        for (int i=0; i<choices.length; i++) {
            out.println("<input type=\"radio\" name=\"survey\" " +
            	"value=\"" + i + "\">" + choices[i]);
            out.println("<br>");
        }
        out.println("<p>");
        out.println("<input type=\"submit\" value=\"Vote\">");
        out.println("</form>");
        out.println("</body></html>");
    }

Example - SurveyServlet - POST

    public void doPost(HttpServletRequest req,
        HttpServletResponse res)
        throws ServletException, IOException {
        int survey[];
        int total = 0;
        
        // Determine number of survey response so far
        survey = loadSurveyResults();
        for(int i = 0; i < survey.length; i++)
            total += survey[i];

        // Get current survey response
        String choice = req.getParameter("survey");
        ++total;

        // Update survey results and save them to file
        survey[Integer.parseInt(choice)]++;
        saveSurveyResults(survey);
        
        // Calculate resulting percentages
        int result[] = new int[survey.length];
        for (int i = 0; i < survey.length; i++) {
            result[i] = Math.round(
            	100 * survey[i] / total);
        }

        // Send response to the client
        res.setContentType("text/html");
        PrintWriter out = res.getWriter();
        showPage(out, result, survey, total);
    }

Example - Load and Save Survey Results

    private int[] loadSurveyResults() {
        int[] survey = null;
        File f = new File("data/survey.dat");
        if(f.exists()) {
            	try {
            ObjectInputStream is = new ObjectInputStream(
            	new FileInputStream(f));
            survey = (int[]) is.readObject();
            is.close();
            	} catch(IOException e) {
            e.printStackTrace(System.out);
       			}
        } else {
       		survey = new int[choices.length];
        }
        return survey;
    }

    private void saveSurveyResults(int survey[]) {
        File f = new File("data/survey.dat");
            try {
        ObjectOutputStream output = new ObjectOutputStream(
            new FileOutputStream(f));
        output.writeObject(survey);
        output.close();
            } catch (IOException e) {
        e.printStackTrace();
            }
    }

Example - Show Updated Survey Results

    private void showPage(PrintWriter out, 
        int result[], int survey[], int total) {
        out.println("<html><head><title>");
        out.print("Survey Results");
        out.println("</title><body>");
        out.println("Thank you for participating in the survey!");
        out.println("<p>");
        out.println("Results:");
        out.println("<p>");
        out.println("<table>");

        for (int i = 0; i<result.length; i++){
            out.println("<tr>");
       		out.println("<td>" + choices[i]);
       		out.println("<td>" + result[i] + "%");
            out.println("<td>" + survey[i]);
        }
    
        out.println("</table>");
        out.println("<p>");
        out.print("Total number of responses: ");
        out.print(total);
        out.println("</body></html>");

        out.close();
    }

Example - Survey Form

<html><head>
<title>Survey</title>
</head><body>
<form method="POST">
What is your favorite browser?
<p>
<input type="radio" name="survey" value="0">Netscape Communicator
<br>
<input type="radio" name="survey" value="1">Internet Explorer
<br>
<input type="radio" name="survey" value="2">Opera
<br>
<input type="radio" name="survey" value="3">None of the above
<br>
<p>
<input type="submit" value="Vote">
</form>
</body></html>

Example - Survey Result

<html><head><title>
Survey Results</title><body>
Thank you for participating in the survey!
<p>
Results:
<p>
<table>
<tr>
<td>Netscape Communicator
<td>40%
<td>4
<tr>
<td>Internet Explorer
<td>10%
<td>1
<tr>
<td>Opera
<td>30%
<td>3
<tr>
<td>None of the above
<td>20%
<td>2
</table>
<p>
Total number of responses: 10</body></html>

Cookies

The Cookie class can be used by servlets to create cookie objects
```
Cookie cookie = new Cookie(name, value);
```

Cookie support the methods you would expect from earlier sections of the course

cookie.setDomain(domain);
cookie.setMaxAge(maxAge);
cookie.setPath(path);
cookie.setSecure(secure);

The request object can then be asked for a list of the cookies sent with it
```
Cookie[] cookies = req.getCookies();
```

Example - Cookies

The following example comprises two servlets
- SetCookieServlet
- GetCookieServlet
SetCookieServlet sets a cookie with the user's first and last names
It then redirects the browser to the GetCookieServlet

GetCookieServlet gets the cookie and extracts the first and last name from it

<html><head></head>
<body>
<form action="http://localhost:8080/examples/servlet/SetCookieServlet"
    method="POST">
First name<br>
<input type="text" name="firstName"><p>
Last name<br>
<input type="text" name="lastName"></p>
<input type="submit" value="OK">
</form>
</body></html>

Example - SetCookieServlet

The method sendRedirect() of the response object redirects the browser to another page

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class SetCookieServlet extends HttpServlet {
    public void doGet(HttpServletRequest req,
        HttpServletResponse res) {
        doPost(req, res);
    }
    
    public void doPost(HttpServletRequest req,
        HttpServletResponse res) {
            try {
        String firstName =
            req.getParameter("firstName");
        String lastName =
            req.getParameter("lastName");
        String value =
            firstName + ":" + lastName;
        Cookie cookie = new Cookie("user", value);
        cookie.setMaxAge(3600);
        res.addCookie(cookie);
        res.sendRedirect("/examples/servlet/GetCookieServlet");
            } catch (Exception e) { }
    }	
}

Example - GetCookieServlet

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class GetCookieServlet extends HttpServlet {
    public void doGet(HttpServletRequest req,
        HttpServletResponse res) {
        doPost(req, res);
    }
    
    public void doPost(HttpServletRequest req,
        HttpServletResponse res) {
        res.setContentType("text/html");
            try {
        PrintWriter out = res.getWriter();
        out.println("<h2>Cookies</h2>");
        Cookie[] cookies = req.getCookies();
        for (int i=0; i<cookies.length; i++) {
            Cookie c = cookies[i];
            String name = c.getName();
            String value = c.getValue();
            out.println("Cookie name: " + name);
            out.println("<p>");
            if (name.equals("user")) {
            	int index = value.indexOf(":");
            	String firstName =
            		value.substring(0, index);
            	String lastName =
            		value.substring(index+1);
            	out.println("Values retrieved from cookie:");
            	out.println("<br>");
            	out.println("First name: " + firstName);
            	out.println("<br>");
            	out.println("Last name: " + lastName);
            }
        }
        out.close();
            } catch (Exception e) { }
    }
}

Example - GetCookiesServlet Result

<h2>Cookies</h2>
Cookie name: user
<p>
Values retrieved from cookie:
<br>
First name: joe
<br>
Last name: cool

Maintaining Sessions

A session is a connection between client and server that persists over multiple requests
Most common method used for maintaining sessions is through use of cookies
A new session can be established through the getSession method of the servlet request
```
HttpSession session = req.getSession(true);
```
If the argument is "true", the servlet engine will create a new session if one doesn't already exist

HttpSession

We can store values in a session and retrieve them later
- putValue()
- getValue()

To store a value in the session context

Integer counter = new Integer(100);
session.putValue("counter", counter);

To retrieve the value later in the same session

Integer counter = (Integer)
    session.getValue("counter");

We can also remove a value from a session and obtain all session value names
- removeValue()
- getValueNames()
If we want to end the session use the invalidate() method
```
session.invalidate();
```

References

Java Servlet API Tutorial
- http://developer.java.sun.com/developer/onlineTraining/Servlets/Fundamentals/servlets.html