Servlets

Presentation Transcript

CEN 4010

Servlets overview

Servlet Technology
Handling requests
Session tracking
Handling Cookies
JDBC Servlet

Webserver

web site files

html files: text, multi-media and links
Common Gateway Interface (CGI):

cgi scripts: sh, csh, Perl, …
Servlets:

Java program that runs on webserver

JavaServer pages (JSP):

Contains html and Java
Compiled into a servlet and run on webserver

Servlet

Supports request/response model

client send request
server responds (with html page)

Different types of servlets

GenericServlet

not protocol specific

HttpServlet

Serves http requests

Servlet tasks

process html forms
middle-tier processing

connect to sources behind firewall (e.g. DB)

maintain session information between requests
serve as concentration point for multiple clients

Servlet runs on webserver

requested via URL by client

requires special webserver

Tomcat, IBM application server, JRun, Oracle …

can start with webserver

permanent: if startup effort is high

can be started on client request

temporary: if rarely used

servlet is unaware of when it was started

Basic modes of operation

Multi threaded

Single servlet instance runs in many threads
Servlet fields shared
Must consider field contention

Single threaded

Multiple instances, each in one thread
Servlet fields are not shared

Servlet interfaces and classes

Java Servlet packages

javax.servlet
javax.servlet.http

Part of J2EE
to compile include into CLASSPATH

(tomcat)... common\lib\servlet-api.jar

HttpServletservice method

invoked for

“get” and “post”

method in

html form

HttpServlet class

Servlet example: HelloWorld.java

package mypackage;

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class HelloWorld extends HttpServlet {

public void doGet(HttpServletRequest request,

HttpServletResponse response)

throws IOException, ServletException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

out.println(“<html><head><title>Hello</title></head>”);

out.println(“<body>”);

out.println("Hello World“);

out.println(“</body></html>”);

}

Make the servlet known as web application: web.xml

<web-app>

<display-name>HelloWorld example</display-name>

<description>Welcome to HelloWorld</description>

<servlet-name>HelloWorld</servlet-name>

<servlet-class>mypackage.HelloWorld</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>HelloWorld</servlet-name>

<url-pattern>/hello</url-pattern>

</servlet-mapping>

</web-app>

html example: hello.html

<html>

<head>

<title>Hello World Servlet Example</title>

<body>

<form

action=“http://localhost:8080/HelloExample/hello”

method=“get”>

</form>

</body>

</html>

Servlet deployment on Tomcat

Create directory ...\webapps\HelloExample

Copy hello.html into this directory as index.html
Create subdirectory WEB-INF
Create subdirectory WEB-INF\classes
Compile HelloWorld.java to

WEB-INF\classes\mypackage\HelloWorld.class

Copy web.xml into directory: WEB-INF

Open “http://localhost:8080/HelloExample”

Process Form parameters

html form defines fields with

name
type
value(s)

received by servlet via request parameter
HttpServletRequest class defines helper methods

Helper methods

getParameter(“name”)

Returns string (maybe empty) or null if parameter does not exist

getParameterValues(“name”)

Returns array of strings

getParameterNames()

Returns enumeration of parameter names

html example

<html>

<body>

<head>

<title>Request Parameters Example</title>

</head>

<h3>Request Parameters Example</h3>

Please enter

First Name: <input type=text size=20 name=firstname> <br>

Last Name: <input type=text size=20 name=lastname> <br>

</form>

</body>

</html>

doPost method

String first = request.getParameter("firstname");

String last = request.getParameter("lastname");

out.println("<html><head><title>Answer</title></head>");

out.println("<body>Welcome " + first + " " + last);

out.println("</body></html>");

Keep track of session

http is a stateless protocol
but there is need to maintain session information

username, password
shopping cart
…

possible: use hidden field(s)
better: HttpSession

Keep track of session

session can remember values for attributes across servlet invocations
methods:

session = request.getSession(true);
session.setAttribute(“name”, object);
session.getAttribute(“name”);

getSession

session = request.getSession(true);

retrieves current session from request
“true” parameter causes new session to be created if it does not exist
session remains alive until:

it times out (reaches time maximum)
explicit cancellation

Attributes

getAttribute(“name”)
setAttribute(“name”, value)

value can be any object

removeAttribute(“name”)
getAttributeNames()

HttpSession methods

getID()
isNew()
getCreationTime()
getLastAccessedTime()
invalidate()
setMaxInactiveInterval()
getMaxInactiveInterval()

SessionServlet: doPost (1/3)

HttpSession session = request.getSession(true);

String heading;

if (session.isNew()) {

heading = "Welcome, Newcomer";

} else {

heading = "Hello Again";

}

Integer accessCount = (Integer)session.getAttribute("accessCount");

if (accessCount != null) {

accessCount =

new Integer(accessCount.intValue() + 1);

} else {

accessCount = new Integer(0);

}

session.setAttribute("accessCount", accessCount);

out.println("<html><head><title>Answer</title></head>");

out.println("<body> " + heading + "<br>");

out.println("Number of previous accesses " + accessCount );

out.println("</body></html>");

Handle Cookies

allow to store information in the browser
can be retrieved by servlet
2 types

temporary cookies

lasts as long as the browser instance

permanent cookies

must have expiration date
can be deleted

Note: browsers can disable cookies

To establish a Cookie

create instance of class Cookie

Cookie mine = new Cookie(“name”, “value”);

permanent has max age

mine.setMaxAge(seconds);

can have domain and path

mine.setDomain(“.aul.fiu.edu”);

mine.setPath(“/catalog”);

To access Cookies

through request parameter

Cookie all[] = request.getCookies();

find your cookie

for (int i=0; i<all.length; i++) {

out.println(“Cookie name: “ + all[i].getName());

out.println(“ value: “ + all[i].getValue());

}

To save Cookie

through response header

response.addCookie(singleCookie);

Example:

Cookie mine = new Cookie(“user”, “ege”);

mine.setMaxAge(60 * 60 * 24 * 365);

response.addCookie(mine);

Access database

servlet may access database using JDBC

load driver
get connection
formulate statement
execute statement
process result set

Example: database access

Class.forName("com.sybase.jdbc.SybDriver");

conn = DriverManager.getConnection("jdbc:sybase:Tds:ocelot.aul.fiu.edu:7100",

“student", “student");

out.println("<html><head><title>Customers</title></head><body>");

String s1 = "select * from Customers;";

Statement s = conn.createStatement();

ResultSet result = s.executeQuery(s1);

out.println("<table border=1><tr><th>ID<th>name<th>city<th>street</tr>");

while (result.next()) {

out.println("<tr><td>" + result.getInt(1));

out.println("<td>" + result.getString(2));

out.println("<td>" + result.getString(3));

out.println("<td>" + result.getString(4) + "</tr>");

}

out.println("</table></body></html>");

Performance considerations

new connection is created for every request to servlet
other choices

place connection into session

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