1. An introduction to Java Servlet Programming
Java Servlets
An introduction to Java Servlet Programming

2. Containers and Components
Several clients – one system
Today we will talk about one part of the Web layer, the Servlets

3. Web Development The web is static by nature Based on HTTP
A stateless network environment
The stateless nature is what have made the Internet into what it is today
A repetion

4. HTTP A very simple and readable protocol Several methods Text based
GET
The basic request to get a page
POST
Used from forms in the web pages
DELETE
HEAD
Repetition

5. Java Servlets - Introduction
Extensions of the web server
Run inside the Servlet Container
The container can act as a web server
The container can interact with a web server via plugins (Apache and mod_jk for Tomcat for example)
One of the front ends to the J2EE system
The servlet acts as the HTTP Receiver and calls EJBs, JMS and so on
Good for implementing control logic
Captures that request and determines what to do
Initializes Beans, EJBs and other resources
Forward the request to a view (A Java Server Page)

6. Java Servlets - Introduction
Support for sessions
A way around the stateless nature of the web
Stored on the server
Can contain anything
Private for a specific user
Identified with “jsessionid”
As a Cookie or as a request parameter
The stateless nature is crucial for the web, but it’s a problem when developing web based application rather that home pages
A Session is a way around this
A user can have his set of objects in the session

7. Java Servlets - Introduction
Multithreaded by nature
There is only one instance of each Servlet
The Servlet must be thread safe
Only read only Class variable
The Servlet can implement SingleThreadModel to force the container to only use one thread at a time
No concurrent access => bad performance
Not so good for generation nice layout

8. Java Servlets - Different types
A Servlet is a Servlet when in extends one of the abstract Servlet-classes
Currently two types
HttpServlet
GenericServlet

9. Java Servlets - Initialization
A Servlet is only initialized once
And there is only one instance
The init(ServletConfig) is executed at initialization
The ServletConfig can be used to get environment variables from web.xml
The place initialize common objects like connection pools
Destroy() is called when a Servlet is uloaded

10. Javax.servlet.GenericServlet
The GenericServlet can work with any protocol
Not used that much in web development

11. javax.servlet.http.HttpServlet
HttpServlet is specialized for HTTP
Contains methods to intercept most HTTP-methods
doGet(HttpServletRequest, HttpServletResponse)
doPost(HttpServletRequest, HttpServletResponse) …
processRequest(HttpServletRequest…)
Intercepts all method if present

12. HttpServletRequst A representation of the entire HTTP request
Contains all request parameters
POST-parameters and GET-parameters are treated equally
Used to get information about the caller
Hostname
Web browser and OS
Username if any

13. HttpServletRequst Used to get a handle to the session
request.getParameter(“name”)
Gets the value of a parameter.
Returns a String that might be casted
request.setAttribute(obj, “name”)
Used to add an attribute
Usefull when forwarding
request.getAttribute(“name”)
Get an attribute
Returns an Object

14. HttpServletRequst request.getHeader(“name”) request.getLocale()
Return any HTTP header
request.getLocale()
Returns the perfered language
getLocales() return an Enumeration of all supported languages
request.getSession(boolean)
Returns the HttpSession
The argument decides if a new session should be created in none is present

15. HttpServletRequest request.getCookies()
Return an array of all Cookies
request.getRequestDispatcher(url)
Used to dispatch the URL, i.e. to pass it down the request chain
RequestDispatcher.forward()

16. HttpServletResponse
Represents the response that will be sent to the user
Used to write output
response.getWriter()
Returns an PrintWriter
print(), println()…
Used to encode URLs
Used to set content type and other HTTP Headers

17. HttpSession A representation of a Session Retrieved from the request
HttpSession.getAttribute(“name”)
Returns an Object
HttpSession.setAttribute(Object, “name”)
HttpSession.inValidate()
Destroys the Session

18. A very simple Servlet import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;
public class Servlet1 extends HttpServlet {
public void init(ServletConfig config) throws ServletException {
super.init(config); }
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType(“text/html”);
PrintWriter out = response.getWriter();
out.println(“<h1>Yo</h1>");
out.close();

19. J2EE Web applications A web application is standardized
The entire web application is controlled by a single XML-file, web.xml
All files (JSP, Servlets, Tags and so on) is packaged in a WAR file (Web Archive)
A JAR-file with a different ending

20. J2EE Web app – Directory structure
/ The web root. Can contain JSPs, HTML and subdirectories
WEB-INF -- NOT available to the outside
web.xml
Container specific XML-files, orion-web.xml
lib
Jar files that will be available for the application
classes
Compiled classes like servlets, beans, Home and remote interfaces for EJBs

21. web.xml <?xml version = '1.0' encoding = 'windows-1252'?>
<!DOCTYPE web-app PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN" "
<web-app>
<description>Empty web.xml file for Web Application</description>
<servlet>
<servlet-name>ServletOne</servlet-name>
<servlet-class>ServletOne</servlet-class>
<init-param>
<param-name>DBUrl</param-name>
<param-value>jdbc:mimer://lara.mimer.se/fslara82</param-value>
</init-param>
</servlet>
<servlet-mapping>
<url-pattern>/servletone</url-pattern>
</servlet-mapping>
<session-config>
<session-timeout>30</session-timeout>
</session-config>

22. web.xml continued <mime-mapping>
<extension>html</extension>
<mime-type>text/html</mime-type>
</mime-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
<taglib>
<taglib-uri>/jstl</taglib-uri>
<taglib-location>WEB-INF/jstl.tld</taglib-location>
</taglib>

23. web.xml continued <security-constraint>
<web-resource-collection>
<web-resource-name>adminresource</web-resource-name>
<url-pattern>/servlet/*</url-pattern>
</web-resource-collection>
<auth-constraint>
<role-name>admin</role-name>
</auth-constraint>
<user-data-constraint>
<transport-guarantee>NONE</transport-guarantee>
</user-data-constraint>
</security-constraint>
<login-config>
<auth-method>FORM</auth-method>
<form-login-config>
<form-login-page>/login.jsp</form-login-page>
<form-error-page>/errorLogin.jsp</form-error-page>
</form-login-config>
</login-config>
<security-role>
<description>An adminstrator</description>
</security-role>
</web-app>

24. Examples