Transcript Java_Servlet

Comp2513
Java Servlet Basics
Daniel L. Silver, Ph.D.
Objectives
To introduce the basic concepts of Java
Servlets
 To discuss FORMs and Java Servlets
 Review more complex servlets
 Reference: DDEA Ch.7, Sharma p.110-122
and EJP (Ch.4) p.48-63

2002
Daniel L. Silver
2
The Problems
with Applets and CGI

Java Applets have three major drawbacks:
– Take time to load onto client
– May not work as planned (depends on JVM)
– Security risk for client



Server-side code is preferred for business logic
CGI allows an application to run on server but
creates server performance problems
Most notably each time a separate process must be
spawned (for Java a separate JVM is run)
2002
Daniel L. Silver
3
Enter Servlets
Servlets overcome this problem
 Servlets relie on a Servlet Engine
(Application Server) to manage multiple
requests for the same application
 Java servlets have the advantages of Java
applets but run on the server side
 The Jserv engine from SUN was the first
Java servlet engine

2002
Daniel L. Silver
4
Java Servlet Technology




Applications run on the server.
Extend functionality of a web server and provide
structure for a business environment.
Servlets can be operating system and hardware
platform independent.
Servlets process HTTP/HTML requests with all
the benefits of the mature Java language
(portability, performance, reusability, and crash
protection.)
2002
Daniel L. Silver
5
Applet vs. Servlet

Applet

Servlet
– Client side.
– Takes time to load.
– JVM varies with
browser.
– Require compatible
browser
– Security issue if client
side program needs to
access sensitive data via
browser.
2002
Daniel L. Silver
–
–
–
–
Server side.
Runs on request.
Constant JVM
No GUI required, can
generate HTML,
Javascript, Applet code
– Server side
programming and data
access preferred for
business applications.
6
CGIs vs. Servlets

CGI programs

Servlets
– Separate process for each
CGI program
– Mod_perl and FastCGI
improves performance of
CGI but not to level of
servlets
– Have difficult time
maintaining state across
requests
2002
Daniel L. Silver
– Run under single JVM
(better performance)
– Servlets loaded into
memory with first call, and
stay in memory
– Have built in state
preservation methods
– Java's inherent security
– Proprietary source code can
be retained by only giving
up *.class files to the server
7
Websphere Java Servlet
Request Processing
Client
http://eagle.acadiau.ca/demo/servlet/HelloWorld
Browser
HTML
Internet
Tomcat
App. Server
JVM
HTTP
Server
servlet/HelloWorld
HelloWorld.class
demo/servlet/ equates to
…/demo/WEB-INF/classes/HelloWorld.class
2002
Daniel L. Silver
8
Servlet Life-Cycle



The servlet is initialized and is loaded into the
servers memory
The servlet resides in memory and responds to
requests from clients
The servlet is destroyed (by the server engine)
NOTE: When you update a servlet, Tomcat
checks the modification date of the .class file and
if it is more recent then the running version, the
servlet is destroy and then re-initialize
2002
Daniel L. Silver
9
Fundamental parts of a Servlet
1. import javax.servlet.*; and import javax.servlet.http.*;
- packages of servlet classes that implement the Java Servlet API
2. public class HelloWorld extends HttpServlet {
- extends the HTTPServlet class
3. init()
-intializes servlet after loading into memory
- place to perform operations done only once at start-up
- reading a current properties
- clearing log files, notifying other services that the servlet is running
4. service(), doGet(), doPost()
- this is where work is done
- each time the servlet is called a new thread of execution begins
- input is passed to the Java code via either HTTP GET or POST commands
5. destoy()
- executed when server engine calls servlet to terminate
- used to flush I/O, disconnect from database
2002
Daniel L. Silver
10
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>");
out.println("<body>");
out.println("<head>");
out.println("<title>Hello World!</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Hello World!</h1>");
out.println("</body>");
out.println("</html>");
}2002
Daniel L. Silver
}
Structure of
a Servlet:
HelloWorld
11
Some Basic Java Servlets

Several examples …
http://eagle.acadiau.ca/demo/servletexample.html
http://eagle.acadiau.ca:8080/examples/servlets/index.html
http://www.servlets.com/jservlet2/examples/index.html

Java Developers Almanac:
http://javaalmanac.com/egs/javax.servlet/pkg.html
2002
Daniel L. Silver
12
FORMS and Java Servlets

Within HTML code on the client: A
FORM tag similar to CGI is used to pass
data from a browser to a Java servlet:
…
<FORM ACTION = 'http://eagle/store35/servlet/ShowFormVariables'
METHOD=‘GET'>
Enter your first name <INPUT TYPE=text NAME='firstName'
VALUE=' '><br>
…

This will cause the web browser to generate
a URL (if “Danny” is entered):
http://eagle/demo/servlet/ShowFormVariables?firstName=Danny
2002
Daniel L. Silver
13
FORMS and Java Servlets

Within Java code on the server: The doGet servlet
method is used to obtain the two objects:
– HttpServletRequest and HttpServletResponse


HttpServletRequest is used to get the INPUT
parameter names and values:
– getParameterNames()
– getParameterValues()
HttpServletResponse used to return HTML
response
– printlin()
2002
Daniel L. Silver
14
An Example of a Java Servlet
handling FORM input

From page 117 of Sharma – servlet that
returns browser input passed via a FORM
tag
http://eagle.acadiau.ca/danstech/ShowFormVariables.html

Here is the java source code :
http://eagle/danstech/ShowFormVariables.java
2002
Daniel L. Silver
15
A more complex Servlet Example


AddToShopppingCart.java from your store
provides a larger and more complex servlet
example
Also see ClearShoppingCart.java
2002
Daniel L. Silver
16
Problems with Servlets ?

Do you see any problems with servlets from
an E-Commerce perspective?
2002
Daniel L. Silver
17
Additional Web Info





http://java.sun.com/products/servlet/
http://developer.java.sun.com/developer/onlineTra
ining/Servlets/Fundamentals/
http://hotwired.lycos.com/webmonkey/99/07/inde
x3a.html?tw=programming
http://www.servlets.com/jservlet2/index.html
http://www.servlets.com/jservlet2/examples/index.
html
2002
Daniel L. Silver
18
THE END
[email protected]