Knowledge Sharing

Agenda
What is Servlet?
Servlet hierarchy & lifecycle
Servlet program structure
Deploying servlets on Tomcat
HTTP Servlets and HTTP request methods
Understanding servlet API
Responding to requests
Accessing form input data
Working with header fields
URL redirecting

What is servlet?
A Java class which conforms to the Java Servlet API, a protocol by which a Java class may respond to HTTP requests.
A Servlet is an object that receives a request and generates a response based on that request
Servlets are the Java counterpart to non-Java dynamic Web content technologies such as CGI and ASP.NET
( http://en.wikipedia.org/wiki/Java_Servlet )

Advantages of Servlets
Faster than CGI scripts because use a different process model.
Use standard API that is supported by many Web servers.
Have all of the advantages of the Java languages, including ease of development and platform independence.
Can access the large set of APIs available for the Java platform.
( http://java.sun.com )

Servlet V.S. CGI
A s ervlet stays in memory between requests. A CGI program needs to be loaded and started for each CGI request .
A servlet doesn’t run in a separate process. This removes the overhead of creating a new process for each request which CGI does.
A servlet allows each request to be handled by a separate Java thread within the web server process (same amount of threads as request but there only be one copy of the servlet class crea ted in memory).
( http://www.novocode.com/doc/servlet-essentials/chapter1.html )

Servlet Hierarchy Servlets (javax.servlet.Servlet interface) Generic Servlet (javax.servlet.GenericServlet) HTTP Servlet (javax.servlet.http.HttpServlet) MyServlet

Servlet Lifecycle

Servlet Lifecycle (cont.)

Servlet Program Structure

[Screencast] Create simple servlet

[Showcase] Servlet Lifecycle

[Showcase] Servlet Lifecycle (cont.)

Deploying Servlets on Tomcat
Create a servlet
Compile the servlet :
No different from compiling java program
( javac OurServlet.java )
javax.servlet.* and javax.servlet.http.* must be added in Classpath.
Create web application folder under webapps folder in Tomcat (web container).
Create WEB-INF folder under web application folder.
http://www.roseindia.net/servlets/introductiontoconfigrationservlet.shtml

Deploying Servlets on Tomcat (cont)
Create ‘web.xml’ file and ‘classes’ folder under WEB-INF.
Copy the servlet class file into ‘classes’ folder
Edit web.xml to include servlet’s name and url pattern .

Deploying Servlets on Tomcat (cont)
Run Tomcat server and then execute the Servlet

HTTP Servlet and HTTP request method

HTTP Servlet and HTTP request method (cont.)
HTTP is a request-response oriented protocol.
An HTTP request consists of :
Request method ( GET, HEAD, PUT, POST, DELETE, OPTIONS and TRACE)
URI
Header fields
Body
An HTTP response contains :
Result code
Header fields and body
http://www.novocode.com/doc/servlet-essentials/chapter1.html

Understanding Servlet API
Packages in the Java Servlet API 2.1
http://java.sun.com/products/servlet/2.1/servlet-2.1.pdf

Understanding Servlet API (cont)
Packages in the Java Servlet API 2.1
http://java.sun.com/products/servlet/2.1/servlet-2.1.pdf

Request Heade r
Accept
Accept-charset
Accept-encoding
Accept-language
Authorization
Connection
Content-length
Cookie
The most common headers :
From
Host
If-modified-since
Pragma
Referer
User-agent
UA-Pixels, UA-color, UA-OS, UA-CPU
http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/Servlet-Tutorial-Request-Headers.html

Request Heade r (cont.)
Call the getHeader() method of the HttpServletRequest, which returns a String if the header was supplied on this request, null otherwise.
Call the getHeaderNames() to get an Enumeration of all header names received on the particular request.
Reading request header from Servlets : http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/Servlet-Tutorial-Request-Headers.html

Accessing form input data
Call getParameter() method of the HttpServletRequest, supplying the parameter name (case sensitive) as an argument.
Call getParameterValues() method to get the parameter that probably have more than one value.
Call getParameterNames() to get full list of all parameters received on the particular request.
http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/Servlet-Tutorial-Form-Data.html

URL Redirecting
Using sendRedirect() method of HttpServletResponse class.
Using forward() method of RequestDispatcher class.
http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/Servlet-Tutorial-Form-Data.html

URL Redirecting (cont.)
sendRedirect() It is a new request from the client, and the way to pass data is through the session or with web parameters (url?name=value)
RequestDispatcher The target servlet/JSP receives the same request/response objects as the original servlet/JSP. Therefore, can pass data between them using request.setAttribute().
http://www.theserverside.com/discussions/thread.tss?thread_id=26425

URL Redirecting (cont.)
sendRedirect() It will updates the browser history.
RequestDispatcher If use RequestDispatcher to forward from Servlet-2 to JSP-3, the user's address bar will read http://[host]/Servlet-2. A reload/refresh will execute both Servlet-2 and JSP-3.
Both kinds of redirections are useful, depending on the precise effect you want.
http://www.theserverside.com/discussions/thread.tss?thread_id=26425

Thank you Fahmi Jafar (fahmijafar@fahmijafar.net)

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Knowledge Sharing : Java Servlet

by Fahmi Jafar on May 29, 2010

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