Servlets Life Cycle have the following Components:
Handled by the servlets container.
Create and initialize the servlets.
Handle zero or more service call.
Destroy and garbage collect the servlets.
A single servlets instance to handle every request
A D V E R T I S E M E N T
The Basic Servlet Architecture or Life Cycle
Servlet container create only one instance of each servlet but the request of each user is handled with a separate thread.
Each thread then calls the doGet or doPost methods.
This idea is shown in the above Figure-5.
1.The init method of the servlets is called when the servlets is first created and
each time the server receives a request for a servlets, the server spawns a
new thread calls service method.
2.The service method check the HTTP request type (GET,SET, PUT, DELETE, etc.) and
calls doGet, doPost, doPut,doDelete, etc. method as appropriate.
3.Finally, the server may decide to remove a previous loaded servlet.
In this case, the server calls destroy method of the servlets.
Before we can start writing the first Servlets, we need to know some
basics of HTTP ("HyperText Transfer Protocol"), the protocol which is
used by a WWW client (e.g. a browser) to send a request to a Web Server.
HTTP is the request-response oriented protocol. An HTTP request consist
of a request method, a URI, header fields and a body (which can be
empty). An HTTP response contain a result and again header fields
and a body.
The service method of HttpServlet
dispatch a request to different Java methods for different HTTP
request methods. It recognize the standard HTTP/1.1 methods and should
not be overridden in subclasses unless you need to implement additional
methods. The recognized methods are GET, PUT,HEAD, POST, DELETE,
OPTIONS and TRACE. Other methods are answered with a Bad Request
HTTP error. An HTTP method XXX is dispatched to the Java method
doXxx, e.g. GET -> doGet.
All these method expect the parameters "(HttpServletRequest req,
HttpServletResponse res)". The method doOptions and
doTrace have suitable default implementations and are
usually not overridden. The HEAD method (which is supposed to return the
same header lines that a GET method would return, but doesn't include a
body) is performed by calling the doGet and ignoring any output
that is written by this method. That leaves us with the method
doGet doPut doPost and doDelete whose default implementations in HttpServlet return a
Bad Request HTTP error. A subclass of HttpServlet
overrides one or more of these method to provide a meaningful
The request data is passed to all the methods through the first argument
of type HttpServletRequest (which is a subclass of the more
general ServletRequest class). The response can be created
with the methods of the second argument of type HttpServletResponse
(a subclass of ServletResponse).
When you request a URL in the Web Browser, the GET method is used for
the request. A GET request does not have the body (i.e. the body is
empty). The response should contain the body with the response data and
header fields which describe the body (especially Content-Type
and Content-Encoding). When you send an HTML form, either
GET or POST action can be used. With the GET request the parameters are end
in the URL, with a POST request they are transmited in the body. HTML
editors and upload tools use PUT requests to the upload resources to a Web
Server and DELETE requests to delete resources.
Classic Servlets-service() Method
JSP's-Java Embeded in HTML Templates
Http Servlets-doGet & doPost()
Visual Servlets-Generated by Visual Age
Packages in Servlets
There are two types of package available in servlets which are as follows:
Interfaces in javax.servlet.*
classes in javax.servlet.*
Interfaces in javax.servlet.http.*
classes in javax.servlet.http.*
Servlet Scope Objects
The <minOccurs> indicator specify the minimum number of time an element can occur:
There are four scope objects in servlets which enables the sharing information between web components.
The scope objects and their corresponding Java classes are listed below:
Web Context javax.servlet.ServletContext
You can get the attribute values from servlet scope objects using getAttribute method and set new values of attributes using setAttribute method.
For example, you can get the clients IP address by calling the getRemoteAddr method of HttpServletRequest class.
Following Examples prints the Hello World in the browser
public class HelloWorld extends HttpServlet
public void doGet(HttpServletRequest request,
throws ServletException, IOException
PrintWriter out = response.getWriter();
To be a servlet, a class should extend the HttpServlet and override doGet
or doPost (or both), depending on whether the data is being sent by
GET or by POST. These methods take two arguments: an
HttpServletRequest and an HttpServletResponse. The HttpServletRequest
has methods that let you find out about the incoming information such as FORM data,
HTTP request headers, and the like.
The HttpServletResponse has
method that lets you specify the HTTP response line (200, 404, etc.), response
headers (Content-Type, Set-Cookie, etc.), and, most
importantly, lets you obtain a PrintWriter used to send output back to the
client. For simple servlet, most of the effort is spent in println
statements that generate the desired page. Note that doGet and
doPost throw two exception, so you are required to include them in
the declaration. Also note that you have to import classes in the java.io
(for PrintWriter, etc.), javax.servlet (for HttpServlet, etc.), and
javax.servlet.http (for HttpServletRequest and HttpServletResponse). Finally, note
that doGet and doPost are called by service
method, and sometimes you may want to override service directly,
e.g. for a servlet that handles both GET and POST
Compiling and Installing the Servlet
Note that the specific details for installing servlet vary from Web server
to Web server. Please refer to your Web server documentation for the definitive
directions. The on-line examples are running on Java Web Server (JWS) 2.0, where
servlet are expected to be in a directory called servlets in the
JWS installation hierarchy. However, I placed this servlets in a separate package
(hall) to avoid conflicts with other servlets on this server;
you'll want to do the same if you are using a Web server that is used by other
people and doesn't have a good infrastructure for the "virtual servers" to prevent
these conflicts automatically. Thus, HelloWorld.java actually goes
in a subdirectory called hall in the servlets
Note that setup on most other servers is similar, and the servlets and
JSP examples in the tutorial have also been tested using BEA WebLogic and IBM
WebSphere 3.0. WebSphere has an excellent mechanism for virtual servers, and it
is not necessary to use packages solely to prevent name conflicts with other
If you've never used the packages before, there are two main ways to compile
classes that are in packages.
One way is to set your CLASSPATH to point to directory
above the one actually containing your servlets. You can them compile
normally from within the directory. For example, if your base directory is
C:\JavaWebServer\servlets and your package name (and thus subdirectory
name) is the hall, and you were on Windows, you'd do:
DOS> set CLASSPATH=C:\JavaWebServer\servlets;%CLASSPATH%
DOS> cd C:\JavaWebServer\servlets\hall
DOS> javac YourServlet.java
The first part, setting CLASSPATH, you probably want to
do permanently, rather than each time you start a new DOS window. On Windows
95/98 you'd typically put the "set CLASSPATH=..." statement in your
autoexec.bat file somewhere after the line that set
CLASSPATH to point to servlet.jar and jsp.jar.
On Windows NT, you'd go to Start menu, select Settings, select Control
Panel, select System, select Environment, then enter the variable and value.
Note also that if your package were of the form name1.name2.name3
rather than simply name1 as here, you'd still have the
CLASSPATH point to the top-level directory of your package hierarchy (the
one containing name1).
A second way to compile classes that are in packages is to go to directory above
the one containing your servlets, and then do "javac
directory\YourServlet.java" (Windows; note the backslash) or "javac
directory/YourServlet.java" (Unix; note the forward slash). For
example, suppose again that your base directory is C:\JavaWebServer\servlets
and your package name (and thus subdirectory name) is hall, and you
were on Windows. In that case, you'd do following:
DOS> cd C:\JavaWebServer\servlets
DOS> javac hall\YourServlet.java
Note that, on Windows, most JDK 1.1 versions of javac
require a backslash, not a forward slash, after directory name. This is
fixed in JDK 1.2, but since many Web servers are configured to use the JDK 1.1, many
servlet authors stick with JDK 1.1 for portability.
Finally, another advanced option is to keep source code in a location
distinct from the .class files, and use javac's "-d"
option to install them in the location the Web server expects.
Running the Servlet
With the Java Web Server, servlet are placed in the servlets
directory within the main JWS installation directory, and are invoked via
http://host/servlet/ServletName. Note that the directory is servlets
plural, while URL refers to servlet, singular. Since this
example was placed in the hall package, it would be invoked via
http://host/servlet/hall.HelloWorld. Other Web servers may have
slightly different conventions on where to install servlets and how to invoke
them. Most server also let you define aliases for servlets, so that a servlet
can be invoked via http://host/any-path/any-file.html. The process
for doing this is completely server-specific; check your server's documentation
for the details.
A Servlet that Generates HTML
Most servlet generate HTML, not plain text as in the previous example. To
do that, you need two additional steps: tell the browser that you're sending
back HTML, and modify the println statements to build a legal Web
page. The first step is done by setting Content-Type response
header. In general, headers can be set via the setHeader method of
HttpServletResponse, but setting the content type is such a common
task that there is also a special setContentType method just for
this purpose. Note that you need to set the response headers before actually
returning any of the content via the PrintWriter. Here's an
The following program called HelloWWW.java will print Hellow WWW in the browser in the HTML format.