servlets basics:
What is web application?
A web application is a server side
application, runs on a server and produces the web-pages on to the browser.
Web-applications are 2 types.
1.
Presentation oriented web-applications.
2.
Service oriented web-applications.
1. Presentation oriented
web-applications:
Ø
Generally presentation oriented web-applications are
created by using static html pages.
Ø
These applications are concentrated only on
presentation or display on to the browser.
2. Service oriented web-applications:
Ø
Service oriented applications will concentrate only on
providing services to the clients.
Ø
These are implemented using server-side technologies
like servlet’s and jsp’s.
A web-application is a collection of both passive and active resources. We call these are web resources.
Passive resource means it do not require
server to process the request. These require browser to process the request.
Example: html pages and images both are
passive resources.
Servlet’s and jsp’s are active resources
they need server to process the requests.
When a request is given for a resource of a web-application to the server, then 1st server receives the request and identifies whether the request is given for passive resource or active resource.
If the
request is given for passive resource then server takes care about the request
and provides the required page to the browser.
If the client request is given for active resource, then server will hand over the request to another application called web-container. This container will do the remaining work and provides response back to the server. Now the server will send this response back to the browser/client.
What is a servlet?
Ø
A servlet is a server side web-component, managed by
the web-container for the generation of dynamic information.
Ø
Web-container will take care about entire low level
activities like identification of
required servlet, load the corresponding servlet class, instantiation,
invocation of init(),service() methods.
Ø
A servlet is a platform independent java class runs at
server side and increases the functionality of a server.
Sun Micro System released the servlet technology for two types of people.
1.
1.Application writers
2.
2.server writers
Application writers are called programmers or developers and server writers are called vendors.
Servlet API has 2 packages.
1.
Javax.servlet
2.
Javax.servlet.http
For application writers i.e for programmers sun provided the servlet API.
For server writers sun provided the
servlet specification.it contains set of guidelines to be followed while
providing implementation for interfaces.
Different ways to create a Servlet:
v
By implementing the servlet interface.
v
By extending the GenericServlet class.(abstract class)
v
By extending HttpServlet class(abstract class)
Servlet interface:
v
It acts as a root interface for entire servlet
specification.
v
Every servlet in java compulsory should implement this
interface either directly or indirectly.
v
The methods in the servlet interface are
1. init()
2. service()
3. destroy()
4. getServletConfig()
5. getServletInfo()
init():
·
This method will be executed by the web-container only
once just after instantiating the servlet immediately.
·
The main purpose of this method is to perform
initialization.
·
After compilation of init() method only the servlet is
in a position to process client request.
Web-container
won’t place the servlet into service in the following cases.
1. The init() method throws ServletException.
2. If init()
method not completed within the time period specified by the web-container.
In the above cases the web-container
makes that servlet object eligible for
garbage collection without calling any other life-cycle methods.
Service() :
Public void
service(ServletRequest request,ServletResponse response) throws
ServletException,IOException
·
This method will be executed for every client request.
·
Entire service logic we have to write in this method
only.
·
If we are defining servlet by extending the
GenericServlet abstract class then compulsory we should provide service()
method in our servlet class.
·
If we are creating the servlet class by extending the
HttpServlet abstract class then it is
not recommended to override service() method in our class. we have to provide
the service logic in either doGet() or doPost() methods.
Destroy():
·
This method will be executed only once just before
taking the servlet object from the out
of service.
·
The main purpose of destroy() method is for clean-up
activities.
·
While executing
destroy() method if any exception has raised JVM ignores that exception.
·
Whenever web-container calls the destroy method, it
will wait until all the threads in the service() method complete their
execution.
Note:
from the sevice() method we can call destroy() method
explicitly in that case it will be executed just like a normal method call and
servlet object cannot be destroyed.
getSevletConfig():
public ServletConfig getServletConfig();
this method returns ServletConfig object and by using
that object servlet object get its all
information.
getServletInfo():
this method returns information about the servlet
like purpose, author, copy right
information.etc
SERVLET LIFE-CYCLE:
·
For our servlet web-container creates the object, in
order to create the object for our class
the class must be visible to the web-container so we must create the class with
public modifier.
Phases in life-cycle:
1. Not
available
2. Instantiation
3. Initialization
4. Service
5. Destroy
1.Not available:
·
For our servlet
class objects will be created by the web-container.
·
Web-container is a part of a server
·
Before creating the object for a servlet ,the servlet
in a position called the not available.
·
By default the container creates an object to our servlet
class, whenever the first request is given to
it.
·
We can also tell to the container, create the object
to our servlet class before first request come to it. i.e at the time of server
start-up.
2.Instantiation & 3.Initialization:
·
Whenever the object of servlet is created then it is
moved from the stage not available to instantiation.
·
After instantiated ,the web-container calls the
constructor of the servlet class and then init() method.
·
By using the init() method web-container initializes
the servlet object by using init() method of the servlet class.
4.Service:
·
After initialization done, the web-container calls the
service() method of the servlet object for providing the services for the given
client request.
·
For every request given by client web-container calls the service() method.
5.Destroy:
·
Whenever the server is shout-down or servlet is
removed from the server, then it’s object will be destroyed.
·
In order to de initializes the resources web-container
calls destroy() method.
Simple understanding of servlet lifecycle:
1. Loading the
servlet class.
2. Servlet
Instantiation.
3. Execution of
init() method.
4. Execution of
service() method.
5. Execution of
destroy() method.
All of the above 5 tasks are done by the
web-container.
v Except the
service() method init() and destroy() methods are one time activities.
v For every
servlet web-container creates ServletConfig object just before calling init()
method
v For every
request web-container creates the ServletRequest and ServletResponse object s
just before invoking the service()
method.
v For every
web-application web-container creates the ServletContext object at the time of
application deployment.
GenericServlet:
v GenericServlet
implements the Servlet interface and provides the implementation for every
method except service() method. Hence GenericServlet is declared as abstract
class.
v
v GenericServlet implements ServletConfig,
Servlet and Serializable interfaces.
v If you
create the servlet class by extending GenericServlet class then you must override
the service() method.
public class abstract GenericServlet
implements Servlet,ServletConfig,Serializable
{
private transient ServletConfig config;
public
void init(ServletConfig config)throws ServletException
{
this.config=config;
init();
}
Public void init() throws
ServletException
{
}
Public servletConfig getServletInfo()
{
return config;
}
}
GenericServlet contains 2 init(ServletConfig)
methods one is for web-container purpose
And other init() method for programmer
purpose.
HttpServlet:
HttpServlet acts as base class for developing HttpServlets.
HttpServlet contains 2 service() methods.
1. public void service(ServletRequest request,ServletResponse response) throws ServletException
2. protected void service(ServletRequest request,ServletResponse response) throws ServletException,IOException
LifeCycle of HttpServlet:
1. when ever we sending the form ,the browser prepares the http request object and send its to the server.
2.server checks whether the request is for static or dynamic information.
3.if the request ius for dynamic information then webserver forwards the request to the web-container.
4.web-container identifies the corresponding servlet by usnig web.xml
5.web-container checks whether the servlet object is already available or not,if not web-container loads the servlet class followed by instantiation and execution of init() method.
6.then web-container invokes public service() methos.if the service() method is available it will execute otherwise the HttpServlet service() method will be executed. whic is implemented as follows
public void service(ServletRequest request,ServletResponse response) throws ServletException,IOException
{
HttpServletRequest=(HttpServletRequest)request;;
HttpServletResponse=(HttpServletResponse)response;
service(request,response);
}
7.web-container checks whether protected service() method available or not,if not HttpServlet service() will be executed like this
protected void service( HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException
{
String method=request.getMethod();
if(method=="GET")
{
doGet(request,response);
}
else if(method=="POST")
{
doPost(request,response);
}
String method=request.getMethod();
if(method=="GET")
{
doGet(request,response);
}
else if(method=="POST")
{
doPost(request,response);
}
if(method=="HEAD")
{
doHead(request,response);
}
'
'
'
'
else
provide 501 status response saying invalid http method
}
8.from service() method web-container identifies the request method and invokes the corresponding doxxx() method.
9.
web-container checks whether this doxxx() method is available or not if notit will be execute the HttpServlet class doxxx() method which will having error information..
methods flow by the container:
public service()------->protected service()-------->doxxx()
{
doHead(request,response);
}
'
'
'
'
else
provide 501 status response saying invalid http method
}
8.from service() method web-container identifies the request method and invokes the corresponding doxxx() method.
9.
web-container checks whether this doxxx() method is available or not if notit will be execute the HttpServlet class doxxx() method which will having error information..
methods flow by the container:
public service()------->protected service()-------->doxxx()