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Servlets
What is the servlet?
Servlet is a script, which resides and executes
on server side, to create dynamic HTML. In servlet
programming we will use java language. A servlet
can handle multiple requests concurrently.
What is the architechture of servlet
package?
Servlet Interface is the central abstraction.
All servlets implements this Servlet
Interface either direclty or indirectly
( may implement or extend Servlet Interfaces sub
classes or sub interfaces)
Servlet
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Generic Servlet
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HttpServlet ( Class ) -- we will extend this class
to handle GET / PUT HTTP requests
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MyServlet
What is the difference between HttpServlet
and GenericServlet?
A GenericServlet has a service() method to handle
requests.
HttpServlet extends GenericServlet added new methods
doGet()
doPost()
doHead()
doPut()
doOptions()
doDelete()
doTrace() methods
Both these classes are abstract.
What's the difference between servlets
and applets?
Servlets executes on Servers. Applets executes
on browser. Unlike applets, however, servlets
have no graphical user interface.
What are the uses of Servlets?
A servlet can handle multiple requests concurrently,
and can synchronize requests. Servlets can forward
requests to other servers and servlets. Thus servlets
can be used to balance load among several servers.
When doGET() method will going to
execute?
When we specified method='GET' in HTML
Example : < form name='SSS' method='GET'>
When doPOST() method will going to
execute?
When we specified method='POST' in HTML
< form name='SSS' method='POST' >
What is the difference between Difference
between doGet() and doPost()?
GET Method : Using get method we can able to pass
2K data from HTML
All data we are passing to Server will be displayed
in URL (request string).
POST Method : In this method we does
not have any size limitation.
All data passed to server will be hidden, User
cannot able to see this info
on the browser.
What is the servlet life cycle?
When first request came in for the servlet , Server
will invoke init() method of the servlet. There
after if any user request the servlet program,
Server will directly executes the service() method.
When Server want to remove the servlet from pool,
then it will execute the destroy() method
Which code line must be set before
any of the lines that use the PrintWriter?
setContentType() method must be set.
Which protocol will be used by browser
and servlet to communicate?
HTTP
In how many ways we can track the
sessions?
Method 1) By URL rewriting
Method 2) Using Session object
Getting Session form HttpServletRequest
object
HttpSession session = request.getSession(true);
Get a Value from the session
session.getValue(session.getId());
Adding values to session
cart = new Cart();
session.putValue(session.getId(), cart);
At the end of the session, we can
inactivate the session by using the following
command
session.invalidate();
Method 3) Using cookies
Method 4) Using hidden fields
How Can You invoke other web resources
(or other servelt / jsp ) ?
Servelt can invoke other Web resources in two
ways: indirect and direct.
Indirect Way : Servlet will return
the resultant HTML to the browser which will point
to another Servlet (Web resource)
Direct Way : We can call another Web
resource (Servelt / Jsp) from Servelt program
itself, by using RequestDispatcher object.
You can get this object using getRequestDispatcher("URL")
method. You can get this object from either a
request or a Context.
Example :
RequestDispatcher dispatcher = request.getRequestDispatcher("/jspsample.jsp");
if (dispatcher != null)
dispatcher.forward(request, response);
}
How Can you include other Resources
in the Response?
Using include method of a RequestDispatcher object.
Included WebComponent (Servlet / Jsp)
cannot set headers or call any method (for example,
setCookie) that affects the headers of the response.
Example : RequestDispatcher dispatcher
= getServletContext().getRequestDispatcher("/banner");
if (dispatcher != null)
dispatcher.include(request, response);
}
What is the difference between the
getRequestDispatcher(String path) ServletRequest
interface and ServletContext interface?
The getRequestDispatcher(String path) method of
ServletRequest interface accepts parameter the
path to the resource to be included or forwarded
to, which can be relative to the request of the
calling servlet. If the path begins with a "/"
it is interpreted as relative to the current context
root.
The getRequestDispatcher(String path)
method of ServletContext interface cannot accepts
relative paths. All path must sart with a "/"
and are interpreted as relative to curent context
root. If the resource is not available, or if
the server has not implemented a RequestDispatcher
object for that type of resource, getRequestDispatcher
will return null. Your servlet should be prepared
to deal with this condition.
What is the use of ServletContext
?
Using ServletContext, We can access data from
its environment. Servlet context is common to
all Servlets so all Servlets share the information
through ServeltContext.
can we create more than ServletContext
and ServletConfig in your application??
ServletContext which is managed by
webcontainer, For the
whole web application only one servletcontext
will be
there...
In ServletConfig , this is also managed
by webcontainer,
for every request object one servletconfig object
will be
there..
Question: What is ServletContext?
Answer: ServletContext is an Interface that defines
a set of methods that a servlet uses to communicate
with its servlet container, for example, to get
the MIME type of a file, dispatch requests, or
write to a log file. There is one context per
"web application" per Java Virtual Machine.
(A "web application" is a collection
of servlets and content installed under a specific
subset of the server's URL namespace such as /catalog
and possibly installed via a .war file.)
Question: What is meant by Pre-initialization
of Servlet?
Answer: When servlet container is loaded, all
the servlets defined in the web.xml file does
not initialized by default. But the container
receives the request it loads the servlet. But
in some cases if you want your servlet to be initialized
when context is loaded, you have to use a concept
called pre-initialization of Servlet. In case
of Pre-initialization, the servlet is loaded when
context is loaded. You can specify <load-on-startup>1</load-on-startup>
in between the <servlet></servlet>
tag.
Question: What mechanisms are used
by a Servlet Container to maintain session information?
Answer: Servlet Container uses Cookies, URL rewriting,
and HTTPS protocol information to maintain the
session.
Question: What do you understand by
servlet mapping?
Answer: Servlet mapping defines an association
between a URL pattern and a servlet. You can use
one servlet to process a number of url pattern
(request pattern). For example in case of Struts
*.do url patterns are processed by Struts Controller
Servlet.
Question: What must be implemented
by all Servlets?
Answer: The Servlet Interface must be implemented
by all servlets.
Question: What are the differences
between Servlet and Applet?
Answer: Servlets are server side components that
runs on the Servlet container. Applets are client
side components and runs on the web browsers.
Servlets have no GUI interface.
Question: What are the uses of Servlets?
Answer: * Servlets are used to process the client
request.
* A Servlet can handle multiple request concurrently
and be used to develop high performance system
* A Servlet can be used to load balance among
serveral servers, as Servlet can easily forward
request.
Question: What are the objects that
are received when a servlets accepts call from
client?
Answer: The objects are ServeltRequest and ServletResponse
. The ServeltRequest encapsulates the communication
from the client to the
server. While ServletResponse encapsulates the
communication from the Servlet back to the client.
Question: What is a Session?
Answer: A Session refers to all the request that
a single client makes to a server. A session is
specific to the user and for each user a new session
is created to track all the request from that
user. Every user has a separate session and separate
session variable is associated with that session.
In case of web applications the default time-out
value for session variable is 20 minutes, which
can be changed as per the requirement.
Question: What is Session ID?
Answer: A session ID is an unique identification
string usually a long, random and alpha-numeric
string, that is transmitted between the client
and the server. Session IDs are usually stored
in the cookies, URLs (in case url rewriting) and
hidden fields of Web pages.
Question: What is Session Tracking?
Answer: HTTP is stateless protocol and it does
not maintain the client state. But there exist
a mechanism called "Session Tracking"
which helps the servers to maintain the state
to track the series of requests from the same
user across some period of time.
Question: What are different types
of Session Tracking?
Answer: Mechanism for Session Tracking are:
a) Cookies
b) URL rewriting
c) Hidden form fields
d) SSL Sessions
Question: What is HTTPSession Class?
Answer: HttpSession Class provides a way to identify
a user across across multiple request. The servlet
container uses HttpSession interface to create
a session between an HTTP client and an HTTP server.
The session lives only for a specified time period,
across more than one connection or page request
from the user.
Question: Why do u use Session Tracking
in HttpServlet?
Answer: In HttpServlet you can use Session Tracking
to track the user state. Session is required if
you are developing shopping cart application or
in any e-commerce application.
Question: What are the advantage of
Cookies over URL rewriting?
Answer: Sessions tracking using Cookies are more
secure and fast. Session tracking using Cookies
can also be used with other mechanism of Session
Tracking like url rewriting.
Cookies are stored at client side
so some clients may disable cookies so we may
not sure that the cookies may work or not.
In url rewriting requites large data
transfer from and to the server. So, it leads
to network traffic and access may be become slow.
Question: What is session hijacking?
Answer: If you application is not very secure
then it is possible to get the access of system
after acquiring or generating the authentication
information. Session hijacking refers to the act
of taking control of a user session after successfully
obtaining or generating an authentication session
ID. It involves an attacker using captured, brute
forced or reverse-engineered session IDs to get
a control of a legitimate user's Web application
session while that session is still in progress.
Question: What is Session Migration?
Answer: Session Migration is a mechanism of moving
the session from one server to another in case
of server failure. Session Migration can be implemented
by:
a) Persisting the session into database
b) Storing the session in-memory on multiple servers.
Question: How to track a user session
in Servlets?
Answer: The interface HttpSession can be used
to track the session in the Servlet. Following
code can be used to create session object in the
Servlet: HttpSession session = req.getSession(true);
Question: How you can destroy the
session in Servlet?
Answer: You can call invalidate() method on the
session object to destroy the session. e.g. session.invalidate();
Collection of large number of Servlet
Interview Questions. These questions are frequently
asked in the Java Interviews.
Question: What is a Servlet?
Answer: Java Servlets are server side components
that provides a powerful mechanism for developing
server side of web application. Earlier CGI was
developed to provide server side capabilities
to the web applications. Although CGI played a
major role in the explosion of the Internet, its
performance, scalability and reusability issues
make it less than optimal solutions. Java Servlets
changes all that. Built from ground up using Sun's
write once run anywhere technology java servlets
provide excellent framework for server side processing.
Question: What are the types of Servlet?
Answer: There are two types of servlets, GenericServlet
and HttpServlet. GenericServlet defines the generic
or protocol independent servlet. HttpServlet is
subclass of GenericServlet and provides some http
specific functionality linke doGet and doPost
methods.
Question: What are the differences
between HttpServlet and Generic Servlets?
Answer: HttpServlet Provides an abstract class
to be subclassed to create an HTTP servlet suitable
for a Web site. A subclass of HttpServlet must
override at least one method, usually one of these:
doGet, if the servlet supports HTTP
GET requests
doPost, for HTTP POST requests
doPut, for HTTP PUT requests
doDelete, for HTTP DELETE requests
init and destroy, to manage resources that are
held for the life of the servlet
getServletInfo, which the servlet uses to provide
information about itself
There's almost no reason to override the service
method. service handles standard HTTP requests
by dispatching them to the handler methods for
each HTTP request type (the doXXX methods listed
above). Likewise, there's almost no reason to
override the doOptions and doTrace methods.
GenericServlet defines a generic,
protocol-independent servlet. To write an HTTP
servlet for use on the Web, extend HttpServlet
instead.
GenericServlet implements the Servlet
and ServletConfig interfaces. GenericServlet may
be directly extended by a servlet, although it's
more common to extend a protocol-specific subclass
such as HttpServlet.
GenericServlet makes writing servlets
easier. It provides simple versions of the lifecycle
methods init and destroy and of the methods in
the ServletConfig interface. GenericServlet also
implements the log method, declared in the ServletContext
interface.
To write a generic servlet, you need
only override the abstract service method.
Question: Differentiate between Servlet
and Applet.
Answer: Servlets are server side components that
executes on the server whereas applets are client
side components and executes on the web browser.
Applets have GUI interface but there is not GUI
interface in case of Servlets.
Question: Differentiate between doGet
and doPost method?
Answer: doGet is used when there is are requirement
of sending data appended to a query string in
the URL. The doGet models the GET method of Http
and it is used to retrieve the info on the client
from some server as a request to it. The doGet
cannot be used to send too much info appended
as a query stream. GET puts the form values into
the URL string. GET is limited to about 256 characters
(usually a browser limitation) and creates really
ugly URLs.
POST allows you to have extremely
dense forms and pass that to the server without
clutter or limitation in size. e.g. you obviously
can't send a file from the client to the server
via GET. POST has no limit on the amount of data
you can send and because the data does not show
up on the URL you can send passwords. But this
does not mean that POST is truly secure. For real
security you have to look into encryption which
is an entirely different topic
Question: What are methods of HttpServlet?
Answer: The methods of HttpServlet class are :
* doGet() is used to handle the GET, conditional
GET, and HEAD requests
* doPost() is used to handle POST requests
* doPut() is used to handle PUT requests
* doDelete() is used to handle DELETE requests
* doOptions() is used to handle the OPTIONS requests
and
* doTrace() is used to handle the TRACE requests
Question: What are the advantages
of Servlets over CGI programs?
Answer: Question: What are methods of HttpServlet?
Answer: Java Servlets have a number of advantages
over CGI and other API's. They are:
Platform Independence
Java Servlets are 100% pure Java, so it is platform
independence. It can run on any Servlet enabled
web server. For example if you develop an web
application in windows machine running Java web
server. You can easily run the same on apache
web server (if Apache Serve is installed) without
modification or compilation of code. Platform
independency of servlets provide a great advantages
over alternatives of servlets.
Performance
Due to interpreted nature of java, programs written
in java are slow. But the java servlets runs very
fast. These are due to the way servlets run on
web server. For any program initialization takes
significant amount of time. But in case of servlets
initialization takes place very first time it
receives a request and remains in memory till
times out or server shut downs. After servlet
is loaded, to handle a new request it simply creates
a new thread and runs service method of servlet.
In comparison to traditional CGI scripts which
creates a new process to serve the request. This
intuitive method of servlets could be use to develop
high speed data driven web sites.
Extensibility
Java Servlets are developed in java which is robust,
well-designed and object oriented language which
can be extended or polymorphed into new objects.
So the java servlets takes all these advantages
and can be extended from existing class the provide
the ideal solutions.
Safety
Java provides a very good safety features like
memory management, exception handling etc. Servlets
inherits all these features and emerged as a very
powerful web server extension.
Secure
Servlets are server side components, so it inherits
the security provided by the web server. Servlets
are also benefited with Java Security Manager.
Question: What are the lifecycle methods
of Servlet?
Answer: The interface javax.servlet.Servlet, defines
the three life-cycle methods. These are:
public void init(ServletConfig config) throws
ServletException
public void service( ServletRequest req, ServletResponse
res) throws ServletException, IOException
public void destroy()
The container manages the lifecycle of the Servlet.
When a new request come to a Servlet, the container
performs the following steps.
1. If an instance of the servlet does not exist,
the web container
* Loads the servlet class.
* Creates an instance of the servlet class.
* Initializes the servlet instance by calling
the init method. Initialization is covered in
Initializing a Servlet.
2. The container invokes the service method, passing
request and response objects.
3. To remove the servlet, container finalizes
the servlet by calling the servlet's destroy method.
Question: What are the type of protocols
supported by HttpServlet?
Answer: It extends the GenericServlet base class
and provides an framework for handling the HTTP
protocol. So, HttpServlet only supports HTTP and
HTTPS protocol.
Question: What are the directory Structure
of Web Application?
Answer: Web component follows the standard directory
structure defined in the J2EE specification
Question: What is ServletContext?
Answer: ServletContext is an Interface that defines
a set of methods that a servlet uses to communicate
with its servlet container, for example, to get
the MIME type of a file, dispatch requests, or
write to a log file. There is one context per
"web application" per Java Virtual Machine.
(A "web application" is a collection
of servlets and content installed under a specific
subset of the server's URL namespace such as /catalog
and possibly installed via a .war file.)
Question: What is meant by Pre-initialization
of Servlet?
Answer: When servlet container is loaded, all
the servlets defined in the web.xml file does
not initialized by default. But the container
receives the request it loads the servlet. But
in some cases if you want your servlet to be initialized
when context is loaded, you have to use a concept
called pre-initialization of Servlet. In case
of Pre-initialization, the servlet is loaded when
context is loaded. You can specify <load-on-startup>1</load-on-startup>
in between the <servlet></servlet>
tag.
Question: What mechanisms are used
by a Servlet Container to maintain session information?
Answer: Servlet Container uses Cookies, URL rewriting,
and HTTPS protocol information to maintain the
session.
Question: What do you understand by
servlet mapping?
Answer: Servlet mapping defines an association
between a URL pattern and a servlet. You can use
one servlet to process a number of url pattern
(request pattern). For example in case of Struts
*.do url patterns are processed by Struts Controller
Servlet.
Question: What must be implemented
by all Servlets?
Answer: The Servlet Interface must be implemented
by all servlets.
Question: What are the differences
between Servlet and Applet?
Answer: Servlets are server side components that
runs on the Servlet container. Applets are client
side components and runs on the web browsers.
Servlets have no GUI interface.
Question: What are the uses of Servlets?
Answer: * Servlets are used to process the client
request.
* A Servlet can handle multiple request concurrently
and be used to develop high performance system
* A Servlet can be used to load balance among
serveral servers, as Servlet can easily forward
request.
Question: What are the objects that
are received when a servlets accepts call from
client?
Answer: The objects are ServeltRequest and ServletResponse
. The ServeltRequest encapsulates the communication
from the client to the
server. While ServletResponse encapsulates the
communication from the Servlet back to the client.
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