Servlets Interview Questions

1.What is the Servlet?
A servlet is a Java programming language class that is used to extend the capabilities of servers
that host applications accessed by means of a request- response programming model.

2.What are the new features added to Servlet 2.5?
Following are the changes introduced in Servlet 2.5:
 A new dependency on J2SE 5.0
 Support for annotations
 Loading the class
 Several web.xml conveniences
 A handful of removed restrictions
 Some edge case clarifications

Learn more about Servlets 2.5 features

3.What are the uses of Servlet?
Typical uses for HTTP Servlets include:
 Processing and/or storing data submitted by an HTML form.
 Providing dynamic content, e.g. returning the results of a database query to the client.
 A Servlet can handle multiple request concurrently and be used to develop high
performance system
 Managing state information on top of the stateless HTTP, e.g. for an online shopping cart
system which manages shopping carts for many concurrent customers and maps every
request to the right customer.

4.What are the advantages of Servlet over CGI?
Servlets have several advantages over CGI:
 A Servlet does not run in a separate process. This removes the overhead of creating a new
process for each request.
 A Servlet stays in memory between requests. A CGI program (and probably also an
extensive runtime system or interpreter) needs to be loaded and started for each CGI
request.
 There is only a single instance which answers all requests concurrently. This saves
memory and allows a Servlet to easily manage persistent data.
 Several web.xml conveniences
 A handful of removed restrictions
 Some edge case clarifications

5.What are the phases of the servlet life cycle?
The life cycle of a servlet consists of the following phases:
 Servlet class loading : For each servlet defined in the deployment descriptor of the Web
application, the servlet container locates and loads a class of the type of the servlet. This
can happen when the servlet engine itself is started, or later when a client request is
actually delegated to the servlet.

 Servlet instantiation : After loading, it instantiates one or more object instances of the
servlet class to service the client requests.

 Initialization (call the init method) : After instantiation, the container initializes a
servlet before it is ready to handle client requests. The container initializes the servlet by
invoking its init() method, passing an object implementing the ServletConfig interface. In
the init() method, the servlet can read configuration parameters from the deployment
descriptor or perform any other one-time activities, so the init() method is invoked once
and only once by the servlet container.

 Request handling (call the service method) : After the servlet is initialized, the
container may keep it ready for handling client requests. When client requests arrive, they
are delegated to the servlet through the service() method, passing the request and
response objects as parameters. In the case of HTTP requests, the request and response
objects are implementations of HttpServletRequest and HttpServletResponse
respectively. In the HttpServlet class, the service() method invokes a different handler
method for each type of HTTP request, doGet() method for GET requests, doPost()
method for POST requests, and so on.

 Removal from service (call the destroy method) : A servlet container may decide to
remove a servlet from service for various reasons, such as to conserve memory resources.
To do this, the servlet container calls the destroy() method on the servlet. Once the
destroy() method has been called, the servlet may not service any more client requests.
Now the servlet instance is eligible for garbage collection
The life cycle of a servlet is controlled by the container in which the servlet has been
deployed.



6.Why do we need a constructor in a servlet if we use the init method?
Even though there is an init method in a servlet which gets called to initialize it, a constructor is
still required to instantiate the servlet. Even though you as the developer would never need to
explicitly call the servlet's constructor, it is still being used by the container (the container still
uses the constructor to create an instance of the servlet). Just like a normal POJO (plain old java
object) that might have an init method, it is no use calling the init method if you haven't
constructed an object to call it on yet.

7.How the servlet is loaded?
A servlet can be loaded when:
 First request is made.
 Server starts up (auto-load).
 There is only a single instance which answers all requests concurrently. This saves
memory and allows a Servlet to easily manage persistent data.
 Administrator manually loads.

8.How a Servlet is unloaded?
A servlet is unloaded when:
 Server shuts down.
 Administrator manually unloads.

9.What is Servlet interface?
The central abstraction in the Servlet API is the Servlet interface. All servlets implement this
interface, either directly or , more commonly by extending a class that implements it.


Note: Most Servlets, however, extend one of the standard implementations of that interface,
namely javax.servlet.GenericServlet and javax.servlet.http.HttpServlet.

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10.What is the GenericServlet class?
GenericServlet is an abstract class that
implements the Servlet interface and the ServletConfig interface. In addition to the methods
declared in these two interfaces, this class also provides simple versions of the lifecycle methods
init and destroy, and implements the log method declared in the ServletContext interface.
Note: This class is known as generic servlet, since it is not specific to any protocol.

11.What's the difference between GenericServlet and HttpServlet?
GenericServlet HttpServlet
The GenericServlet is an abstract class that is
extended by HttpServlet to provide HTTP
protocol-specific methods.
An abstract class that simplifies writing HTTP
servlets. It extends the GenericServlet base
class and provides an framework for handling
the HTTP protocol.
The GenericServlet does not include protocol-
specific methods for handling request
parameters, cookies, sessions and setting
response headers.
The HttpServlet subclass passes generic
service method requests to the relevant doGet()
or doPost() method.
GenericServlet is not specific to any protocol.
HttpServlet only supports HTTP and HTTPS
protocol.

12.Why is HttpServlet declared abstract?
The HttpServlet class is declared abstract because the default implementations of the main
service methods do nothing and must be overridden. This is a convenience implementation of the
Servlet interface, which means that developers do not need to implement all service methods. If
your servlet is required to handle doGet() requests for example, there is no need to write a
doPost() method too.

13.Can servlet have a constructor ?
One can definitely have constructor in servlet.Even you can use the constrctor in servlet for
initialization purpose,but this type of approch is not so common. You can perform common
operations with the constructor as you normally do.The only thing is that you cannot call that
constructor explicitly by the new keyword as we normally do.In the case of servlet, servlet
container is responsible for instantiating the servlet, so the constructor is also called by servlet
container only.

14.What are the types of protocols supported by HttpServlet ?
 Servlets Interview Questions
It extends the GenericServlet base class and provides a framework for handling the HTTP
protocol. So, HttpServlet only supports HTTP and HTTPS protocol.

15.What is the difference between doGet() and doPost()?
# doGet() doPost()
1
In doGet() the parameters are appended to
the URL and sent along with header
information.
In doPost(), on the other hand will (typically)
send the information through a socket back to
the webserver and it won't show up in the
URL bar.
2
The amount of information you can send
back using a GET is restricted as URLs can
only be 1024 characters.
You can send much more information to the
server this way - and it's not restricted to
textual data either. It is possible to send files
and even binary data such as serialized Java
objects!
3
doGet() is a request for information; it does
not (or should not) change anything on the
server. (doGet() should be idempotent)
doPost() provides information (such as
placing an order for merchandise) that the
server is expected to remember
4 Parameters are not encrypted Parameters are encrypted
5
doGet() is faster if we set the response
content length since the same connection is
used. Thus increasing the performance
doPost() is generally used to update or post
some information to the server.doPost is
slower compared to doGet since doPost does
not write the content length
6
doGet() should be idempotent. i.e. doget
should be able to be repeated safely many
times
This method does not need to be idempotent.
Operations requested through POST can have
side effects for which the user can be held
accountable.
7
doGet() should be safe without any side
effects for which user is held responsible
This method does not need to be either safe
8 It allows bookmarks. It disallows bookmarks.

16.When to use doGet() and when doPost()?
Always prefer to use GET (As because GET is faster than POST), except mentioned in the
following reason:
 If data is sensitive
 Data is greater than 1024 characters
 If your application don't need bookmarks.

17.How do I support both GET and POST from the same Servlet?
The easy way is, just support POST, then have your doGet method call your doPost method:

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doPost(request, response);
}

18.Should I override the service() method?
We never override the service method, since the HTTP Servlets have already taken care of it .
The default service function invokes the doXXX() method corresponding to the method of the
HTTP request.For example, if the HTTP request method is GET, doGet() method is called by
default. A servlet should override the doXXX() method for the HTTP methods that servlet
supports. Because HTTP service method check the request method and calls the appropriate
handler method, it is not necessary to override the service method itself. Only override the
appropriate doXXX() method.

19.How the typical servlet code look like ?


20.What is a servlet context object?
A servlet context object contains the information about the Web application of which the servlet
is a part. It also provides access to the resources common to all the servlets in the application.
Each Web application in a container has a single servlet context associated with it.

21.What are the differences between the ServletConfig interface and the ServletContext
interface?
ServletConfig ServletContext
The ServletConfig interface is implemented by
the servlet container in order to pass
configuration information to a servlet. The
server passes an object that implements the
ServletConfig interface to the servlet's init()
method.
A ServletContext defines a set of methods that
a servlet uses to communicate with its servlet
container.
There is one ServletConfig parameter per
servlet.
There is one ServletContext for the entire
webapp and all the servlets in a webapp share
it.
The param-value pairs for ServletConfig object
are specified in the <init-param> within the
<servlet> tags in the web.xml file
The param-value pairs for ServletContext
object are specified in the <context-param>
tags in the web.xml file.


22.What's the difference between forward() and sendRedirect() methods?
forward() sendRedirect()
A forward is performed internally by the
servlet.
A redirect is a two step process, where the web
application instructs the browser to fetch a
second URL, which differs from the original.
The browser is completely unaware that it has
taken place, so its original URL remains intact.
The browser, in this case, is doing the work
and knows that it's making a new request.
Any browser reload of the resulting page will
simple repeat the original request, with the
original URL
A browser reloads of the second URL ,will not
repeat the original request, but will rather fetch
the second URL.
Both resources must be part of the same
context (Some containers make provisions for
cross-context communication but this tends not
to be very portable)
This method can be used to redirect users to
resources that are not part of the current
context, or even in the same domain.
Since both resources are part of same context,
the original request context is retained
Because this involves a new request, the
previous request scope objects, with all of its
parameters and attributes are no longer
available after a redirect.
(Variables will need to be passed by via the
session object).
Forward is marginally faster than redirect.
redirect is marginally slower than a forward,
since it requires two browser requests, not one.


23.What is the difference between the include() and forward() methods?
include() forward()
The RequestDispatcher include() method
inserts the the contents of the specified
resource directly in the flow of the servlet
The RequestDispatcher forward() method
is used to show a different resource in place of
the servlet that was originally called.
response, as if it were part of the calling
servlet.
If you include a servlet or JSP document, the
included resource must not attempt to change
the response status code or HTTP headers, any
such request will be ignored.
The forwarded resource may be another
servlet, JSP or static HTML document, but the
response is issued under the same URL that
was originally requested. In other words, it is
not the same as a redirection.
The include() method is often used to
include common "boilerplate" text or template
markup that may be included by many servlets.
The forward() method is often used where a
servlet is taking a controller role; processing
some input and deciding the outcome by
returning a particular response page.

24.What's the use of the servlet wrapper
classes??
The HttpServletRequestWrapper and
HttpServletResponseWrapper classes are
designed to make it easy for developers to
create custom implementations of the
servlet request and response types. The
classes are constructed with the standard
HttpServletRequest and HttpServletResponse instances respectively and their default
behaviour is to pass all method calls directly to the underlying objects.


25.What is the directory structure of a WAR file?
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26.What is a deployment descriptor?
A deployment descriptor is an XML document with an .xml extension. It defines a component's
deployment settings. It declares transaction attributes and security authorization for an enterprise
bean. The information provided by a deployment descriptor is declarative and therefore it can be
modified without changing the source code of a bean.
The JavaEE server reads the deployment descriptor at run time and acts upon the component
accordingly.

27.What is the difference between the getRequestDispatcher(String path) method of
javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface?
ServletRequest.getRequestDispatcher(Strin
g path)
ServletContext.getRequestDispatcher(Strin
g path)
The getRequestDispatcher(String path) The getRequestDispatcher(String path)
method of javax.servlet.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.
method of javax.servlet.ServletContext
interface cannot accept relative paths. All path
must start with a “/” and are interpreted as
relative to current context root.

28.What is preinitialization of a servlet?
A container does not initialize the servlets as soon as it starts up, it initializes a servlet when it
receives a request for that servlet first time. This is called lazy loading. The servlet specification
defines the element, which can be specified in the deployment descriptor to make the servlet
container load and initialize the servlet as soon as it starts up. The process of loading a servlet
before any request comes in is called preloading or preinitializing a servlet.

29.What is the <load-on-startup> element?
The <load-on-startup> element of a deployment descriptor is used to load a servlet file when
the server starts instead of waiting for the first request. It is also used to specify the order in
which the files are to be loaded. The <load-on-startup> element is written in the deployment
descriptor as follows:
<servlet>
<servlet-name>ServletName</servlet-name>
<servlet-class>ClassName</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>
Note: The container loads the servlets in the order specified in the <load-on-startup> element.

30.What is session?
A session refers to all the requests that a single client might make to a server in the course of
viewing any pages associated with a given application. Sessions are specific to both the
individual user and the application. As a result, every user of an application has a separate
session and has access to a separate set of session variables.


31.What is Session Tracking?
Session tracking is a mechanism that servlets use to maintain state about a series of requests from
the same user (that is, requests originating from the same browser) across some period of time.

32.What is the need of Session Tracking in web application?
HTTP is a stateless protocol i.e., every request is treated as new request. For web applications to
be more realistic they have to retain information across multiple requests. Such information
which is part of the application is reffered as "state". To keep track of this state we need session
tracking.

Typical example: Putting things one at a time into a shopping cart, then checking out--each page
request must somehow be associated with previous requests.

33.What are the types of Session Tracking ?
Sessions need to work with all web browsers and take into account the users security
preferences. Therefore there are a variety of ways to send and receive the identifier:
 URL rewriting : URL rewriting is a method of session tracking in which some extra data
(session ID) is appended at the end of each URL. This extra data identifies the session.
The server can associate this session identifier with the data it has stored about that
session. This method is used with browsers that do not support cookies or where the user
has disabled the cookies.

 Hidden Form Fields : Similar to URL rewriting. The server embeds new hidden fields
in every dynamically generated form page for the client. When the client submits the
form to the server the hidden fields identify the client.

 Cookies : Cookie is a small amount of information sent by a servlet to a Web browser.
Saved by the browser, and later sent back to the server in subsequent requests. A cookie
has a name, a single value, and optional attributes. A cookie's value can uniquely identify
a client.

 Secure Socket Layer (SSL) Sessions : Web browsers that support Secure Socket Layer
communication can use SSL's support via HTTPS for generating a unique session key as
part of the encrypted conversation.

Learn more about Session Tracking

34.How do I use cookies to store session state on the client?
In a servlet, the HttpServletResponse and HttpServletRequest objects passed to method
HttpServlet.service() can be used to create cookies on the client and use cookie information
transmitted during client requests. JSPs can also use cookies, in scriptlet code or, preferably,
from within custom tag code.
 To set a cookie on the client, use the addCookie() method in class HttpServletResponse.
Multiple cookies may be set for the same request, and a single cookie name may have
multiple values.
 To get all of the cookies associated with a single HTTP request, use the getCookies()
method of class HttpServletRequest

35.What are some advantages of storing session state in cookies?
 Cookies are usually persistent, so for low-security sites, user data that needs to be stored
long-term (such as a user ID, historical information, etc.) can be maintained easily with
no server interaction.
 For small- and medium-sized session data, the entire session data (instead of just the
session ID) can be kept in the cookie.

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36.What are some disadvantages of storing
session state in cookies?
 Cookies are controlled by
programming a low-level API, which is more difficult to implement than some other
approaches.
 All data for a session are kept on the client. Corruption, expiration or purging of cookie
files can all result in incomplete, inconsistent, or missing information.
 Cookies may not be available for many reasons: the user may have disabled them, the
browser version may not support them, the browser may be behind a firewall that filters
cookies, and so on. Servlets and JSP pages that rely exclusively on cookies for client-side
session state will not operate properly for all clients. Using cookies, and then switching to
an alternate client-side session state strategy in cases where cookies aren't available,
complicates development and maintenance.
 Browser instances share cookies, so users cannot have multiple simultaneous sessions.
 Cookie-based solutions work only for HTTP clients. This is because cookies are a feature
of the HTTP protocol. Notice that the while package javax.servlet.http supports
session management (via class HttpSession), package javax.servlet has no such
support.

37.What is URL rewriting?
URL rewriting is a method of session tracking in which some extra data is appended at the end of
each URL. This extra data identifies the session. The server can associate this session identifier
with the data it has stored about that session.
Every URL on the page must be encoded using method HttpServletResponse.encodeURL().
Each time a URL is output, the servlet passes the URL to encodeURL(), which encodes session
ID in the URL if the browser isn't accepting cookies, or if the session tracking is turned off.
E.g., http://abc/path/index.jsp;jsessionid=123465hfhs
Advantages
 URL rewriting works just about everywhere, especially when cookies are turned off.
 Multiple simultaneous sessions are possible for a single user. Session information is local
to each browser instance, since it's stored in URLs in each page being displayed. This
scheme isn't foolproof, though, since users can start a new browser instance using a URL
for an active session, and confuse the server by interacting with the same session through
two instances.
 Entirely static pages cannot be used with URL rewriting, since every link must be
dynamically written with the session state. It is possible to combine static and dynamic
content, using (for example) templating or server-side includes. This limitation is also a
barrier to integrating legacy web pages with newer, servlet-based pages.

 Servlets Interview Questions
DisAdvantages
 Every URL on a page which needs the session information must be rewritten each time a
page is served. Not only is this expensive computationally, but it can greatly increase
communication overhead.
 URL rewriting limits the client's interaction with the server to HTTP GETs, which can
result in awkward restrictions on the page.
 URL rewriting does not work well with JSP technology.
 If a client workstation crashes, all of the URLs (and therefore all of the data for that
session) are lost.

38.How can an existing session be invalidated?
An existing session can be invalidated in the following two ways:
 Setting timeout in the deployment descriptor: This can be done by specifying timeout
between the <session-timeout>tags as follows:
<session-config>
<session-timeout>10</session-timeout>
</session-config>
This will set the time for session timeout to be ten minutes.

 Setting timeout programmatically: This will set the timeout for a specific session. The
syntax for setting the timeout programmatically is as follows:
public void setMaxInactiveInterval(int interval)
The setMaxInactiveInterval() method sets the maximum time in seconds before a
session becomes invalid.
Note :Setting the inactive period as negative(-1), makes the container stop tracking
session, i.e, session never expires.

39.How can the session in Servlet can be destroyed?
An existing session can be destroyed in the following two ways:
 Programatically : Using session.invalidate() method, which makes the container
abonden the session on which the method is called.
 When the server itself is shutdown.

40.A client sends requests to two different web components. Both of the components access the
session. Will they end up using the same session object or different session ?
Creates only one session i.e., they end up with using same session .
Sessions is specific to the client but not the web components. And there is a 1-1 mapping
between client and a session.

41.What is servlet lazy loading?
 A container doesnot initialize the servlets ass soon as it starts up, it initializes a servlet
when it receives a request for that servlet first time. This is called lazy loading.
 The servlet specification defines the <load-on-startup> element, which can be specified
in the deployment descriptor to make the servlet container load and initialize the servlet
as soon as it starts up.
 The process of loading a servlet before any request comes in is called preloading or
preinitializing a servlet.

42.What is Servlet Chaining?
Servlet Chaining is a method where the output of one servlet is piped into a second servlet. The
output of the second servlet could be piped into a third servlet, and so on. The last servlet in the
chain returns the output to the Web browser.

43.How are filters?
Filters are Java components that are used to intercept an incoming request to a Web resource and
a response sent back from the resource. It is used to abstract any useful information contained in
the request or response. Some of the important functions performed by filters are as follows:
 Security checks
 Modifying the request or response
 Data compression
 Logging and auditing
 Response compression
Filters are configured in the deployment descriptor of a Web application. Hence, a user is not
required to recompile anything to change the input or output of the Web application.

44.What are the functions of an intercepting filter?
The functions of an intercepting filter are as follows:
 It intercepts the request from a client before it reaches the servlet and modifies the
request if required.
 It intercepts the response from the servlet back to the client and modifies the request if
required.
 There can be many filters forming a chain, in which case the output of one filter becomes
an input to the next filter. Hence, various modifications can be performed on a single
request and response.



45.What are the functions of the Servlet container?
The functions of the Servlet container are as follows:
 Lifecycle management : It manages the life and death of a servlet, such as class loading,
instantiation, initialization, service, and making servlet instances eligible for garbage
collection.
 Communication support : It handles the communication between the servlet and the
Web server.
 Multithreading support : It automatically creates a new thread for every servlet request
received. When the Servlet service() method completes, the thread dies.
 Declarative security : It manages the security inside the XML deployment descriptor
file.
 JSP support : The container is responsible for converting JSPs to servlets and for
maintaining them.