A servlet is a Java
programming language class used to extend the capabilities of servers that host
applications accessed via a request-response programming model. Although
servlets can respond to any type of request, they are commonly used to extend the
applications hosted by Web servers. For such applications, Java Servlet
technology defines HTTP-specific servlet classes.
The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines life-cycle methods.
The javax.servlet and javax.servlet.http packages provide interfaces and classes for writing servlets. All servlets must implement the Servlet interface, which defines life-cycle methods.
When
implementing a generic service, you can use or extend the GenericServlet class provided with the Java Servlet API. The
HttpServlet class provides methods, such as
doGet
and doPost
, for handling
HTTP-specific services. Servlet Life Cycle
The
life cycle of a servlet is controlled by the container in which the servlet has
been deployed. When a request is mapped to a servlet, the container performs
the following steps.
- 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.
- Invokes the service method, passing a request and response object.
- If the container needs to remove the servlet, it finalizes the servlet by calling the servlet's destroy method.
Few questions from javarevisited.
Question 1: In web.xml file <load-on-startup>1</load-on-startup> is defined between <servlet></servlet> tag what does it means.
Ans: whenever we request for any servlet the servlet container will initialize the servlet and load it which is defined in our config file called web.xml by default it will not initialize when our context is loaded .defining like this <load-on-startup>1</load-on-startup> is also known as pre initialization of servlet means now the servlet for which we have define this tag has been initialized in starting when context is loaded before getting any request.When this servlet question was asked to me in an interview few years back , I was not even aware of this element but this questions pointed me to look DTD of web.xml and understand other elements as well..
Question 2: How can we create deadlock condition on our servlet?
Ans: one simple way to call doPost() method inside doGet() and doGet()method inside doPost() it will create deadlock situation for a servlet. This is rather simple servlet interview questions but yet tricky if you don’t think of it J
Question 3: For initializing a servlet can we use constructor in place of init ().
Ans: No, we can not use constructor for initializing a servlet because for initialization we need an object of servletConfig using this object we get all the parameter which are defined in deployment descriptor for initializing a servlet and in servlet class we have only default constructor according to older version of java so if we want to pass a
Config object we don’t have parametrized constructor and apart from this servlet is loaded and initialized by container so ots a job of container to call the method according to servlet specification they have lifecycle method so init() method is called firstly.
More important Java doesn't allow interfaces to declare constructors. These kinds of servlet interview questions are quite popular on service based companies who just want to dig one level more.
Question 4: Why super.init (config) wiil be the first statement inside init(config) method.
Ans: This will be the first statement if we are overriding the init(config ) method by this way we will store the config object for future reference and we can use by getServletConfig () to get information about config object if will not do this config object will be lost and we have only one way to get config object because servlet pass config object only in init method . Without doing this if we call the servletConfig method will get NullPointerException.
Question5: Can we call destroy() method inside the init() method is yes what will happen?
Ans:Yes we can call like this but if we have not override this method container will call the default method and nothing will happen.after calling this if any we have override the method then the code written inside is executed.
Question 6: How can we refresh servlet on client and server side automatically?
Ans: On client side we can use Meta http refresh and server side we can use server push.
Question 7: How can you get the information about one servlet context in another servlet?
Ans: In context object we can set the attribute which we want on another servlet and we can get that attribute using their name on another servlet.
Context.setAttribute (“name”,” value”)
Context.getAttribute (“name”)
Question 8: Why we need to implement Single Thread model in case of Servlet.
Ans: In J2EE we can implement our servlet on two different ways either by using:
1. Single Thread Model
2. Multithread Model
Depending upon our scenario, if we have implemented single thread means only one instance is going handle one request at a time no two thread will concurrently execute service method of servlet.
Example in banking account where sensitive data is handle mostly this scenario was used this interface is deprecated in Servlet API version 2.4.
As the name signifies multi thread means a servlet is capable to handle multiple requests at same time. This servlet interview question was quite popular few years back on entry level but now its loosing its shine.
Question 9: what is servlet collaboration?
Ans communication between two servlet is called servlet collaboration which is achieved by 3 ways.
1. RequestDispatchers include () and forward() method .
2. Using sendRedirect()method of Response object.
3. Using servlet Context methods
Question 10: What is the difference between ServletConfig and ServletContext?
Ans: ServletConfig as the name implies provide the information about configuration of a servlet which is defined inside the web.xml file or we can say deployment descriptor.its a specific object for each servlet.
ServletContext is application specific object which is shared by all the servlet belongs to one application in one JVM .this is single object which represent our application and all the servlet access application specific data using this object.servlet also use their method to communicate with container.
These Servlet interview questions are good for quick recap of important concept before appearing on any J2EE interview. Please share if you have come across any other interesting interview question on Servlets.
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
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.
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.
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 HTTPprotocol-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 ?
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);
}
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.
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 response, as if it were part of the
calling servlet.
|
The RequestDispatcher
forward() method is used to show a different resource in place of the
servlet that was originally called.
|
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?
25.What is the directory structure of a WAR file?
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.
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(String
path)
|
ServletContext.getRequestDispatcher(String
path)
|
The getRequestDispatcher(String
path) method of javax.servlet.ServletRequestinterface 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 javax.servlet.ServletContextinterface 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>
<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.
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.
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.
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 packagejavax.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
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.
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>
<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.
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.
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