Servlet Interview Questions

 

Servlet Interview Questions - 1

1- How many objects of a servlet is created?
Only one object at the time of first request by servlet or web container.

2- What is the life-cycle of a servlet?
The web container maintains the life cycle of a servlet instance. Let's see the life cycle of the servlet: 
Servlet class is loaded. 
Servlet instance is created. 
init method is invoked. 
service method is invoked. 
destroy method is invoked. 


As displayed in the above diagram, there are three states of a servlet: new, ready and end. The servlet is in new state if servlet instance is created. After invoking the init() method, Servlet comes in the ready state. In the ready state, servlet performs all the tasks. When the web container invokes the destroy() method, it shifts to the end state.

1) Servlet class is loaded
The classloader is responsible to load the servlet class. The servlet class is loaded when the first request for the servlet is received by the web container.

2) Servlet instance is created
The web container creates the instance of a servlet after loading the servlet class. The servlet instance is created only once in the servlet life cycle.

3) init method is invoked
The web container calls the init method only once after creating the servlet instance. The init method is used to initialize the servlet. It is the life cycle method of the javax.servlet.Servlet interface. Syntax of the init method is given below:

public void init(ServletConfig config) throws ServletException

4) service method is invoked
The web container calls the service method each time when request for the servlet is received. If servlet is not initialized, it follows the first three steps as described above then calls the service method. If servlet is initialized, it calls the service method. Notice that servlet is initialized only once. The syntax of the service method of the Servlet interface is given below

public void service(ServletRequest request, ServletResponse response) 

throws ServletException, IOException 

5) destroy method is invoked
The web container calls the destroy method before removing the servlet instance from the service. It gives the servlet an opportunity to clean up any resource for example memory, thread etc. The syntax of the destroy method of the Servlet interface is given below:

public void destroy() 

3- What are the life-cycle methods for a servlet?
public void init(ServletConfig config)

It is invoked only once when first request comes for the servlet. It is used to initialize the servlet.

public void service(ServletRequest request,ServletResponse)throws ServletException,IOException

It is invoked at each request.The service() method is used to service the request.

public void destroy()

It is invoked only once when servlet is unloaded.

4- Who is responsible to create the object of servlet?
The web container or servlet container.

5- When servlet object is created?
At the time of first request.

6- What is difference between Get and Post method?
Get - 
1) Limited amount of data can be sent because data is sent in header.
2) Not Secured because data is exposed in URL bar.

3) Can be bookmarked
4) Idempotent
5) It is more efficient and used than Post

Post
1) Large amount of data can be sent because data is sent in body.
2) Secured because data is not exposed in URL bar.
3) Cannot be bookmarked
4) Non-Idempotent
5) It is less efficient and used

7- What is difference between PrintWriter and ServletOutputStream?
PrintWriter is a character-stream class where as ServletOutputStream is a byte-stream class. The PrintWriter class can be used to write only character-based information whereas ServletOutputStream class can be used to write primitive values as well as character-based information.

8- What is difference between GenericServlet and HttpServlet?
The GenericServlet is protocol independent whereas HttpServlet is HTTP protocol specific. HttpServlet provides additional functionalities such as state management etc.

9- What is servlet collaboration?
When one servlet communicates to another servlet, it is known as servlet collaboration. There are many ways of servlet collaboration: 
RequestDispacher interface 
sendRedirect() method etc. 

10- What is the purpose of RequestDispatcher Interface?
The RequestDispacher interface provides the facility of dispatching the request to another resource it may be html, servlet or jsp. This interceptor can also be used to include the content of antoher resource.

11- Can you call a jsp from the servlet?
Yes, one of the way is RequestDispatcher interface for example:

RequestDispatcher rd=request.getRequestDispatcher("/login.jsp"); 
rd.forward(request,response);

12- Difference between forward() method and sendRedirect() method ?
forward() method

1) forward() sends the same request to another resource.

2) forward() method works at server side.

3) forward() method works within the server only.

sendRedirect() method
1) sendRedirect() method sends new request always because it uses the URL bar of the browser.
2) sendRedirect() method works at client side
3) sendRedirect() method works within and outside the server.

13- What is difference between ServletConfig and ServletContext?
The container creates object of ServletConfig for each servlet whereas object of ServletContext is created for each web application.

14- What is Session Tracking?
Session simply means a particular interval of time.

Session Tracking is a way to maintain state of an user.Http protocol is a stateless protocol.Each time user requests to the server, server treats the request as the new request.So we need to maintain the state of an user to recognize to particular user.

HTTP is stateless that means each request is considered as the new request. It is shown in the figure given below:


Why use Session Tracking?

To recognize the user It is used to recognize the particular user.
Session Tracking Techniques

There are four techniques used in Session tracking:

Cookies 
Hidden Form Field 
URL Rewriting 
HttpSession 

15- What are Cookies?
There are 2 types of cookies in servlets. 
Non-persistent cookie 
Persistent cookie 
Non-persistent cookie

It is valid for single session only. It is removed each time when user closes the browser.
Persistent cookie

It is valid for multiple session . It is not removed each time when user closes the browser. It is removed only if user logout or signout.

Advantage of Cookies 
Simplest technique of maintaining the state. 
Cookies are maintained at client side. 
Disadvantage of Cookies 
It will not work if cookie is disabled from the browser. 
Only textual information can be set in Cookie object. 

16- What is difference between Cookies and HttpSession?
Cookie works at client side whereas HttpSession works at server side.

17- What is filter?
A filter is an object that is invoked at the preprocessing and postprocessing of a request.

It is mainly used to perform filtering tasks such as conversion, logging, compression, encryption and decryption, input validation etc.

18- How can we perform any action at the time of deploying the project?
By the help of ServletContextListener interface.

19- What is the disadvantage of cookies?
It will not work if cookie is disabled from the browser.

20- How can we upload the file to the server using servlet?
One of the way is by MultipartRequest class provided by third party.

21- What is load-on-startup in servlet?
The load-on-startup element of servlet in web.xml is used to load the servlet at the time of deploying the project or server start. So it saves time for the response of first request.

22- What if we pass negative value in load-on-startup?
It will not affect the container, now servlet will be loaded at first request.

23- What is war file?
A war (web archive) file specifies the web elements. A servlet or jsp project can be converted into a war file. Moving one servlet project from one place to another will be fast as it is combined into a single file.

24- How to create war file?
The war file can be created using jar tool found in jdk/bin directory. If you are using Eclipse or Netbeans IDE, you can export your project as a war file.

To create war file from console, you can write following code.

jar -cvf abc.war * 

25- What are the annotations used in Servlet 3?
There are mainly 3 annotations used for the servlet. 
@WebServlet : for servlet class. 
@WebListener : for listener class. 
@WebFilter : for filter class. 

26- Which event is fired at the time of project deployment and undeployment?
ServletContextEvent.

27- Which event is fired at the time of session creation and destroy?
HttpSessionEvent.

28- Which event is fired at the time of setting, getting or removing attribute from application scope?
ServletContextAttributeEvent.

29- What is the use of welcome-file-list?
It is used to specify the welcome file for the project.

30- What is the use of attribute in servlets?
Attribute is a map object that can be used to set, get or remove in request, session or application scope. It is mainly used to share information between one servlet to another.

 

 

 

 

Servlet interview Questions - 2

 

1- What is a Servlet?
Java Servlets is a Server side technology(basically specification) that allow the programmer to develop dynamic web resource program or server side web resource program in Java based web application.

Servlet is the software specification given by sun microsystem that provide set of rule and guideline for vendor company to develop software called servlet container.

Servlet is single instance multiple threads based java component in Java web application to generate dynamic web application.

 

2- What are the types of Servlet?
There are two types of servlets, GenericServlet and HttpServlet. GenericServlet defines the generic or protocol independent servlet. HttpServlet is subclass of GenericServlet and provides http protocl specific functionality.


3- What are the differences between a Servlet and an Applet? 
1) Servlets are server side components that executes on the server whereas applets are client side components and executes on the web browser. 

2)Applets have GUI interface but there is no GUI interface in case of Servlets. 

3)Applets have limited capabilities whereas Servlets are very poweful and can support a wide variety of operations 

4)Servlets can perform operations like interacting with other servlets, JSP Pages, connect to databases etc while Applets cannot do all this. 



4- What are the different methods present in a HttpServlet?
The methods of HttpServlet class are :
doGet() - To handle the GET, conditional GET, and HEAD requests
 
doPost() - To handle POST requests
 
doPut() - To handle PUT requests
 
doDelete() - To handle DELETE requests
 
doOptions() - To handle the OPTIONS requests and
 
doTrace() - To handle the TRACE requests

Apart from these, a Servlet also contains init() and destroy() methods that are used to initialize and destroy the servlet respectively. They are not any operation specific and are available in all Servlets for use during their active life-cycle.


5- What are the advantages of Servlets over CGI programs?
Java Servlets have a number of advantages over CGI and other API's. Some are:

1. Platform Independence - Java Servlets are 100% pure Java, so it is platform independent. 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 without modification code. Platform independency of servlets provide a great advantages over alternatives of servlets.

2. Performance - Anyone who has used CGI would agree that Servlets are much more powerful and quicker than CGI. Because the underlying technology is Java, it is fast and can handle multiple request simultaneously. Also, a servlet gets initialized only once in its lifetime and then continues to serve requests without having to be re-initialized again, hence the performance is much higher than CGIs.

3. 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.

Also, in terms of Safety & Security Servlets are superior when compared to CGI.


6- What are the type of protocols supported by HttpServlet?
It extends the GenericServlet base class and provides an framework for handling the HTTP protocol. So, HttpServlet only supports HTTP and HTTPS protocol.


7- What is ServletContext?
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.



8- What is meant by Pre-initialization of Servlet?
When servlet container is loaded, all the servlets defined in the web.xml file do not get initialized by default. When the container receives a request to hit a particular servlet, it loads that 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 this case, the servlet is loaded when context is loaded. You can specify 1 in between the tag in the Web.xml file in order to pre-initialize your servlet.

Example.

<Load-on-startup>1</Load-on-startup>



9- What mechanisms are used by a Servlet Container to maintain session information?
Servlet Container uses Cookies, URL rewriting, and HTTPS protocol information to maintain the session.

10- What do you understand by servlet mapping?
Servlet mapping defines an association between a URL pattern and a servlet. You can use one servlet to process a number of url patterns. For example in case of Struts *.do url patterns are processed by Struts Controller Servlet.

11- What interface must be implemented by all Servlets?
The Servlet Interface must be implemented by all servlets (either the GenericServlet or the HttpServlet)

12- What are the uses of Servlets?
1) Servlets are used to process the client requests.
2) A Servlet can handle multiple request concurrently and be used to develop high performance system
3) A Servlet can be used to load balance among serveral servers, as Servlet can easily forward request.

13- What are the objects that are received when a servlets accepts call from client? 
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. All the passage of data between the client and server happens by means of these request and response objects. 

 

Servlet interview Questions – 3

 

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

Let us say in a plain java class, you created an init method that gets invoked under some condition, will you be able to invoke it without actually instantiating your class? That is why we need a constructor even for a Servlet.

3- When is the servlet 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. 

4- When is a Servlet unloaded? 
A servlet is unloaded when:
Server shuts down.
Administrator manually unloads.

5- 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 whereas the HttpServlet is specific to the Http Protocol

6- Why is the HttpServlet class declared abstract? 
The HttpServlet class is declared abstract because the default implementations of the main service methods do nothing and must be overridden. Just like any other abstract class, the HttpServlet is a partial skeleton which defines basic behavior and then gives the programmer the freedom to implement any of the other possible features he/she wants.

The simplest example here could be the fact that, any HttpServlet could handle multiple doXXX() methods as we call them which include doGet(), doPost() etc. So, in your applications, if you handle just the POST requests, then you need not implement a doGet() to override the default doGet() implementation.

7- Can a servlet have a constructor? 
Sure Yes. Though we don’t explicitly call the constructor using the new() operator, the container internally invokes the Servlet class’ container when the servlet is initialized.

8- Should I override the service() method while using the HttpServlet? 
We never override the service method, since the HTTP Servlets have already taken care of it. The default service method 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 checks 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.

9- What are the differences between the ServletConfig interface and the ServletContext interface? 

Servlet Config Object- 

1.          It is one per our Servlet/JSP program.

2.          Servlet Config object means it is the object of Servlet container Supplied Java class implement javax.Servlet.ServletConfig interface.

3.          This object is used to pass additional data to Servlet and to read additional details from Servlet.

4.          This object is used to read Servlet init() parameter value of the web application.

5.          Servlet container create the object of Servlet config during instantiation and initialization.

6.          Servlet container destroys the Servlet config object during destruction process of our Servlet object.

Servlet Context Object-

1.          It is one per web application so it is called as the global memory of the web application.

2.          Servlet Context object means it is the object of java class implementing javax.Servlet.ServletContext interface.

3.          Servlet container create this object during during the deployement of web application.

4.          Servlet container destroy this object automatically when web application is undeployed or reloaded or Stopped.

5.          Using this object we can know the detail of underline Server like Server-name,version and Servlet api version.

10- What are the differences between forward() and sendRedirect() methods?
Some key differences between the two methods are:

a. A Forward is performed internally by the servlet whereas a redirect is a two-step process where the web application instructs the web browser to fetch a second/different URL
b. The browser and the user is completely unaware that a forward has taken place (The URL Remains intact) whereas in case of redirect, both the browser and the user will be made aware of the action including a change to the URL
c. The resource to which control is being forwarded has to be part of the same context as the one that is actually calling it whereas in case of redirect this is not a restriction.
d. Forwards are faster than redirects. Redirects are slower because it is actually handling two browser requests in place of forward’s one.

11- What are the difference between the include() and forward() methods? 
The key differences between the two methods are:

a. The include() method inserts the contents of the specified resource directly into the flow of the servlet response, as if it were part of the calling servlet, whereas the forward() is used to show a different resource in place of the servlet that was originally called
b. The include() is often used to include common text or template markup that may be included in many servlets whereas forward() is often used where a servlet plays the role of a controller, processes some input and decides the outcome by returning a particular page response where control is transferred to a different resource

12- What is 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 behavior is to pass all method calls directly to the underlying objects.

13- What is a deployment descriptor? 
A deployment descriptor is an XML document. 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 Java EE server reads the deployment descriptor at run time and acts upon the components accordingly.

14- What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface? 
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, whereas, the getRequestDispatcher(String path) 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.

15- What is the < load-on-startup > element of the deployment descriptor? 
The element of a deployment descriptor is used to load a servlet file when the server starts instead of waiting for the first request. This setting by which a servlet is loaded even before it gets its first request is called pre-initialization of a servlet. It is also used to specify the order in which the files are to be loaded. The container will load the servlets in the order specified in this element.

16- What is servlet lazy loading? 
Lazy servlet loading means – the container does not initialize the servlets as soon as it starts up. Instead it initializes servlets when they receive their first ever request. This is the standard or default behavior. If you want the container to load your servlet at start-up then use pre-initialization using the load-on-startup element in the deployment descriptor.

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

18- What are filters? 
Filters are Java components that are used to intercept an incoming request to a Web resource or the response that is 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:
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.

19- What are the functions of the Servlet container? 
The functions of the Servlet container are as follows:

1.          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. 

2.          Communication support: It handles the communication between the servlet and the Web server. 

3.          Multithreading support: It automatically creates a new thread for every servlet request received. When the Servlet service() method completes, the thread dies. 

4.          Declarative security: It manages the security inside the XML deployment descriptor file. 

5.          JSP support: The container is responsible for converting JSPs to servlets and for maintaining them.

 

Session Based Interview Questions

 

1. What is a Session?
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 requests from that particular user. Sessions are not shared among users and each user of the system will have a seperate session and a unique session Id. In most cases, the default value of time-out* is 20 minutes and it can be changed as per the website requirements.

*Time-Out - The Amount of time after which a session becomes invalidated/destroyed if the session has been inactive.

2. What is Session ID?
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.

3. What is Session Tracking?
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.

4. What are different types of Session Tracking?
Mechanism for Session Tracking are:
a) Cookies
b) URL rewriting
c) Hidden form fields
d) SSL Sessions

5. What is HTTPSession Class?
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.

6. Why do we need Session Tracking in a Servlet based Web Application?
In HttpServlet you can use Session Tracking to track the user state. Simply put, it is used to store information like users login credentials, his choices in previous pages (like in a shopping cart website) etc

7. What are the advantage of Cookies over URL rewriting?
Sessions tracking using Cookies are more secure and fast. It keeps the website URL clean and concise instead of a long string appended to the URL everytime you click on any link in the website. Also, when we use url rewriting, it requites large data transfer from and to the server. So, it may lead to significant network traffic and access to the websites may be become slow.

8. What is session hijacking?
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.

9. What is Session Migration?
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.

10. How to track a user session in Servlets?
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);

Using this session object, the servlet can gain access to the details of the session.

11. How can you destroy the session in Servlet?
You can call invalidate() method on the session object to destroy the session.

e.g. session.invalidate();