Servlet Interview Questions

1. What is Servlet?

Answer:- A servlet is a Java technology-based Web component, managed by a container called servlet container or servlet engine, that generates dynamic content and interacts with web clients via a request\/response paradigm.

2. Why is Servlet so popular?

Answer:- Because servlets are platform-independent Java classes that are compiled to platform-neutral byte code that can be loaded dynamically into and run by a Java technology-enabled Web server.

3. What is servlet container?

Answer:- The servlet container is a part of a Web server or application server that provides the network services over which requests and responses are sent, decodes MIME-based requests, and formats MIME-based responses. A servlet container also contains and manages servlets through their lifecycle.

4. When a client request is sent to the servlet container, how does the container choose which servlet to invoke?

Answer:- The servlet container determines which servlet to invoke based on the configuration of its servlets, and calls it with objects representing the request and response.

5. If a servlet is not properly initialized, what exception may be thrown?

Answer:- During initialization or service of a request, the servlet instance can throw an UnavailableException or a ServletException.

6. Given the request path below, which are context path, servlet path and path info?

Answer:- /bookstore/education/index.html

context path: /bookstore

servlet path: /education

path info: /index.html

7. What is filter? Can filter be used as request or response?

Answer:- A filter is a reusable piece of code that can transform the content of HTTP requests,responses, and header information. Filters do not generally create a response or respond to a request as servlets do, rather they modify or adapt the requests for a resource, and modify or adapt responses from a resource.

8. When using servlets to build the HTML, you build a DOCTYPE line, why do you do that?

Answer:- I know all major browsers ignore it even though the HTML 3.2 and 4.0 specifications require it. But building a DOCTYPE line tells HTML validators which version of HTML you are using so they know which specification to check your document against. These validators are valuable debugging services, helping you catch HTML syntax errors.

9. What is new in ServletRequest interface ? (Servlet 2.4)

Answer:- The following methods have been added to ServletRequest 2.4 version:

public int getRemotePort()

public java.lang.String getLocalName()

public java.lang.String getLocalAddr()

public int getLocalPort()

10. Request parameter How to find whether a parameter exists in the request object?

Answer:- 1.boolean hasFoo = !(request.getParameter(“foo”) == null || request.getParameter(“foo”).equals(“”));

2. boolean hasParameter = request.getParameterMap().contains(theParameter);

(which works in Servlet 2.3+)

11. How can I send user authentication information while making URL Connection?

Answer:- You’ll want to use HttpURLConnection.setRequestProperty and set all the appropriate headers to HTTP authorization.

12. Can we use the constructor, instead of init(), to initialize servlet?

Answer:- Yes , of course you can use the constructor instead of init(). There’s nothing to stop you. But you shouldn’t. The original reason for init() was that ancient versions of Java couldn’t dynamically invoke constructors with arguments, so there was no way to give the constructur a ServletConfig. That no longer applies, but servlet containers still will only call your no-arg constructor. So you won’t have access to a ServletConfig or ServletContext.

13. How can a servlet refresh automatically if some new data has entered the database?

Answer:- You can use a client-side Refresh or Server Push

14. The code in a finally clause will never fail to execute, right?

Answer:- Using System.exit(1); in try block will not allow finally code to execute.

15. What mechanisms are used by a Servlet Container to maintain session information?

Answer:- Cookies, URL rewriting, and HTTPS protocol information are used to maintain session information

16. Difference between GET and POST

Answer:- In GET your entire form submission can be encapsulated in one URL, like a hyperlink. query length is limited to 260 characters, not secure, faster, quick and easy.

In POST Your name/value pairs inside the body of the HTTP request, which makes for a cleaner URL and imposes no size limitations on the form’s output. It is used to send a chunk of data to the server to be processed, more versatile, most secure.

17. What is session?

Answer:- The session is an object used by a servlet to track a user’s interaction with a Web application across multiple HTTP requests.

18. What is servlet mapping?

Answer:- The servlet mapping defines an association between a URL pattern and a servlet. The mapping is used to map requests to servlets.

19. What is servlet context ?

Answer:- The servlet context is an object that contains a servlet’s view of the Web application within which the servlet is running. Using the context, a servlet can log events, obtain URL references to resources, and set and store attributes that other servlets in the context can use. (answer supplied by Sun’s tutorial).

20. Which interface must be implemented by all servlets?

Answer:- Servlet interface.

21. Explain the life cycle of Servlet.

Answer:- Loaded(by the container for first request or on start up if config file suggests load-on-startup), initialized( using init()), service(service() or doGet() or doPost()..), destroy(destroy()) and unloaded.

22. When is the servlet instance created in the life cycle of servlet? What is the importance of configuring a servlet?

Answer:- An instance of servlet is created when the servlet is loaded for the first time in the container. Init() method is used to configure this servlet instance. This method is called only once in the life time of a servlet, hence it makes sense to write all those configuration details about a servlet which are required for the whole life of a servlet in this method.

23. Why don’t we write a constructor in a servlet?

Answer:- Container writes a no argument constructor for our servlet.

24. When we don’t write any constructor for the servlet, how does container create an instance of servlet?

Answer:- Container creates instance of servlet by calling Class.forName(className).newInstance().

25. Once the destroy() method is called by the container, will the servlet be immediately destroyed? What happens to the tasks(threads) that the servlet might beAnswer:- executing at that time?

Yes, but Before calling the destroy() method, the servlet container waits for the remaining threads that are executing the servlet’s service() method to finish.

26. What is the difference between callling a RequestDispatcher using ServletRequest and ServletContext?

Answer:- We can give relative URL when we use ServletRequest and not while using ServletContext.

27. Why is it that we can’t give relative URL’s when using ServletContext.getRequestDispatcher() when we can use the same while calling ServletRequest.getRequestDispatcher()?

Answer:- Since ServletRequest has the current request path to evaluae the relative path while ServletContext does not.