SERVLET INTERVIEW QUESTIONS

1. What is a servlet ?

Servlet is a java program thatruns inside a web container.

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

Yes. But you will not get the servlet specific things from constructor. 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 constructor 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.

3. What is servlet context ?

The servlet context is an object that contains a information about the Web application and container.  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.

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

Cookies, URL rewriting, and HTTPS protocol information are used to maintain session information.

5. Difference between GET and POST?

In GET your entire form submission can be encapsulated in one URL, like a hyperlink. query length is limited to 255 characters, not secure, faster, quick and easy. The data is submitted as part of URL.      In POST data is submitted inside body of the HTTP request. The data is not visible on the URL and it is more secure.

6. What is session?

The session is an object used by a servlet to track a user's interaction with a Web application across multiple HTTP requests. The session is stored on the server.

7. What is servlet mapping?

The servlet mapping defines an association between a URL pattern and a servlet. The mapping is used to map requests to Servlets.

8. What are the different ways for session tracking?

Cookies, URL rewriting, HttpSession, Hidden form fields.

9. What is the difference in using request.getRequestDispatcher() and context.getRequest Dispatcher()?

In request.getRequestDispatcher(path) in order to create it we need to give the relative path of the resource. But in   resourcecontext.getRequestDispatcher(path) in order to create it we need to give the absolute path of the resource.

10. What are context initialization parameters?

Context initialization parameters are specified by the <context-param> in the web.xml file, these are initialization parameter for the whole application.

11. What's the Servlet Interface?

A 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 such as HttpServlet.

12. What is the meaning of response has already been committed error?

You will get this error only when you try to redirect a page after you already have flushed the output buffer. This happens because HTTP specification force the header to be set up before the lay out of the page can be shown. When you try to send a redirect status, your HTTP server cannot send it right now if it hasn't finished to set up the header. Simply it is giving the error due to the specification of HTTP 1.0 and 1.1

13. What is the difference between ServletContext and ServletConfig?

The ServletConfig gives the information about the servlet initialization parameters. The servlet engine implements the ServletConfig interface 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. The ServletContext gives information about the container. The ServletContext interface provides information to servlets regarding the environment in which they are running. It also provides standard way for servlets to write events to a log file.

14. How can a servlet refresh automatically?

A We can use a client-side Refresh or Server Push.

15. What is Server side push?

Server Side push is useful when data needs to change regularly on the clients application or browser, without intervention from client. The mechanism used is, when client first connects to Server, then Server keeps the TCP/IP connection open.

16. What is client side refresh?

The standard HTTP protocols ways of refreshing the page, which is normally supported by all browsers. <META HTTP-EQUIV="Refresh" CONTENT="5; URL=/servlet/MyServlet/"> This will refresh the page in the browser automatically and loads the new data every 5 seconds.

17. What is the Max amount of information that can be saved in a Session Object ?

There is no such limit on the amount of information that can be saved in a Session Object.  The only limit is the Session ID length , which should not exceed more than 4K.

18. Why should we go for inter servlet communication?

The three major reasons to use inter servlet communication are: a) Direct servlet manipulation - allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse - allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration - requires to communicate with each other by sharing specific information (through method invocation).

19. What are the differences between a session and a cookie?

Session is stored in server but cookie stored in client. Session should work regardless of the settings on the client browser. There is no limit on the amount of data that can be stored on session. But it is limited in cookie. Session can store objects and cookies can store only strings. Cookies are faster than session.

20. What is HttpTunneling?

HTTP tunneling is used to encapsulate other protocols within the HTTP or HTTPS protocols. Normally the intranet is blocked by a firewall and the network is exposed to the outer world only through a specific Web server port, that listens for only HTTP requests. To use any other protocol, that by passes the firewall, the protocol is embedded in HTTP and send as HttpRequest.

21. How will you delete a cookie?

Cookie c = new Cookie("name", null);       c.setMaxAge(0);       response.addCookie(killCookie);

22. What is the difference between Context init parameter and Servlet init parameter?

Servlet init parameters are for a single servlet only. No body out side that servlet can access that. It is declared inside the <servlet> tag inside Deployment Descriptor, where as context init parameter is for the entire web application. Any servlet or JSP in that web application can access context init parameter. Context parameters are declared in a tag <context-param> directly inside the <web-app> tag. The methods for accessing context init parameter is getServletContext ().getInitParamter (“name”) where as method for accessing servlet init parameter is getServletConfig ().getInitParamter (“name”);

23. What are the different ways for getting a servlet context?

We will get ServletContext by calling getServletConfig ().getServletContext (). This is because a ServletConfig always hold a reference to ServletContext. By calling this.getServletContext () also we will get a ServletContext object.

24. What is the difference between an attribute and a parameter?

The return type of attribute is object, where the return type of parameter is String. The method to retrieve attribute is getAttribute () where as for parameter is getParamter (). We have a method setAttribute to set an attribute. But there is no setters available for setting a parameter.

25. How to make a context thread safe?

Synchronizing the ServletContext is the only solution to make a ServletContext thread safe.       Eg: synchronized (getServletContext ())       {       // do whatever you want with thread safe context.       }

26. What is the difference between setting the session time out in deployment descriptor and setting the time out programmatically?

In DD time out is specified in terms of minutes only. But in programmatically it is specified in seconds. A session time out value of zero or less in DD means that the session will never expire. To specify session will never expire programmatically it must be negative value.

27. How many cookies can one set in the response object of the servlet? Also, are there any restrictions on the size of cookies?

If the client is using Netscape, the browser can receive and store 300 total cookies and 4 kilobytes per cookie. And the no of cookie is restricted to 20 cookies per server or domain.

28. How can my application get to know when a HttpSession is removed?

You can define a class which implements HttpSessionBindingListener and override the valueUnbound() method.

29. How can we set the inactivity period on a per-session basis?

We can set the session time out programmatically by using the method setMaxInactiveInterval() of HttpSession.

30. What is URL Encoding and URL Decoding ?

URL encoding is the method of replacing all the spaces and other extra characters into their corresponding Hex Characters and Decoding is the process of converting all Hex Characters back to their normal form.

31. What is the difference between an applet and a servlet?

Servlets run inside a web server and applets run inside web browsers. Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.

32. What are the different web servers available name few of them?

Tomcat, Java Web Server, JRun.

33. What is a Java Bean?

A Java Bean is a software component that has been designed to be reusable in a variety of different environments.

34. What are the different types of ServletEngines?

The different types of ServletEngines available are:- Standalone ServletEngine: This is a server that includes built-in support for servlets. Add-on ServletEngine: It is a plug-in to an existing server. It adds servlet support to a server that was not originally designed with servlets in mind.

35. What is a Session Id?

It is a unique id assigned by the server to the user when a user starts a session.

36. What is use of parseQueryString ?

It parses a query string and builds a Hashtable of key-value pairs, where the values are arrays of strings. The query string should have the form of a string packaged by the GET or POST method.

37. When init() and Distroy() will be called.

init() is called whenever the servlet is loaded for the first time into the web server. Destroy will be called whenever the servlet is removed from the web server.

38. What is the use of setComment and getComment methods in Cookies ?

setComment: If a web browser presents this cookie to a user, the cookie's purpose will be described using this comment. This is not supported by version zero cookies. getComment: Returns the comment describing the purpose of this cookie, or null if no such comment has been defined.

39. Why we are used setMaxAge() and getMaxAge() in Cookies ?

setMaxAge : Sets the maximum age of the cookie. The cookie will expire after that many seconds have passed. Negative values indicate the default behavior, and will be deleted when the browser closes.      getMaxAge : Returns the maximum specified age of the cookie. If none was specified, a negative value is returned, indicating the default behavior described with setMaxAge.

40. What is the use of setSecure() and getSecure() in Cookies ?

setSecure method indicates to the web browser that the cookie should only be sent using a secure protocol (https). getSecure method returns the value of the 'secure' flag.

41. How do you communicate between the servlets?

We can communicate between servlets by using RequestDespatcher interface and servlet chaining.

42. What is Servlet chaining?

Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet’s output is the input of next servlet. This process continues until the last servlet is reached. Its output is then sent back to the client. We are achieving Servlet Chaining with the help of RequestDispatcher.

43. How will you communicate from an applet to servlet?

There are three ways to communicate from an applet to servlet and they are: HTTP Communication (Text-based and object-based) , Socket Communication and RMI Communication.

44. Can we call a servlet with parameters in the URL?

Yes. You can call a servlet with parameters in the URL like ?param=value.

45. How do servlets handle multiple simultaneous requests?

When a request comes in, the web server will start a new thread and the request is assigned to a thread, which calls a service method of the servlet.

46. Explain the directory structure of a web application?

The directory structure of a web application consists of two parts. A private directory called WEB-INF and a public resource directory which contains files server to public. WEB-INF folder consists of web.xml (the deployment descriptor), classes directory (where we keeps all our classes and servlets) and lib directory (where we keeps all our jar files). The public folder contains the static resources of the web application.

47. What is pre initialization of a servlet?

A container doesn't initialize the servlets when 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 pre initializing a servlet.

48. What are the uses of ServletRequest?

The ServletRequest gives information such as the names of the parameters passed by the client, the protocol (scheme) being used by the client, and the names of the remote host that made the request and the server that received it. The input stream, ServletInputStream.

49. What are the uses of ServletResponse interface?

ServletResponse allows the servlet to set the content length and MIME type of that response. It provides an output stream, ServletOutputStream and a Writer through which the servlet can send data.

50. How HTTP Servlet handles client requests?

An HTTP Servlet handles client requests through its service method. The service method supports standard HTTP client requests by dispatching each request to a method designed to handle that request.

51. Is HTML page a web component?

No. Static HTML pages and applets are not considered as web components by J2EE specification. Even the server-side utility classes are not considered web components.

52.What is the container?

A container is a runtime support of a system-level entity. Containers provide components with services such as lifecycle management, security, deployment, and threading.

53. What is the web container?

A Servlet and  JSP containers are collectively referred to as Web containers.

54. What is deployment descriptor?

A deployment descriptor is an XML based file which describes a web application's  deployment settings. The name of deployment descriptor of a web application is web.xml.

55. How can you implement singleton pattern in servlets ?

All servlets are singleton only , if the servlet is not implementing SingleThreadModel.

56. What is the difference between an application server and a web server?

Everything in web server must be achieved programmatically. But application server provides a lot of services like security, transaction , scalability. Application server will be having EJB support where web server don't have an EJB support.

57. What is servlet exception?

It indicates that there is an exception occurred in the servlet.

58. Can we implement Runnable interface from within our servlet?

Our servlet can implement the Runnable interface.

59. What is a WAR file?

WAR stands for Web Archive. It is a compressed version of your web application. You can use this WAR file to deploy your web application.

60. What is a servlet filter?

Servlet filters are pluggable Web components that allow us to implement pre-processing and post-processing logic in our Web applications.