Servlet Interview Questions & Answers

Common Servlet Interview Questions with Answers

Question: What is the difference between JSP and Servlets ?
JSP is used mainly for presentation only. A JSP can only be HttpServlet that means the only supported protocol in JSP is HTTP. But a servlet can support any protocol like HTTP, FTP, SMTP etc.

Question: What are the different ways for session tracking?
Cookies, URL rewriting, HttpSession, Hidden form fields

Question: 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

Question: 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.

Question: 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.

Question: 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.

Question: 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.

Question: What is a servlet ?
servlet is a java program that runs inside a web container.

Question: 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.

Question: What is the difference in using request.getRequestDispatcher() and context.getRequestDispatcher()?
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.

Question: What are context initialization parameters?

Context initialization parameters are specified by the in the web.xml file, these are initialization parameter for the whole application.

Question: What is a Expression?
Expressions are act as place holders for language expression, expression is evaluated each time the page is accessed. This will be included in the service method of the generated servlet.

Question: What is a Declaration?
It declares one or more variables or methods for use later in the JSP source file. A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as semicolons separate them. The declaration must be valid in the scripting language used in the JSP file. This will be included in the declaration section of the generated servlet.

Question: What is a Scriptlet?
A scriptlet can contain any number of language statements, variable or expressions that are valid in the page scripting language. Within scriptlet tags, you can declare variables to use later in the file, write expressions valid in the page scripting language, use any of the JSP implicit objects or any object declared with a . Generally a scriptlet can contain any java code that are valid inside a normal java method. This will become the part of generated servlet's service method.

Question: What are the implicit objects?
Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects are: request, response, pageContext, session, application, out, config, page, exception

Question: What's the difference between forward and sendRedirect?
forward is server side redirect and sendRedirect is client side redirect. When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completely with in the web container And then returns to the calling method. When a sendRedirect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completely new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward. Client can disable sendRedirect.

Question: What's the 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 such as HttpServlet.

Question: What are two different types of Servlets ?
GenericServlet and HttpServlet. HttpServlet is used to implement HTTP protocol, where as Generic servlet can implement any protocol.

Question: What is the life cycle of servlet?
Each servlet has the same life cycle: first, the server loads and initializes the servlet by calling the init method. This init() method will be executed only once during the life time of a servlet. Then when a client makes a request, it executes the service method. finally it executes the destroy() method when server removes the servlet.

Question: Can we call destroy() method on servlets from service method ?

Question: What is the need of super.init (config) in servlets ?
Then only we will be able to access the ServletConfig from our servlet. If there is no ServletConfig our servlet will not have any servlet nature.

Question: What is the difference between GenericServlet and HttpServlet?
GenericServlet supports any protocol. HttpServlet supports only HTTP protocol. By extending GenericServlet we can write a servlet that supports our own custom protocol or any other protocol.

Question: Can we write a constructor for servlet ?
Yes. But the container will always call the default constructor only. If default constructor is not present , the container will throw an exception.

Question: What are the parameters for service method ?
ServletRequest and ServletResponse

Question: What are cookies ?
Cookies are small textual information that are stored on client computer. Cookies are used for session tracking.

Question: 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

Question: How do I use a scriptlet to initialize a newly instantiated bean?
A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated (Only at the time of instantiation.) Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

Question: 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.

Question: How can a servlet refresh automatically?
We can use a client-side Refresh or Server Push

Question: 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.

Question: What is client side refresh?
The standard HTTP protocols ways of refreshing the page, which is normally supported by all browsers.
<META HTTP-EQuestion:UIV="Refresh" CONTENT="5; URL=/servlet/MyServlet">

This will refresh the page in the browser automatically and loads the new data every 5 seconds.
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