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Servlet Interview Questions and Answers


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

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 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.

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.

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.

What is servlet context ?
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).

Which interface must be implemented by all servlets?
Servlet interface.

Explain the life cycle of Servlet.

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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.

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

Why don't we write a constructor in a servlet?
Container writes a no argument constructor for our servlet.

When we don't write any constructor for the servlet, how does container create an instance of servlet?
Container creates instance of servlet by calling Class.forName(className).newInstance().

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 be executing at that time?
Yes, but Before calling the destroy() method, the servlet container waits for the remaining threads that are executing the servletís service() method to finish.

What is the difference between callling a RequestDispatcher using ServletRequest and ServletContext?
We can give relative URL when we use ServletRequest and not while using ServletContext.

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()?
Since ServletRequest has the current request path to evaluae the relative path while ServletContext does not.

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