Servlets FAQs

Explain the life cycle methods of a Servlet?
The javax.servlet.Servlet interface defines the three methods known as life-cycle method.
 public void init(ServletConfig config) throws ServletException
 public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException 
 public void destroy() 
First the servlet is constructed, then initialized wih the init() method. Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet.
The servlet is removed from service, destroyed with the destroy() methid, then garbaged collected and finalized.

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. The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root.

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, A public resource directory which contains public resource folder. 
WEB-INF folder consists of
1. web.xml
2. classes directory 
3. lib directory 

What are the common mechanisms used for session tracking?
Cookies
SSL sessions
URL- rewriting

Explain ServletContext?
ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web applicationor servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application.

What is preinitialization of a servlet?
A container doesnot initialize the servlets ass soon as 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 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 preinitializing a servlet.


What is the difference between Difference between doGet() and doPost()?
A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following: http://www.allapplabs.com/svt1?
p1=v1&p2=v2&...&pN=vN doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string.

What is the difference between HttpServlet and GenericServlet?
A GenericServlet has a service() method aimed to handle requests. HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1). Both these classes are abstract.


What is the difference between ServletContext and ServletConfig?
ServletContext: 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.The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized
ServletConfig: The object created after a servlet is instantiated and its default constructor is read. It is created to pass initialization information to the servlet.