Which HTTP method is non-idempotent?
An idempotent operation can be repeated an arbitrary number of times and the result will be the same as if it had been done only once. In arithmetic, adding zero to a number is idempotent.

A HTTP method is said to be idempotent if it returns the same result every time. HTTP methods GET, PUT, DELETE, HEAD, and OPTIONS are idempotent method and we should implement our application to make sure these methods always return same result. HTTP method POST is non-idempotent method and we should use post method when implementing something that changes with every request.

For example, to access an HTML page or image, we should use GET because it will always return the same object but if we have to save customer information to database, we should use POST method. Idempotent methods are also known as safe methods and we don’t care about the repetitive request from the client for safe methods.

What is ServletContext object?

1. javax.servlet.ServletContext interface provides access to web application parameters to the servlet.
2. The ServletContext is unique object and available to all the servlets in the web application. When we want some init parameters to be available to multiple or all of the servlets in the web application, we can use ServletContext object and define parameters in web.xml using element.
3. We can get the ServletContext object via the getServletContext() method of ServletConfig. Servlet containers may also provide context objects that are unique to a group of servlets and which is tied to a specific portion of the URL path namespace of the host.

   @Override
       protected void doGet(HttpServletRequest request, HttpServletResponse response)
               throws ServletException, IOException 
      {
               HttpSession session = request.getSession(true);
               ServletContext context = session.getServletContext();
      }
   

ServletConfig vs ServletContext

Servlet Config	Servlet Context
Servlet config object represent single servlet	It represent whole web application running on particular JVM and common for all the servlet
Its like local parameter associated with particular servlet	Its like global parameter associated with whole application
It’s a name value pair defined inside the servlet section of web.xml file so it has servlet wide scope	ServletContext has application wide scope so define outside of servlet tag in web.xml file.
getServletConfig() method is used to get the config object	getServletContext() method is used to get the context object.
for example shopping cart of a user is a specific to particular user so here we can use servlet config	To get the MIME type of a file or application session related information is stored using servlet context object.

What is difference between ServletResponse sendRedirect() and RequestDispatcher forward() method?

1. RequestDispatcher forward() is used to forward the same request to another resource whereas ServletResponse sendRedirect() is a two step process. In sendRedirect(), web application returns the response to client with status code 302 (redirect) with URL to send the request. The request sent is a completely new request.
2. forward() can’t be used to invoke a servlet in another context, we can only use sendRedirect() in this case.
3. forward() is handled internally by the container whereas sednRedirect() is handled by browser.
4. We should use forward() when accessing resources in the same application because it’s faster than sendRedirect() method that required an extra network call.
5. In forward() browser is unaware of the actual processing resource and the URL in address bar remains same whereas in sendRedirect() URL in address bar change to the forwarded resource.

Difference between web server, web container and application server
Web server: serves content to the web using http protocol.
Web container also known as a Servlet container is the component of a web server that interacts with Java servlets. A web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights.

Application server: hosts and exposes business logic and processes.
An application server is a server that hosts the business logic for a system. It often hosts both long running/batch processes and/or a interop services not meant for human consumption (REST/JSON services, SOAP, RPC, etc).
An application server is typically designed and deployed to facilitate longer running processes that will also be more resource intensive.

Real World Scenarios
Web server
You have an online store with only a web server and no application server. The site will provide a display where you can choose a product from. When you submit a query, the site performs a lookup and returns an HTML result back to its client. The web server sends your query directly to the database server (be patient, I will explain this one in our next nugget) and waits for a response. Once received, the web server formulates the response into an HTML file and sends it to your web browser. This back and forth communication between the server and database server happens every time a query is run.

Application server
if the query you want to run has already been done previously and no data has changed since then, the server will generate the results without having to send the request to the database server. This allows a real-time query where a second client can access the same info and receive real time, reliable information without sending another duplicate query to the database server. The server basically acts as an intermediate between the database server and the web server. This allows the information pulled to be reusable while in the first scenario, since this info is embedded in a particular and “customized” HTML page, this is not a reusable process. A second client will have to request the info again and receive another HTML embedded page with the info requested -highly inefficient.

How to make sure a servlet is loaded at the application startup?
Usually servlet container loads a servlet on the first client request but sometimes when the servlet is heavy and takes time to loads, we might want to load it on application startup. We can use load-on-startup element with servlet configuration in web.xml file or use WebServlet annotation loadOnStartup variable to tell container to load the servlet on system startup.

<servlet>
	<servlet-name>foo</servlet-name>
	<servlet-class>com.foo.servlets.Foo</servlet-class>
	<load-on-startup>5</load-on-startup>
</servlet> 

If there are multiple servlets with load-on-startup value such as 0,1,2,3 then lower integer value servlet will be loaded first.

How to get the actual path of servlet in server?

getServletContext().getRealPath(request.getServletPath())

Why is _jspService() method starting with an ‘_’ while other life cycle methods do not?

_jspService() cannot be overridden where as other methods jspInit(), jspDestroy() can be overridden.

Difference between Filter and Listener in Servlet
Servlet Filter is used for monitoring request and response from client to the servlet, or to modify the request and response, or to audit and log.

Servlet Listener is used for listening to events in a web containers, such as when you create a session, or place an attribute in an session or if you passivate and activate in another container, to subscribe to these events you can configure listener in web.xml, for example HttpSessionListener.