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An Overview of Servlet

The Servlet is a class used in Java applications for enhancing the capabilities of the server. Java servlets run on web application server container and are used to generate HTML content that shows on the web browser. Servlets respond to request from the browser and return response content. Servets are mostly implemented for HTTP protocol and are also called as HTTP Servlet.

In Java, servlets generate dynamic HTML content and respond with excel, XML, pdf, and JSON and other formats. Initially, servlets were meant for creating HTML content in applications, but now they are mostly used at the controller layers. Our new collection of Servlet interview questions is a must read for all developers!

Our Servlet interview questions and answers for experienced professionals is a huge hit amongst potential candidates looking for new jobs. Have you read them yet?

Development History of Servlet

Servlet 1.0 was released in June 1997. You may be asked this information in Servlet interview questions for freshers.

Latest Version

Latest Version: Servlet 4.0 is the latest version that was released in September 2017 on the Java EE 8 platform. You should read in-depth about different versions and their features if you are preparing for tricky Servlet interview questions and answers.

Servlet Interview questions and answers

Looking for a new job? Do not miss to read our Servlet Interview questions and answers. Whether you are a fresher or an experienced, these questions and answers that can help you to crack your interview.

The Servlet is a class used in Java applications for enhancing the capabilities of the server. Servlets respond to a request from the browser and return response content and also run on web application server container and are used to generate HTML content that shows on the web browser.

There are two types of servlets - generic and HTTP

  Generic Servlet HTTP Servlet
1. Subclass of javax.servlet.GenericServlet Subclass of javax.servlet.HttpServlet
2. Protocol independent Protocol dependent
3. Belongs to javax.servlet package Belongs to javax.servlet.http package

Servlert’s life cycle is the entire process from its creation until its destruction. A Servlet follows this path:

  • Initialized by calling init( ) method
  • Calls service( ) method to process request
  • Terminated by destroy( ) method

Finally, the servlet is nothing but the garbage that gets collected by JVM's garbage collector.

CGI or Common Gateway Interface provides dynamic content to users by executing a program that resides in the server. This program processes data to produce dynamic content. Other programs that reside in the server can be written in the native OS such as C++.

  Servlet CGI
1. Portable Not portable
2. Requests handled by lightweight Java Thread Requests handled by heavyweight OS process
3. Data sharing possible Data sharing not available
4. Link directly to the Web server Cannot link directly to the Web server
  GET method POST method
1. A limited amount of data can be sent as data is sent in the header Lot of data can be sent as data is sent in the body
2. Not secured because data is exposed Secured because data is not exposed
3. Can be bookmarked Cannot be bookmarked
  ServletConfig ServletContext
1. Parameters specified for a specific servlet. Parameters specified for the entire application.
2. The object created during the initialization process. The object created during web application deployment.
3. Available as long as the servlet is executing. Available as long as the application is executing.

Session tracking is a technique that servlets use to maintain a record of a series of requests originating from the same user or the same browser across a period of time. These sessions are shared between the servlets that are accessed by a client.

There are 4 techniques to track sessions:

  • Cookies
  • Hidden Fields.
  • URL Rewriting.
  • Session Tracking API.
  Sessions Cookies
1. Server-side files. Client-side files.
2. Stores unlimited content in user's browser. Stores limited content of limited users.
3. Data is not easy to modify. Data is easy to modify.

A filter is an object that filters tasks that are either request to a resource, or response from a resource, or both. Filtering is performed in doFilter method. Filters get configured in deployment descriptor of your application.

RequestDispatcher interface defines an object that can dispatch the request to resources like HTML, JSP, and Servlet, on the server. This interface provides two methods

forward (ServletRequest request, ServletResponse response) will forward a request from Servlet to another resource such as a Servlet, JSP, or HTML, on the server.

Include (ServletRequest request, ServletResponse response) will include the content of a resource such as Servlet, JSP, or HTML, in the response.

  forward() sendRedirect()
1. Server-side redirect. Client-side redirect.
2. Redirect is at the server end, not visible to the client. Redirection is at the client end and visible to the client.
3. Original URL remains intact. Original URL changes.

Both WAR and JAR have the same file structure of a single file that is compressed and contains multiple files inside it. WAR files combine servlets, JSPs, Java and XML files, JAR libraries, javascript libraries, static web pages, and other resources that are needed to run the applications.

HTTP outlines a set of request methods that indicate the action to be performed for a resource. Each request method implements a different semantic, but they also share some common features. Here are the 7 HTTP request methods every web developer preparing for Servlet interview questions and answers should know:

  • GET
  • HEAD
  • POST
  • PUT
  • DELETE
  • OPTIONS
  • PATCH

In order to create a WAR file, use JDK's JAR tool. Use -c switch of jar for creating the war file. Enter the project directory of your project and write this command: jar -cvf projectname.war * Here, -c is for creating file, -v for generating verbose output and -f for specifying name of the archive file.

The SingleThreadModel interface ensures that only one thread is executed at a given point of time in a given servlet service method. This is a marker interface and contains no methods. This interface also ensures thread-safe servlets.

URL Encoding process transforms user input to a Common Gateway Interface (CGI) form. It is fit to travel across the network through stripping spaces, punctuation and replacement with escape characters.

The url-pattern associates a servlet with a set of URLs. When a request from the user arrives, the container uses basic process to match the URL in the request with a URL pattern in the web.xml file. A URL pattern may sometimes contains subsets of US-ASCII characters.

Here are 2 methods for getting client’s IP address in Servlet:

HttpServletRequest.getRemoteAddr method returns IP of the client that sends request.

HttpServletRequestgetRemoteHost method returns the hostname of the client that is sending the request. However, it may return an empty string if hostname is unknown.

Servlet 3.0 is an update of Servlet 2.5. Servlet 3.0 focuses on extensibility and pluggability. Key features in Servlet 3.0 can be categorized into the following areas:

  • Easier development with annotations.
  • Web application modularity.
  • Asynchronous Servlet.
  • Programmatic instantiation of Servlets/Filters.
  • Security Enhancements.

There are ways of servlet authentication:

  • HTTP basic authentication.
  • HTTP digest authentication.
  • HTTPS client authentication.
  • Form-based authentication.

Servlet 3.0 has introduced javax.servlet.annotation that provides annotation types, which can annotate servlet classes. Annotation also represents metadata. So, if you are using annotation, you may not require deployment descriptor.

  Web Server Web Container
1. Provides Request and response for HTTP . Ensures servlet objects life style is maintained.
2. Handles client request through HTTP. Uses service method for some servlet objects .
3. Delivers resources and data of web pages . Responsible for running the servlets .
4. Serves static content. Serves dynamic content
5. Superset Only a part of a superset
  Servlet JSP
1. Java code HTML-based code
2. Acts like a controller in MVC. View for showing output in MVC approach.
3. Accepts protocol requests. Accepts only HTTP requests.
4. May override service() method. Not possible to override service() method.

Accepting a request from the browser and processing it using multiple servlets in a chain is called Servlet Chaining. In this process, communication occurs between chains of servlets and servlet programs in order to process the client request. This process may make the user requests visible to multiple servlets.

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