notes-shivam

overview - servlet

4 min read

1. Introduction to Servlets

  • Servlets are Java programs that run on a web server and generate dynamic web content. They are part of the Java EE (Enterprise Edition) specification and enable the development of web applications.
  • Servlets typically handle requests from web clients (usually browsers), process those requests, and return responses.
  • They work based on the client-server architecture, where the server (web container) processes the request and sends back a response.

2. Hierarchy of Servlets

  • The javax.servlet package defines the standard API for Java Servlets.
  • Key classes/interfaces in the hierarchy include:
    • Servlet (the base interface that all servlets implement)
    • GenericServlet (abstract class that implements the Servlet interface)
    • HttpServlet (extends GenericServlet and provides methods for HTTP request/response handling)
    • ServletRequest and ServletResponse (interfaces used to handle requests and responses, respectively)

3. Lifecycle of a Servlet

A servlet goes through the following stages:

  1. Instantiation: The servlet container loads the servlet class into memory and creates an instance of it.
  2. Initialization (init method): The init() method is called once when the servlet is first loaded into memory. It initializes the servlet.
  3. Request Handling (service method): The service() method is invoked for every request made to the servlet. It handles the HTTP request by calling methods like doGet(), doPost(), etc.
  4. Destruction (destroy method): The destroy() method is called before the servlet is removed from memory. It allows for cleanup resources.

4. Tomcat Configuration

  • Apache Tomcat is a widely-used open-source servlet container that implements the Java Servlet and JavaServer Pages (JSP) specifications.

  • Key configuration files:

    • web.xml: This file, located in the WEB-INF directory, contains the configuration for servlets, mappings, filters, and listeners.
    • server.xml: The main configuration file for Tomcat where server-level configurations like port settings and host settings are defined.

  • Servlets are mapped in web.xml, where you define URL patterns and associate them with a servlet

5. Handling GET and POST Requests (HTTP)

  • GET Request:
    • The doGet() method in a servlet handles HTTP GET requests. It’s used to retrieve data from the server.
    • Data is passed through the URL (query string).
  • POST Request:
    • The doPost() method handles HTTP POST requests, typically used when submitting form data.
    • Data is passed in the body of the request (not in the URL), making it more secure for sending sensitive information.
  • Difference:
    • GET is idempotent (can be cached, repeated without side effects), while POST is not.
    • POST can send larger amounts of data.

6. Handling Data from HTML to Servlet

  • Forms in HTML use the <form> tag with attributes method="GET" or method="POST".

  • When the form is submitted, data is sent to a servlet for processing.

    • In a GET request, data is passed through the URL.
    • In a POST request, data is passed in the body of the request.

  • In the servlet, you retrieve the form data using request.getParameter().

7. Session Tracking

  • Cookies:

    • A cookie is a small piece of data sent by the server and stored on the client’s browser. It can be used to store session information or preferences.
    • In a servlet, you can create cookies using Cookie objects and send them in the response using response.addCookie().
    • Cookies are automatically sent with every request to the server for the domain they are associated with.

  • HttpSession:

    • The HttpSession interface is used to maintain session state across multiple requests.
    • Sessions are created by the servlet container and associated with a unique session ID.
    • You can store and retrieve session attributes using methods like session.setAttribute() and session.getAttribute()
    • Sessions can be tracked via cookies or URL rewriting (if cookies are disabled).

Summary

  • Servlets enable dynamic content generation and interaction between web clients and servers.
  • The servlet lifecycle involves initialization, request processing, and cleanup stages
  • Tomcat is a popular servlet container for running Java web applications.
  • HTTP methods like GET and POST are handled by doGet() and doPost() methods, respectively.
  • Data from HTML forms is passed to servlets via query strings (GET) or request bodies (POST).
  • Session tracking in servlets can be achieved with cookies or HTTP sessions, helping maintain user state across requests.

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