Understanding Servlet Architecture for Java Web Apps

Understanding Servlet Architecture for Java Web Applications

Servlet architecture is a core component of Java EE (Enterprise Edition) used for building dynamic web applications. A Servlet is a Java class that runs on a web server and acts as a middle layer between client requests (typically from a browser) and server responses (usually from a database or application logic).

What is a Servlet?

A Servlet is a Java class used to handle HTTP requests and responses in web applications. It runs on a server, receives requests from a client (usually a browser), processes them (e.g., reads form data, interacts with a database), and sends back a dynamic response (like HTML or JSON).

Key Components of Servlet Architecture

• Client (Browser): Sends an HTTP request to the web server. Typically uses forms, hyperlinks, or JavaScript/AJAX to communicate with the server.
• Web Server/Servlet Container (e.g., Apache Tomcat):
  • Receives the HTTP request.
  • Maps the request to a specific servlet based on the URL pattern defined in web.xml or using annotations.
  • Manages servlet lifecycle and multithreading.
  • Passes the request to the corresponding servlet.
• Servlet:
  • A Java class that extends HttpServlet.
  • Processes the request (e.g., reads parameters, interacts with the database).
  • Generates a response, typically HTML, JSON, or XML.
  • Uses HttpServletRequest and HttpServletResponse objects.
• Response to Client:
  • The Servlet sends the response back to the web server.
  • The web server sends the HTTP response to the client browser.

Servlet Lifecycle Explained

The lifecycle of a Servlet involves several distinct phases:

1. Loading and Instantiation: The servlet class is loaded when first requested or at server startup (using the <load-on-startup> tag in web.xml).
2. Initialization (init() method): Called once after the servlet is instantiated. Used to initialize resources (e.g., database connections, configuration).
3. Request Handling (service() method): Called for each client request. This method delegates the request to specific HTTP method handlers like doGet(), doPost(), doPut(), doDelete(), etc., depending on the HTTP method used.
4. Destruction (destroy() method): Called when the servlet is being taken out of service (e.g., server shutdown or application undeployment). Used to release resources and perform cleanup.

Example Servlet Request Flow

Let's illustrate a typical request flow:

1. Client submits a form via POST to the /login URL.
2. The Web server receives the request for /login and looks up the corresponding LoginServlet.
3. The Servlet container calls the doPost() method of the LoginServlet.
4. The Servlet processes the request, checks user credentials, and then either forwards to another page or returns an error response.
5. The final response is sent back to the client browser.