tasks performed by Servlet Container
A servlet container, also known as a servlet engine, is a part of a web server or an application server that interacts with Java servlets. It provides an environment for servlets to run and includes several key responsibilities:
Table of Contents
1. Lifecycle Management
- Â Loading and Instantiation: The servlet container loads the servlet class and creates an instance of it.
- Â Initialization: It calls the init method to initialize the servlet.
- Â Request Handling: It invokes the service method to handle client requests.
- Â Destruction: It calls the destroy method when the servlet is no longer needed.
2. Request Handling
- The container manages the request-response cycle, receiving client requests and sending responses back to the client.
- It delegates requests to the appropriate servlet based on the URL mapping defined in the deployment descriptor (web.xml).
3. Concurrency Management
  The container handles multiple requests concurrently using threads, ensuring that each request is processed by a separate thread.
4. Session Management
  The container manages user sessions by creating and managing session objects, ensuring that data is maintained across multiple requests from the same client.
5. Security
- Â It provides authentication and authorization mechanisms to secure web applications.
- Â It supports transport layer security (TLS) for secure communication.
6. Resource Management
  The container manages resources like JDBC connections, ensuring efficient use and proper release of resources.
7. Logging and Monitoring
- It provides logging mechanisms to track servlet execution and errors.
- It monitors the performance and health of the servlet and the web application.
Example
Java Servlet Example
Here's an example servlet demonstrating how a servlet container handles common tasks:
java
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
@WebServlet("/example")
public class ExampleServlet extends HttpServlet {
@Override
public void init() throws ServletException {
// Initialization code
System.out.println("Servlet initialized");
}
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// Handling a GET request
HttpSession session = request.getSession();
session.setAttribute("message", "Hello, World!");
response.setContentType("text/html");
response.getWriter().println("<h1>" + session.getAttribute("message") + "</h1>");
}
@Override
public void destroy() {
// Cleanup code
System.out.println("Servlet destroyed");
}
}
Detailed Breakdown
- Lifecycle Management:
- Loading and Instantiation: The servlet container loads ExampleServlet and creates an instance of it.
- Initialization: It calls the init method, printing “Servlet initialized”.
- Request Handling: It handles a GET request by invoking the doGet method, managing the session, and sending an HTML response.
- Destruction: It calls the destroy method when the servlet is no longer needed, printing “Servlet destroyed”.
- Request Handling:
- The doGet method processes the client request, accesses the session, and sends an HTML response back to the client.
- Concurrency Management
- Each request to /example is handled by a separate thread managed by the servlet container.
- Session Management:
- The servlet accesses the session using request.getSession() and sets a session attribute.
- Security:
- Although not explicitly shown in the example, the container can secure the /example URL using authentication and authorization mechanisms defined in the deployment descriptor or annotations.
- Resource Management:
- The servlet container manages resources like session objects and JDBC connections if used within the servlet.
- Logging and Monitoring:
- The servlet container logs messages printed by System.out.println and can monitor the servlet’s performance.