What is Servlets in java?

Java Servlets are server-side components that process client requests, manage server-side logic, and generate dynamic web content. They are part of the Java Enterprise Edition (Java EE) and typically run within a servlet container like Apache Tomcat. Servlets enable web applications to handle various protocols, with HTTP being the most common.

Lifecycle of Servlets in java

Before diving into the types of servlets, it’s important to understand the servlet lifecycle, which consists of the following phases:

1. Loading and Instantiation:
   • Loading: When a servlet is requested for the first time, the servlet container loads the servlet class into memory.
   • Instantiation: The container creates an instance of the servlet class. This step involves calling the default constructor of the servlet class.
2. Initialization (init method):
   • Purpose: This phase prepares the servlet to handle client requests. It is called once and only once when the servlet is first loaded.
   • Method: init(ServletConfig config)
     • ServletConfig: Provides configuration information to the servlet, such as initialization parameters and a reference to the servlet’s context.
     • The servlet can override the init() method to perform initialization tasks, such as setting up resources (e.g., database connections) or reading configuration files.
3. Servicing Requests (service method)
   • Purpose: This phase handles client requests and generates responses.
   • Method: service(ServletRequest req, ServletResponse res)
     • ServletRequest: Encapsulates the client’s request, providing methods to read input and get request information.
     • ServletResponse: Encapsulates the servlet’s response to the client, providing methods to write output and set response attributes.
     • The service method is called multiple times, once for each request. The servlet can override this method to handle requests.
4. Request Handling (service method):
   • The service method is invoked for each client request. It determines the request type (GET, POST, etc.) and calls the corresponding method (doGet, doPost, etc.).
     • Purpose: These methods handle specific types of HTTP requests.Methods:
     • doGet: Handles GET requests, typically used for requesting data.
     • doPost: Handles POST requests, typically used for submitting data to the server.
     • doPut: Handles PUT requests, used for updating resources.
     • doDelete: Handles DELETE requests, used for deleting resources.
     • Each of these methods can be overridden by the servlet to handle specific request types.
5. Destruction (destroy method):
   • Purpose: This phase allows the servlet to clean up resources before it is removed from service.
   • Method: destroy()
     • Called once when the servlet is being taken out of service, either due to server shutdown or explicit removal.
     • The servlet can override the destroy method to release resources, such as closing database connections or stopping background threads.

Life Cycle Overview of Java Servlets

        +-----------------+
        |    Load Class   |
        +-----------------+
                 |
                 v
        +-----------------+
        |  Instantiate    |
        +-----------------+
                 |
                 v
        +-----------------+
        |   Initialize    |
        |   (init())      |
        +-----------------+
                 |
                 v
        +-----------------+
        |  Service        |
        |  (service())    |
        |  (doGet, doPost)|
        +-----------------+
                 |
                 v
        +-----------------+
        |   Destroy       |
        |   (destroy())   |
        +-----------------+

Types of Servlet in java

Java Servlets can be broadly classified into two main categories:

• Generic Servlets
• HTTP Servlets.

Let’s explore each type in detail:

1. Generic Servlets

GenericServlet is an abstract class that implements the Servlet interface and provides a framework for creating servlets that can handle any type of protocol. It is not tied to any specific protocol, making it versatile for handling various types of requests.

Key Features:

• Protocol Independence: Can handle non-HTTP protocols.
• Flexibility: Useful for creating custom protocol handlers.
• Reusability: Serves as a base class for other specialized servlets.

2. HTTP Servlets

HttpServlet is a subclass of GenericServlet that provides HTTP-specific functionality. It is the most commonly used type of servlet and is designed to handle HTTP requests and responses. HttpServlet includes methods for handling different HTTP request methods, such as GET, POST, PUT, DELETE, etc.

Key Features:

• HTTP-Specific: Optimized for handling HTTP requests.
• Built-in Methods: Provides methods like doGet, doPost, doPut, and doDelete for handling specific types of HTTP requests.

Why we use servlets in java

Servlets in Java are a powerful and versatile technology used for creating dynamic web applications.

1. Dynamic Content Generation

Servlets are used to create dynamic web content. They can generate HTML, XML, JSON, or any other type of response based on user input or other dynamic data, making them essential for interactive web applications.

2. Handling HTTP Requests

Servlets can handle various types of HTTP requests (GET, POST, PUT, DELETE, etc.). This allows for complex web interactions, like form submissions, data updates, and file uploads.

3. Session Management

Servlets can manage sessions effectively. They help in tracking user sessions across multiple requests, allowing for functionalities like login systems, shopping carts, and user preferences.

4. Platform Independence

Java Servlets are platform-independent. They can run on any server that supports Java and the Java Servlet API, making them a flexible choice for web application development.

5. Integration with Java EE

Servlets are a core component of the Java EE platform, which means they integrate seamlessly with other Java EE technologies like JSP (JavaServer Pages), JDBC (Java Database Connectivity), and EJB (Enterprise JavaBeans).

6. Scalability and Performance

Servlets are highly scalable. They can handle a large number of concurrent requests, making them suitable for high-traffic web applications. Their performance is enhanced by the use of threads, which allows them to handle multiple requests simultaneously without creating new processes for each request.

7. Security

Servlets support various security features such as SSL/TLS for secure data transmission, role-based authentication, and authorization. This makes them suitable for building secure web applications.

Example Usage of Servlets:

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h1>Hello, World!</h1>");
    }
}

Conclusion

GenericServlet provides a powerful and flexible framework for creating versatile and reusable servlet components. Its protocol independence and simplicity make it an ideal choice for developing general-purpose servlets or custom protocol handlers. By mastering GenericServlet, you can unlock new possibilities in Java web development, building applications that are adaptable, efficient, and ready for the demands of today’s dynamic web environment.