Introduction to Java Servlets

In the world of Web Development, Java Servlets plays a crucial role. It is a part of Java Enterprise Edition which plays a vital role in developing interactive, dynamic, and scalable web applications. In this blog, we will explore Servlets, their lifecycle, and their features that help develop web applications.

What is Java Servlet?

Java Servlet extends the capabilities of a web server. It provides a way to handle the HTTP requests to generate dynamic web content. Unlike static web pages which remain constant until it gets updated, servlets allow developers to develop applications that respond to the user input dynamically. Servlets are managed by a servlet container, which can be a standalone web server or an application server like Apache Tomcat.

Features of Java Servlets

1. Multithreading Support:
   Servlets can handle multiple client requests simultaneously. The web container manages the lifecycle of servlet instances to achieve this capability.

2. HTTP Protocol Support:
   Servlets can handle the HTTP protocol requests and responses.

3. Dynamic content generation:
   Servlets can dynamically generate content on the fly, which is especially useful for creating personalized responses to user requests.

4. Session Management:
   Servlets can maintain user sessions, which allows them to keep track of user-specific data across multiple requests.

5. Scalability:
   Servlets are designed to be scalable, making them suitable for handling a large number of concurrent user requests efficiently.

How do servlets work?

When the client (browser) makes the HTTP request to the server (apache tomcat) the server routes the request to the appropriate servlet based on the 'web.xml' file. The servlet then processes the request and sends the response back to the client. The Web Container manages the life cycle of the servlet from creating the instances to destroying them when no longer needed.

Life-cycle of a servlet

1. Servlet class is loaded

   When the web container receives a request it immediately loads the servlet class after that it loads the web.xml file.

2. Servlet instance is created

   The instance of servlet is created after loading the servlet class. The servlet instance is created only once in the servlet life cycle.

3. init() method invoked

   After an instance is created the web container calls the init() method.

    @Override
        public void init() throws ServletException {
            // TODO Auto-generated method stub
            super.init();
        }
   

4. service() method is invoked

   When a server receives the request it invokes the service method. The service method checks the HTTP request type and calls the appropriate methods doGet, doPost, doPut, doDelete e.t.c.

    public void doGet(HttpServletRequest req, HttpServletResponse res)
       throws ServletException, IOException {
       // Servlet code
    }
   

5. destroy() method invoked

   After currently running threads have completed their jobs, the Servlet container calls the destroy() method on the Servlet instance.

        public void destroy() {
            // TODO Auto-generated method stub
            super.destroy();
        }
   

Conclusion

Java Servlets have been a crucial technology for building web applications for many years. Their ability to handle dynamic content, session management, and scalability make them a robust choice for developers. Moreover, their platform independence and standard API ensure that servlet-based applications can run on various servers without modifications.

As the web development landscape evolves, new technologies like JavaServer Pages (JSP) and frameworks like Spring MVC have gained popularity. However, understanding the core concepts of Java Servlets remains essential, as many of these newer technologies build upon the foundation provided by Servlets. By mastering servlets, developers can build powerful and flexible web applications that cater to the needs of modern users.