Java.Servlet.How does a servlet container manage the servlet life cycle, when and what methods are called?

🔥 Let’s explore how a servlet container manages the servlet lifecycle step by step — including when and what methods are called.

🌀 Servlet Life Cycle Overview

The servlet container (like Tomcat or Jetty) controls the life of a servlet. It goes through three main phases:

1. Loading & Instantiation
2. Initialization (init)
3. Request Handling (service → doGet/doPost)
4. Destruction (destroy)

📦 1. Loading & Instantiation

• When: The first time the servlet is requested, or at startup if load-on-startup is set in web.xml.
• What: The container loads the servlet class and creates a single instance.

public class MyServlet extends HttpServlet {
    // Constructor is optional
    public MyServlet() {
        super();
        System.out.println("Constructor: Servlet instance created");
    }
}

Servlet container ensures only one instance per servlet — it’s multithreaded!

⚙️ 2. Initialization – init()

• When: Called once after instantiation
• What: Initializes the servlet — great place to read config params, set up DB connections, etc.

@Override
public void init(ServletConfig config) throws ServletException {
    super.init(config);
    System.out.println("init(): Called once after servlet instantiation");
}

Only called once per servlet lifecycle.

🔁 3. Request Handling – service()

• When: Every time a request is made to the servlet (e.g., from browser or REST client)
• What: The container spawns a new thread and calls the service() method.

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
    res.getWriter().write("Hello from doGet!");
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse res)
        throws ServletException, IOException {
    res.getWriter().write("Hello from doPost!");
}

The service() method routes to doGet(), doPost(), doPut(), etc., based on the HTTP method.

❌ 4. Destruction – destroy()

• When: When the server is shutting down or the servlet is being undeployed.
• What: Cleans up resources (e.g., closing DB connections, stopping threads).

@Override
public void destroy() {
    System.out.println("destroy(): Called once before servlet is removed");
}

Only called once — time to release heavy resources.

🚀 Summary Flowchart

   +------------------+
   | Class Loaded     |
   +------------------+
           ↓
   +------------------+
   | new MyServlet()  |
   +------------------+
           ↓
   +------------------+
   | init()           |
   +------------------+
           ↓
   +-------------------------------+
   | service() → doGet()/doPost() |
   +-------------------------------+
           ↓  (many times!)
   +------------------+
   | destroy()        |
   +------------------+

🧠 Pro Tips

• Thread-safety: Because multiple requests are handled by threads on a single servlet instance, make sure shared data is synchronized.
• init() vs constructor: Prefer init() for any config logic — container has more control here.
• destroy() cleanup: Essential for avoiding memory leaks.