Java Notes

Servlet Life Cycle

Deep dive into the Servlet lifecycle including initialization, request handling, and destruction phases with practical code examples and diagrams.

Overview

The Servlet Life Cycle defines how a servlet is loaded, initialized, handles requests, and is finally destroyed by the servlet container. Understanding this lifecycle is crucial for managing resources, database connections, and application state.

Phase 1: Loading and Instantiation

The servlet container:

Finds the servlet class (from web.xml or @WebServlet)
Loads the class using ClassLoader
Creates ONE instance using the no-arg constructor

java example

public class MyServlet extends HttpServlet {
    // Constructor called ONCE by container
    public MyServlet() {
        System.out.println("Servlet instantiated!");
    }
}

Output

Servlet instantiated!

When Does Loading Happen?

Trigger	Explanation
First request	Default behavior — lazy loading
Server startup	When `loadOnStartup >= 0` is set

java example

@WebServlet(urlPatterns = "/my-servlet", loadOnStartup = 1)
public class MyServlet extends HttpServlet { }

Example

<servlet> <servlet-name>MyServlet</servlet-name> <servlet-class>com.example.MyServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet>

loadOnStartup values:

Negative (default): Load on first request
0 or positive: Load at server startup (lower number = higher priority)

Phase 2: Initialization — init()

Called exactly once after instantiation. Use it to perform expensive one-time setup.

java example

@WebServlet("/users")
public class UserServlet extends HttpServlet {

    private Connection dbConnection;
    private String appName;

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config); // IMPORTANT: always call super.init()

        // Read initialization parameters
        appName = config.getInitParameter("appName");

        // Open database connection (one-time setup)
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            dbConnection = DriverManager.getConnection(
                "jdbc:mysql://localhost:3306/mydb", "root", "password"
            );
            System.out.println("Database connected in init()");
        } catch (Exception e) {
            throw new ServletException("DB connection failed", e);
        }
    }
}

Output

Database connected in init()

Init Parameters in web.xml

xml example

<servlet>
    <servlet-name>UserServlet</servlet-name>
    <servlet-class>com.example.UserServlet</servlet-class>
    <init-param>
        <param-name>appName</param-name>
        <param-value>MyWebApp</param-value>
    </init-param>
</servlet>

Two Versions of init()

java example

// Version 1: With ServletConfig (called by container)
public void init(ServletConfig config) throws ServletException {
    super.init(config);
    // your setup code
}

// Version 2: Convenience method (no need to call super)
public void init() throws ServletException {
    // your setup code — simpler, recommended
}

Phase 3: Request Handling — service()

Called for every client request. The container creates a new thread for each request.

java example

// The service() method dispatches to appropriate handler
protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String method = req.getMethod();
    if (method.equals("GET")) {
        doGet(req, resp);
    } else if (method.equals("POST")) {
        doPost(req, resp);
    } else if (method.equals("PUT")) {
        doPut(req, resp);
    } else if (method.equals("DELETE")) {
        doDelete(req, resp);
    }
}

Handling Multiple HTTP Methods

java example

@WebServlet("/products")
public class ProductServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle GET — list products
        response.setContentType("application/json");
        PrintWriter out = response.getWriter();
        out.println("{\"products\": []}");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle POST — create product
        String name = request.getParameter("name");
        String price = request.getParameter("price");
        // Save to database...
        response.setStatus(HttpServletResponse.SC_CREATED);
        response.getWriter().println("Product created: " + name);
    }

    @Override
    protected void doPut(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle PUT — update product
        response.getWriter().println("Product updated");
    }

    @Override
    protected void doDelete(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle DELETE — remove product
        response.getWriter().println("Product deleted");
    }
}

Thread Safety Issue

Example

// ❌ WRONG — Instance variable shared across threads @WebServlet("/counter") public class UnsafeServlet extends HttpServlet { private int count = 0; // SHARED! Race condition! protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException { count++; // NOT thread-safe resp.getWriter().println("Count: " + count); } } // ✅ CORRECT — Use local variables or synchronization @WebServlet("/counter") public class SafeServlet extends HttpServlet { private final AtomicInteger count = new AtomicInteger(0); protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException { int currentCount = count.incrementAndGet(); // Thread-safe resp.getWriter().println("Count: " + currentCount); } }

Phase 4: Destruction — destroy()

Called once when the container shuts down or the servlet is unloaded.

java example

@Override
public void destroy() {
    // Close database connection
    try {
        if (dbConnection != null && !dbConnection.isClosed()) {
            dbConnection.close();
            System.out.println("Database connection closed in destroy()");
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }

    // Release other resources
    System.out.println("Servlet destroyed — cleanup complete");
}

Output

Database connection closed in destroy()
Servlet destroyed — cleanup complete

Complete Lifecycle Example

Example

@WebServlet(urlPatterns = "/lifecycle", loadOnStartup = 1) public class LifecycleServlet extends HttpServlet { private int requestCount = 0; public LifecycleServlet() { System.out.println("1. Constructor called — Servlet instantiated"); } @Override public void init() throws ServletException { System.out.println("2. init() called — One-time initialization"); } @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { requestCount++; System.out.println("3. service()/doGet() called — Request #" + requestCount); resp.setContentType("text/html"); resp.getWriter().println("<h1>Request #" + requestCount + "</h1>"); } @Override public void destroy() { System.out.println("4. destroy() called — Total requests served: " + requestCount); } }

Output

1. Constructor called — Servlet instantiated
2. init() called — One-time initialization

Console Output:

Example

1. Constructor called — Servlet instantiated 2. init() called — One-time initialization 3. service()/doGet() called — Request #1 3. service()/doGet() called — Request #2 3. service()/doGet() called — Request #3 4. destroy() called — Total requests served: 3

ServletConfig vs ServletContext

Feature	ServletConfig	ServletContext
Scope	Per servlet	Per web application
Access	`getServletConfig()`	`getServletContext()`
Purpose	Servlet-specific params	App-wide shared data
Set by	`<init-param>`	`<context-param>`

java example

// Reading context parameters (application-wide)
@Override
public void init() throws ServletException {
    ServletContext context = getServletContext();
    String dbUrl = context.getInitParameter("DATABASE_URL");
}

Example

<context-param> <param-name>DATABASE_URL</param-name> <param-value>jdbc:mysql://localhost:3306/mydb</param-value> </context-param>

Interview Key Points

Servlet lifecycle: Constructor → init() → service() → destroy()
Only ONE instance of a servlet is created (singleton pattern)
Multiple threads call service() concurrently
init() is called once; destroy() is called once
Use loadOnStartup to control when initialization happens
Instance variables are NOT thread-safe in servlets
init(ServletConfig) must call super.init(config) to save config

Summary Table

Method	Called	Times	Purpose
Constructor	On first request or startup	Once	Create instance
init()	After construction	Once	Setup resources
service()	On each HTTP request	Many	Handle request
destroy()	On shutdown/unload	Once	Cleanup resources

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Servlet Life Cycle.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, servlets, jsp, servlet

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Java Notes

Servlet Life Cycle

Deep dive into the Servlet lifecycle including initialization, request handling, and destruction phases with practical code examples and diagrams.

Overview

Phase 1: Loading and Instantiation

The servlet container:

Finds the servlet class (from web.xml or @WebServlet)
Loads the class using ClassLoader
Creates ONE instance using the no-arg constructor

java example

public class MyServlet extends HttpServlet {
    // Constructor called ONCE by container
    public MyServlet() {
        System.out.println("Servlet instantiated!");
    }
}

Output

Servlet instantiated!

When Does Loading Happen?

Trigger	Explanation
First request	Default behavior — lazy loading
Server startup	When `loadOnStartup >= 0` is set

java example

@WebServlet(urlPatterns = "/my-servlet", loadOnStartup = 1)
public class MyServlet extends HttpServlet { }

Example

<servlet> <servlet-name>MyServlet</servlet-name> <servlet-class>com.example.MyServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet>

loadOnStartup values:

Negative (default): Load on first request
0 or positive: Load at server startup (lower number = higher priority)

Phase 2: Initialization — init()

Called exactly once after instantiation. Use it to perform expensive one-time setup.

java example

@WebServlet("/users")
public class UserServlet extends HttpServlet {

    private Connection dbConnection;
    private String appName;

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config); // IMPORTANT: always call super.init()

        // Read initialization parameters
        appName = config.getInitParameter("appName");

        // Open database connection (one-time setup)
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            dbConnection = DriverManager.getConnection(
                "jdbc:mysql://localhost:3306/mydb", "root", "password"
            );
            System.out.println("Database connected in init()");
        } catch (Exception e) {
            throw new ServletException("DB connection failed", e);
        }
    }
}

Output

Database connected in init()

Init Parameters in web.xml

xml example

<servlet>
    <servlet-name>UserServlet</servlet-name>
    <servlet-class>com.example.UserServlet</servlet-class>
    <init-param>
        <param-name>appName</param-name>
        <param-value>MyWebApp</param-value>
    </init-param>
</servlet>

Two Versions of init()

java example

// Version 1: With ServletConfig (called by container)
public void init(ServletConfig config) throws ServletException {
    super.init(config);
    // your setup code
}

// Version 2: Convenience method (no need to call super)
public void init() throws ServletException {
    // your setup code — simpler, recommended
}

Phase 3: Request Handling — service()

Called for every client request. The container creates a new thread for each request.

java example

// The service() method dispatches to appropriate handler
protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String method = req.getMethod();
    if (method.equals("GET")) {
        doGet(req, resp);
    } else if (method.equals("POST")) {
        doPost(req, resp);
    } else if (method.equals("PUT")) {
        doPut(req, resp);
    } else if (method.equals("DELETE")) {
        doDelete(req, resp);
    }
}

Handling Multiple HTTP Methods

java example

@WebServlet("/products")
public class ProductServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle GET — list products
        response.setContentType("application/json");
        PrintWriter out = response.getWriter();
        out.println("{\"products\": []}");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle POST — create product
        String name = request.getParameter("name");
        String price = request.getParameter("price");
        // Save to database...
        response.setStatus(HttpServletResponse.SC_CREATED);
        response.getWriter().println("Product created: " + name);
    }

    @Override
    protected void doPut(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle PUT — update product
        response.getWriter().println("Product updated");
    }

    @Override
    protected void doDelete(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Handle DELETE — remove product
        response.getWriter().println("Product deleted");
    }
}

Thread Safety Issue

Example

Phase 4: Destruction — destroy()

Called once when the container shuts down or the servlet is unloaded.

java example

@Override
public void destroy() {
    // Close database connection
    try {
        if (dbConnection != null && !dbConnection.isClosed()) {
            dbConnection.close();
            System.out.println("Database connection closed in destroy()");
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }

    // Release other resources
    System.out.println("Servlet destroyed — cleanup complete");
}

Output

Database connection closed in destroy()
Servlet destroyed — cleanup complete

Complete Lifecycle Example

Example

Output

1. Constructor called — Servlet instantiated
2. init() called — One-time initialization

Console Output:

Example

ServletConfig vs ServletContext

Feature	ServletConfig	ServletContext
Scope	Per servlet	Per web application
Access	`getServletConfig()`	`getServletContext()`
Purpose	Servlet-specific params	App-wide shared data
Set by	`<init-param>`	`<context-param>`

java example

// Reading context parameters (application-wide)
@Override
public void init() throws ServletException {
    ServletContext context = getServletContext();
    String dbUrl = context.getInitParameter("DATABASE_URL");
}

Example

<context-param> <param-name>DATABASE_URL</param-name> <param-value>jdbc:mysql://localhost:3306/mydb</param-value> </context-param>

Interview Key Points

Servlet lifecycle: Constructor → init() → service() → destroy()
Only ONE instance of a servlet is created (singleton pattern)
Multiple threads call service() concurrently
init() is called once; destroy() is called once
Use loadOnStartup to control when initialization happens
Instance variables are NOT thread-safe in servlets
init(ServletConfig) must call super.init(config) to save config

Summary Table

Method	Called	Times	Purpose
Constructor	On first request or startup	Once	Create instance
init()	After construction	Once	Setup resources
service()	On each HTTP request	Many	Handle request
destroy()	On shutdown/unload	Once	Cleanup resources

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Servlet Life Cycle.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, servlets, jsp, servlet

Continue learning this concept

PreviousServlet Introduction NextSession Tracking