Servlet Development Guide: Hello World Example

Creating a Basic Servlet

To begin learning Servlet development, we start with a simple Hello World example. Create a dynamic web project in you're IDE, then define a new Servlet class.

package com.example.demo;

import java.io.*;
import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.*;

@WebServlet("/HelloServlet")
public class GreetingServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.setContentType("text/html; charset=UTF-8");
        PrintWriter writer = resp.getWriter();
        try {
            writer.println("<html>");
            writer.println("<head><title>Greeting</title></head>");
            writer.println("<body>");
            writer.println("<h2>Hello World from Servlet</h2>");
            writer.println("</body></html>");
        } finally {
            writer.close();
        }
    }
}

Configuring URL Patterns with Annotations

The @WebServlet annotation, introduced in Servlet 3.0, simplifies deployment by eliminating the need for web.xml configuration. It supports multiple URL patterns.

@WebServlet(
    name = "MainServlet",
    urlPatterns = {"/hello", "/greeting"}
)
public class MainServlet extends HttpServlet {
    // Servlet implementation
}

Traditional Configuration via web.xml

For pre-3.0 environments or explicit configuration, you can define servlets in the deployment descriptor.

<servlet>
    <servlet-name>DemoServlet</servlet-name>
    <servlet-class>com.example.DemoServlet</servlet-class>
    <init-param>
        <param-name>encoding</param-name>
        <param-value>UTF-8</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>DemoServlet</servlet-name>
    <url-pattern>/demo/*</url-pattern>
</servlet-mapping>

Accessing Configuration Parameters

Servlets can retrieve initialization parameters defined in web.xml or via annotations.

public class ConfigReaderServlet extends HttpServlet {

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        // Get servlet-specific init parameter
        String encoding = getInitParameter("encoding");

        // Get global context parameter
        ServletContext context = getServletContext();
        String imageTypes = context.getInitParameter("ImageType");

        resp.setContentType("text/html");
        PrintWriter out = resp.getWriter();
        out.println("<p>Encoding: " + encoding + "</p>");
        out.println("<p>Allowed Image Types: " + imageTypes + "</p>");
    }
}

Resource Injection with @Resource

Java EE provides dependency injection for resources defined in the deployment descriptor.

<env-entry>
    <env-entry-name>welcomeMessage</env-entry-name>
    <env-entry-type>java.lang.String</env-entry-type>
    <env-entry-value>Welcome to the Application</env-entry-value>
</env-entry>

public class InjectedServlet extends HttpServlet {

    @Resource(name = "welcomeMessage")
    private String greetingText;

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.getWriter().println(greetingText);
    }
}

Servlet Lifecycle Annotations

The @PostConstruct and @PreDestroy annotations allow you to hook into the servlet lifecycle.

public class LifecycleServlet extends HttpServlet {

    @PostConstruct
    public void initialize() {
        System.out.println("Servlet initialized - @PostConstruct");
    }

    @PreDestroy
    public void cleanup() {
        System.out.println("Servlet about to be destroyed - @PreDestroy");
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        resp.getWriter().println("Lifecycle demo");
    }
}

Request Dispatching and Redirection

Servlets can forward requests to other resources or redirect clients to different URLs.

public class NavigationServlet extends HttpServlet {

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        // Forward to another resource
        RequestDispatcher dispatcher = req.getRequestDispatcher("/result.jsp");
        dispatcher.forward(req, resp);

        // Alternative: Redirect to external URL
        // resp.sendRedirect("https://example.com");
    }
}

Response Header Manipulation

You can control client behavior by setting specific HTTP headers.

public class HeaderServlet extends HttpServlet {

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {

        // Set refresh header for auto-redirect
        resp.setHeader("Refresh", "5; URL=https://example.com");

        // Alternative redirection method
        resp.setStatus(HttpServletResponse.SC_MOVED_TEMPORARILY);
        resp.setHeader("Location", "https://example.com");

        resp.getWriter().println("Redirecting in 5 seconds...");
    }
}

Thread Safety Considerations

Since servlets are typically multithreaded, avoid using instance variables for request-specific data to prevent concurrency issues.

public class ThreadSafeServlet extends HttpServlet {

    // Unsafe: shared across all requests
    // private int requestCount = 0;

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        // Safe: local variable for each request
        int currentCount = 0;
        // Process request
        resp.getWriter().println("Request processed");
    }
}

Tags: servlet Java-EE WebServlet Annotation web.xml

Posted on Sun, 10 May 2026 09:41:30 +0000 by cvincent

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