Javalin Servlets and Filters

In this post, we’ll explore Servlets and Filters in a lightweight way using Javalin, a modern Java web framework that simplifies HTTP handling. We’ll write easy-to-follow code examples and show their outputs.

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1. What Are Servlets and Filters?

Servlet

• A Servlet is a Java class that handles HTTP requests (GET, POST, etc.) and generates responses.
• It’s the backbone of Java web applications (before frameworks like Spring took over).
• Example use cases: API endpoints, dynamic content generation.

Filter

• A Filter intercepts requests/responses before they reach a Servlet (or after they leave it).
• Useful for:
  • Authentication/Authorization
  • Logging
  • Input validation
  • Response compression

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2. Setting Up Javalin

Javalin is a minimalist framework that simplifies Servlet/Filter-like behavior without XML configurations. First, add the dependency (Maven):

<dependency>
    <groupId>io.javalin</groupId>
    <artifactId>javalin</artifactId>
    <version>6.0.0</version>
</dependency>

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3. Basic Servlet Example in Javalin

In Javalin, a Servlet-like behavior is achieved using get(), post(), etc., methods.

Example: Simple GET Endpoint

import io.javalin.Javalin;

public class SimpleServlet {
    public static void main(String[] args) {
        Javalin app = Javalin.create().start(7070);

        app.get("/hello", ctx -> {
            ctx.result("Hello from Javalin Servlet!");
        });
    }
}

Output

• Visit http://localhost:7070/hello in a browser.
• Response:
  Hello from Javalin Servlet!

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4. Basic Filter Example in Javalin

Javalin doesn’t have a direct Filter interface, but we can mimic it using middleware (before() and after() hooks).

Example: Logging Filter

import io.javalin.Javalin;

public class LoggingFilter {
    public static void main(String[] args) {
        Javalin app = Javalin.create()
            .before(ctx -> {
                System.out.println("[Filter] Request received: " + ctx.method() + " " + ctx.path());
            })
            .after(ctx -> {
                System.out.println("[Filter] Response sent: " + ctx.status());
            })
            .start(7070);

        app.get("/greet", ctx -> {
            ctx.result("Greetings from Javalin!");
        });
    }
}

Output

• Visit http://localhost:7070/greet.
• Console Logs:

  [Filter] Request received: GET /greet  
  [Filter] Response sent: 200

• Browser Response:
  Greetings from Javalin!

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5. Advanced Filter: Authentication Middleware

Let’s implement a simple auth filter that checks for a token.

Example: Auth Middleware

import io.javalin.Javalin;
import io.javalin.http.Handler;

public class AuthFilter {
    public static void main(String[] args) {
        Javalin app = Javalin.create().start(7070);

        // Auth middleware
        Handler authMiddleware = ctx -> {
            String token = ctx.header("Authorization");
            if (token == null || !token.equals("secret123")) {
                ctx.status(401).result("Unauthorized");
                return;
            }
            ctx.next(); // Proceed if authorized
        };

        // Protected endpoint
        app.before("/protected/*", authMiddleware);
        app.get("/protected/data", ctx -> {
            ctx.result("This is secret data!");
        });

        // Public endpoint
        app.get("/public", ctx -> {
            ctx.result("Anyone can see this.");
        });
    }
}

Output

1. Without Token:

   • curl http://localhost:7070/protected/data
   • Response: 401 Unauthorized

2. With Token:

   • curl -H "Authorization: secret123" http://localhost:7070/protected/data
   • Response: This is secret data!

3. Public Endpoint:

   • curl http://localhost:7070/public
   • Response: Anyone can see this.

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6. Conclusion

• Servlets in Javalin are handled via get(), post(), etc., methods.
• Filters can be implemented using middleware (before()/after() hooks).
• Javalin’s simplicity makes it great for learning web fundamentals.

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Further Reading

• Javalin Official Docs
• Java Servlet & Filter Basics (Oracle Docs)

Happy coding! 🚀