JWT validation filter in Spring Boot

Introduction

A JWT validation filter checks if a user's token is valid before allowing access to protected parts of a web app. It helps keep the app safe by making sure only authorized users can get in.

When you want to protect API endpoints so only logged-in users can access them.

When you need to check the user's identity on every request automatically.

When you want to reject requests with expired or tampered tokens.

When building a REST API that uses JWT for user authentication.

When you want to add security without changing your controller code.

Syntax

Spring Boot

public class JwtValidationFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        String token = extractToken(request);
        if (token != null && validateToken(token)) {
            // Set authentication in security context
            filterChain.doFilter(request, response);
        } else {
            response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
            return;
        }
    }

    private String extractToken(HttpServletRequest request) {
        String bearer = request.getHeader("Authorization");
        if (bearer != null && bearer.startsWith("Bearer ")) {
            return bearer.substring(7);
        }
        return null;
    }

    private boolean validateToken(String token) {
        // Logic to check token signature and expiry
        return true; // or false
    }
}

The filter extends OncePerRequestFilter to run once per request.

Extract the token from the Authorization header, usually starting with "Bearer ".

Examples

Basic check to allow or deny access based on token validity.

Spring Boot

String token = extractToken(request);
if (token != null && validateToken(token)) {
    // Allow access
} else {
    response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
    return;
}

Extracts the JWT token from the Authorization header if it starts with "Bearer ".

Spring Boot

private String extractToken(HttpServletRequest request) {
    String bearer = request.getHeader("Authorization");
    if (bearer != null && bearer.startsWith("Bearer ")) {
        return bearer.substring(7);
    }
    return null;
}

Placeholder for token validation logic using a JWT library.

Spring Boot

private boolean validateToken(String token) {
    // Use JWT library to check signature and expiry
    return true; // or false
}

Sample Program

This filter checks the Authorization header for a JWT token. If the token equals "valid-token", it lets the request continue. Otherwise, it sends a 401 Unauthorized response with a message.

Spring Boot

package com.example.security;

import jakarta.servlet.FilterChain;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import org.springframework.web.filter.OncePerRequestFilter;
import java.io.IOException;

public class JwtValidationFilter extends OncePerRequestFilter {

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        String token = extractToken(request);
        if (token != null && validateToken(token)) {
            // Token is valid, continue processing
            filterChain.doFilter(request, response);
        } else {
            // Token missing or invalid, reject request
            response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
            response.getWriter().write("Unauthorized: Invalid or missing token");
        }
    }

    private String extractToken(HttpServletRequest request) {
        String bearer = request.getHeader("Authorization");
        if (bearer != null && bearer.startsWith("Bearer ")) {
            return bearer.substring(7);
        }
        return null;
    }

    private boolean validateToken(String token) {
        // Simple dummy validation: token must equal "valid-token" for demo
        return "valid-token".equals(token);
    }
}

Important Notes

Always place the JWT validation filter before your controllers in the filter chain.

Use a real JWT library like jjwt or java-jwt to verify token signature and expiry.

Make sure to handle exceptions and send clear error messages for better debugging.

Summary

A JWT validation filter checks tokens on incoming requests to protect your app.

It extracts the token from the Authorization header and verifies it.

If the token is valid, the request proceeds; if not, it returns 401 Unauthorized.

Practice

1. What is the main purpose of a JWT validation filter in a Spring Boot application?

easy

A. To generate new JWT tokens for users

B. To check and verify JWT tokens on incoming HTTP requests

C. To log all incoming requests without validation

D. To encrypt the response data before sending

Solution

1. Step 1: Understand JWT validation filter role

   A JWT validation filter is designed to intercept incoming requests and check the validity of JWT tokens.

2. Step 2: Identify the correct purpose

   It does not generate tokens or encrypt data; its main job is to verify tokens to allow or deny access.

3. Final Answer:

   To check and verify JWT tokens on incoming HTTP requests -> Option B

4. Quick Check:

   JWT validation filter = Verify tokens [OK]

Hint: JWT filter checks tokens on requests, not generating or logging [OK]

Common Mistakes:

• Confusing validation with token generation
• Thinking filter encrypts data
• Assuming it only logs requests

2. Which method in a Spring Boot filter is typically overridden to implement JWT validation logic?

easy

A. doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain)

B. init(FilterConfig filterConfig)

C. destroy()

D. handleRequest(HttpRequest request)

Solution

1. Step 1: Identify filter method for request processing

   In Spring Boot, filters extend OncePerRequestFilter and override doFilterInternal to process requests.

2. Step 2: Match method to JWT validation

   doFilterInternal is where JWT token extraction and validation happen before continuing the chain.

3. Final Answer:

   doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain) -> Option A

4. Quick Check:

   JWT validation code goes in doFilterInternal [OK]

Hint: JWT validation logic goes in doFilterInternal method [OK]

Common Mistakes:

• Using init() which is for filter setup only
• Confusing destroy() with request handling
• Inventing non-existent handleRequest() method

3. Given this snippet inside a JWT validation filter, what happens if the token is invalid?

String token = request.getHeader("Authorization");
if (token == null || !jwtUtil.validateToken(token)) {
    response.setStatus(HttpServletResponse.SC_UNAUTHORIZED);
    return;
}
chain.doFilter(request, response);

medium

A. The server throws a NullPointerException

B. The request proceeds without validation

C. The request is blocked with 401 Unauthorized status

D. The token is refreshed automatically

Solution

1. Step 1: Analyze token check condition

   If token is missing or invalid, the code sets response status to 401 and returns immediately.

2. Step 2: Understand filter chain behavior

   Because it returns before calling chain.doFilter, the request does not proceed further.

3. Final Answer:

   The request is blocked with 401 Unauthorized status -> Option C

4. Quick Check:

   Invalid token = 401 block [OK]

Hint: Invalid token triggers 401 and stops request chain [OK]

Common Mistakes:

• Assuming request proceeds despite invalid token
• Expecting automatic token refresh
• Thinking NullPointerException occurs here

4. Identify the error in this JWT validation filter snippet:

@Override
protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws ServletException, IOException {
    String token = request.getHeader("Authorization");
    if (token != null && jwtUtil.validateToken(token)) {
        SecurityContextHolder.getContext().setAuthentication(jwtUtil.getAuthentication(token));
    }
    chain.doFilter(request, response);
}

medium

A. It does not handle the case when token is missing or invalid by blocking the request

B. It incorrectly sets authentication before validation

C. It calls chain.doFilter twice causing errors

D. It throws IOException without handling

Solution

1. Step 1: Review token validation logic

   The code sets authentication only if token is valid, but does not block invalid or missing tokens.

2. Step 2: Check filter chain continuation

   It always calls chain.doFilter, so invalid requests proceed without rejection.

3. Final Answer:

   It does not handle the case when token is missing or invalid by blocking the request -> Option A

4. Quick Check:

   Missing block on invalid token = security hole [OK]

Hint: Always block requests with missing or invalid tokens [OK]

Common Mistakes:

• Allowing requests without token validation
• Calling chain.doFilter twice (not here though)
• Misunderstanding exception handling in filters

5. You want to create a JWT validation filter that extracts the token from the Authorization header, validates it, and sets the user authentication in the security context only if valid. Which sequence of actions is correct inside doFilterInternal?

hard

A. Continue filter chain -> Extract token -> Validate token -> Set authentication -> Else respond 401

B. Validate token -> Extract token -> Set authentication -> Continue filter chain -> Else respond 401

C. Set authentication -> Extract token -> Validate token -> Continue filter chain -> Else respond 401

D. Extract token -> Validate token -> Set authentication -> Continue filter chain -> Else respond 401

Solution

1. Step 1: Determine correct order of JWT processing

   First, extract the token from the Authorization header, then validate it to ensure it's correct.

2. Step 2: Set authentication and control flow

   If valid, set user authentication in the security context, then continue the filter chain; otherwise, respond with 401 Unauthorized.

3. Final Answer:

   Extract token -> Validate token -> Set authentication -> Continue filter chain -> Else respond 401 -> Option D

4. Quick Check:

   Correct JWT filter flow = Extract token -> Validate token -> Set authentication -> Continue filter chain -> Else respond 401 [OK]

Hint: Extract first, then validate, set auth, continue or block [OK]

Common Mistakes:

• Validating before extracting token
• Setting authentication before validation
• Continuing filter chain before validation