What is JWT, and how is it used for authentication in Java applications?

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JWT (JSON Web Token) is a compact, URL-safe token format used for securely transmitting information between parties as a JSON object. It is commonly used in modern web applications for authentication and authorization. A JWT typically consists of three parts: a header, a payload, and a signature.

Structure of a JWT:

1. Header: Specifies the algorithm (e.g., HMAC, RSA, etc.) used for signing the token.

2. Payload: Contains claims, which are statements about an entity (typically the user) and additional data. Claims can be standard (e.g., sub for subject) or custom (e.g., user roles).

3. Signature: Ensures that the token has not been tampered with. It's created by signing the header and payload using a secret key or private key.

How JWT is used for authentication in Java applications:

1. User Login: When a user logs in with their credentials (username/password), the backend (usually a Java-based application) verifies the credentials.

2. Token Creation: Upon successful authentication, the server generates a JWT that contains user-related information (e.g., user ID, roles) in the payload. This token is signed with a secret key to prevent tampering.

3. Token Transmission: The server sends the JWT to the client (e.g., a web browser or mobile app). The client stores the token, typically in local storage or session storage.

4. Subsequent Requests: For subsequent requests, the client sends the JWT in the HTTP Authorization header, usually prefixed with Bearer (e.g., Authorization: Bearer <token>).

5. Token Validation: The server receives the JWT, verifies its signature using the secret key, and checks its validity (e.g., expiration time). If valid, the server processes the request; if not, it returns an error (e.g., 401 Unauthorized).

Benefits:

• Stateless: The server does not need to maintain session information, as the token carries the necessary data.

• Scalability: As JWT is stateless, it's ideal for distributed applications, allowing easy scaling across multiple servers.

In Java applications, libraries like Java JWT (jjwt) or Spring Security are commonly used to create, sign, and validate JWTs for secure authentication.

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