JWTUtils.java

@Service
public class JWTUtils {

	/**
	 * Based the secret key it is doing the signing
	 */
    private String secret = "learnitcodeit";

    /**
     * Extracting username from the token
     * @param token
     * @return
     */
    public String extractUsername(String token) {
        return extractClaim(token, Claims::getSubject);
    }

    /**
     * Extracting the expiry of the token
     * @param token
     * @return
     */
    public Date extractExpiration(String token) {
        return extractClaim(token, Claims::getExpiration);
    }

    /**
     * Extracting the claim based on the condition
     * @param <T>
     * @param token
     * @param claimsResolver
     * @return
     */
    public <T> T extractClaim(String token, Function<Claims, T> claimsResolver) {
        final Claims claims = extractAllClaims(token);
        return claimsResolver.apply(claims);
    }
    
    private Claims extractAllClaims(String token) {
        return Jwts.parser().setSigningKey(secret).parseClaimsJws(token).getBody();
    }

    private Boolean isTokenExpired(String token) {
        return extractExpiration(token).before(new Date());
    }

    /**
     * Generating token
     * @param username
     * @return
     */
    public String generateToken(String username) {
        Map<String, Object> claims = new HashMap<>();
        return createToken(claims, username);
    }

    
    private String createToken(Map<String, Object> claims, String subject) {

        return Jwts.builder().setClaims(claims).setSubject(subject).setIssuedAt(new Date(System.currentTimeMillis()))
                .setExpiration(new Date(System.currentTimeMillis() + 1000 * 60 * 60 * 10))
                .signWith(SignatureAlgorithm.HS256, secret).compact();
    }

    /**
     * Validating the token
     * @param token
     * @param userDetails
     * @return
     */
    public Boolean validateToken(String token, UserDetails userDetails) {
        final String username = extractUsername(token);
        return (username.equals(userDetails.getUsername()) && !isTokenExpired(token));
    }
}