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public String doHMACSHA256(String part1AndPart2, String secretKey){
    Mac mac = Mac.getInstance("HmacSHA256");
    mac.init(new SecretKeySpec(secretKey.getBytes(), "HmacSHA256"));

    byte[] hashBytes = mac.doFinal(part1AndPart2.getBytes());
    String hash = doBASE64(hashBytes);
    return hash;
}

public String doBASE64(byte[] bytes) {
    Base64.Encoder encoder = Base64.getEncoder();
    String base64 = encoder.encodeToString(bytes);
    return base64;
}

public String doBASE64(String input) {
    byte[] bytes = input.getBytes(Charset.forName("UTF-8"));
    String base64 = doBASE64(bytes);
    return base64;
}

public String generateJWT(){

    String HEADER = String.join("\n", Files.readAllLines(Paths.get("./header.json")));
    String PAYLOAD = String.join("\n", Files.readAllLines(Paths.get("./payload.json")));

    String PART1 = doBASE64(HEADER);
    String PART2 = doBASE64(PAYLOAD);

    String PART1_PART2 = PART1 + "." + PART2;

    String PART3 = doBASE64(doHMACSHA256(PART1_PART2, SECRET_KEY));

    String JWT_TOKEN = PART1_PART2 + "." + PART3;

    return JWT_TOKEN;
}

public boolean validateJWT(String jwt) {

    String[] parts = jwt.split("\\.");
    String PART1 = parts[0];
    String PART2 = parts[1];
    String PART3 = parts[2];

    String PART1_PART2 = PART1 + "." + PART2;

    String jwtSignature = doBASE64(doHMACSHA256(PART1_PART2, SECRET_KEY));

    return jwtSignature.equals(PART3);

}

String JWT_TOKEN = generateJWT();

boolean isValid = validateJWT(JWT_TOKEN);

System.out.println("Is valid ? " + isValid);