要创建一个JWT，可以使用Java的javax.crypto库来进行加密和解密操作，而不使用nimbusds库。以下是一个示例代码，用于创建和验证JWT：

import java.security.Key;
import java.security.KeyPairGenerator;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.Signature;
import java.util.Base64;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

public class JWTExample {
    private static final String ALGORITHM = "RSA";

    public static void main(String[] args) {
        // 生成公私钥对
        KeyPairGenerator keyPairGenerator;
        try {
            keyPairGenerator = KeyPairGenerator.getInstance(ALGORITHM);
        } catch (NoSuchAlgorithmException e) {
            throw new RuntimeException("Failed to generate key pair", e);
        }
        keyPairGenerator.initialize(2048);
        Key privateKey = keyPairGenerator.generateKeyPair().getPrivate();
        Key publicKey = keyPairGenerator.generateKeyPair().getPublic();

        // 创建JWT
        String token = createJWT(privateKey);
        System.out.println("JWT: " + token);

        // 验证JWT
        boolean isValid = verifyJWT(token, publicKey);
        System.out.println("JWT is valid: " + isValid);
    }

    private static String createJWT(Key privateKey) {
        Map claims = new HashMap<>();
        claims.put("username", "john.doe");
        claims.put("role", "admin");

        // 生成JWT的header
        Map header = new HashMap<>();
        header.put("alg", "RS256");
        header.put("typ", "JWT");

        // 设置JWT的有效期为1小时
        long nowMillis = System.currentTimeMillis();
        long expMillis = nowMillis + 3600000;
        Date exp = new Date(expMillis);

        // 生成JWT的payload
        String jwtPayload = Base64.getUrlEncoder().withoutPadding().encodeToString(claims.toString().getBytes());

        // 生成JWT的signature
        String signature;
        try {
            Signature signer = Signature.getInstance("SHA256withRSA");
            signer.initSign((PrivateKey) privateKey);
            signer.update(jwtPayload.getBytes());
            byte[] signatureBytes = signer.sign();
            signature = Base64.getUrlEncoder().withoutPadding().encodeToString(signatureBytes);
        } catch (Exception e) {
            throw new RuntimeException("Failed to create JWT signature", e);
        }

        // 生成JWT
        String jwt = Base64.getUrlEncoder().withoutPadding().encodeToString(header.toString().getBytes())
                + "." + jwtPayload + "." + signature;

        return jwt;
    }

    private static boolean verifyJWT(String jwt, Key publicKey) {
        String[] parts = jwt.split("\\.");

        // 验证JWT的header和payload
        String header = new String(Base64.getUrlDecoder().decode(parts[0]));
        String payload = new String(Base64.getUrlDecoder().decode(parts[1]));
        System.out.println("JWT Header: " + header);
        System.out.println("JWT Payload: " + payload);

        // 验证JWT的signature
        String signature = parts[2];
        try {
            Signature verifier = Signature.getInstance("SHA256withRSA");
            verifier.initVerify((PublicKey) publicKey);
            verifier.update((parts[0] + "." + parts[1]).getBytes());
            byte[] signatureBytes = Base64.getUrlDecoder().decode(signature);
            return verifier.verify(signatureBytes);
        } catch (Exception e) {
            throw new RuntimeException("Failed to verify JWT signature", e);
        }
    }
}

上述代码使用RSA算法生成公私钥对，然后使用私钥创建JWT，并使用公钥验证JWT的有效性。JWT的格式为header.payload.signature，其中header和payload分别是Base64编码的JSON字符串，signature是通过使用私钥对header和payload进行签名后得到的Base64编码字符串。