如何加密字符串使用AES256，ECB模式与填充pkcs5？

import javax.crypto.Cipher;
import javax.crypto.spec.SecretKeySpec;
import java.util.Base64;

public class AESUtil {
    private static final String ALGORITHM = "AES";
    private static final String MODE = "AES/ECB/PKCS5Padding";

    public static String encrypt(String plaintext, String key) throws Exception {
        SecretKeySpec secretKey = new SecretKeySpec(key.getBytes(), ALGORITHM);
        Cipher cipher = Cipher.getInstance(MODE);
        cipher.init(Cipher.ENCRYPT_MODE, secretKey);
        byte[] encryptedBytes = cipher.doFinal(plaintext.getBytes());
        return Base64.getEncoder().encodeToString(encryptedBytes);
    }

    public static String decrypt(String ciphertext, String key) throws Exception {
        SecretKeySpec secretKey = new SecretKeySpec(key.getBytes(), ALGORITHM);
        Cipher cipher = Cipher.getInstance(MODE);
        cipher.init(Cipher.DECRYPT_MODE, secretKey);
        byte[] decryptedBytes = cipher.doFinal(Base64.getDecoder().decode(ciphertext));
        return new String(decryptedBytes);
    }

    public static void main(String[] args) throws Exception {
        String plaintext = "Hello, world!";
        String key = "0123456789abcdef0123456789abcdef";
        String ciphertext = encrypt(plaintext, key);
        System.out.println("Ciphertext: " + ciphertext);
        String decryptedText = decrypt(ciphertext, key);
        System.out.println("Decrypted text: " + decryptedText);
    }
}