使用Java实现JWE规范下的AES-256-GCM算法签名，使用中文注释

以下是使用Java实现JWE规范下的AES-256-GCM算法签名的示例代码：

import java.security.Key;
import java.security.SecureRandom;
import java.util.Base64;

import javax.crypto.Cipher;
import javax.crypto.KeyGenerator;
import javax.crypto.spec.GCMParameterSpec;
import javax.crypto.spec.SecretKeySpec;

public class JWE_AES_256_GCM {

    public static void main(String[] args) throws Exception {
        // 生成随机密钥
        Key key = generateKey();
        System.out.println("随机密钥：" + Base64.getUrlEncoder().withoutPadding().encodeToString(key.getEncoded()));
        
        // 加密明文数据
        String plaintext = "Hello world!";
        String cipherText = encrypt(plaintext, key);
        System.out.println("加密后的数据：" + cipherText);
        
        // 解密密文数据
        String decryptedText = decrypt(cipherText, key);
        System.out.println("解密后的数据：" + decryptedText);
    }

    /**
     * 生成随机密钥
     */
    public static Key generateKey() throws Exception {
        KeyGenerator keyGenerator = KeyGenerator.getInstance("AES");
        keyGenerator.init(256, new SecureRandom());
        return keyGenerator.generateKey();
    }

    /**
     * 加密明文数据
     */
    public static String encrypt(String plaintext, Key key) throws Exception {
        byte[] plaintextBytes = plaintext.getBytes("UTF-8");
        
        // 生成随机IV
        byte[] iv = generateIv();
        
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        SecretKeySpec keySpec = new SecretKeySpec(key.getEncoded(), "AES");
        GCMParameterSpec parameterSpec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.ENCRYPT_MODE, keySpec, parameterSpec);
        byte[] ciphertextBytes = cipher.doFinal(plaintextBytes);
        
        // 将IV和密文拼接成JWE字符串
        String ivString = Base64.getUrlEncoder().withoutPadding().encodeToString(iv);
        String ciphertextString = Base64.getUrlEncoder().withoutPadding().encodeToString(ciphertextBytes);
        return ivString + "." + ciphertextString;
    }

    /**
     * 解密密文数据
     */
    public static String decrypt(String jwe, Key key) throws Exception {
        // 解析JWE字符串，获取IV和密文
        String[] parts = jwe.split("\\.");
        byte[] iv = Base64.getUrlDecoder().decode(parts[0]);
        byte[] ciphertextBytes = Base64.getUrlDecoder().decode(parts[1]);
        
        Cipher cipher = Cipher.getInstance("AES/GCM/NoPadding");
        SecretKeySpec keySpec = new SecretKeySpec(key.getEncoded(), "AES");
        GCMParameterSpec parameterSpec = new GCMParameterSpec(128, iv);
        cipher.init(Cipher.DECRYPT_MODE, keySpec, parameterSpec);
        byte[] plaintextBytes = cipher.doFinal(ciphertextBytes);
        
        return new String(plaintextBytes, "UTF-8");
    }

    /**
     * 生成随机IV
     */
    public static byte[] generateIv() {
        byte[] iv = new byte[12];
        new SecureRandom().nextBytes(iv);
        return iv;
    }

}