1、小程序开发者调用微信的方法，获取小程序code，微信用户加密信息encryptedData，偏移量iv
2、将code、encryptedData、iv这三个参数传给后端
3、java端通过code请求微信的接口获取到微信用户的认证信息

https://api.weixin.qq.com/sns/jscode2session?appid=小程序appId&secret=" + 小程序secret + "&js_code=" +小程序端传来的code + "&grant_type=authorization_code";

通过调用此接口，微信接口返回的信息如下:

{"session_key":"FdBRYW2SgLBRg==","openid":"oTrxW45rpcmBhQ","session_key":"CB26oXZa6mDTjQw=="}

4、调用getWechetEncryptedData方法，把encryptedData、iv、session_key传入，即可获取用户的微信信息。

public class AesUtil {
    
    public static boolean initialized = false;

    public static String getWechetEncryptedData(String encryptedData, String sessionKey, String iv) {
        byte[] content = Base64.decodeBase64(encryptedData);
        byte[] keyByte = Base64.decodeBase64(sessionKey);
        byte[] ivByte = Base64.decodeBase64(iv);
        String userInfo = null;
        initialize();
        try {
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS7Padding");
            Key sKeySpec = new SecretKeySpec(keyByte, "AES");
            cipher.init(Cipher.DECRYPT_MODE, sKeySpec, generateIV(ivByte));// 初始化
            byte[] result = cipher.doFinal(content);
            if (null != result && result.length > 0) {
                userInfo = new String(result, StandardCharsets.UTF_8);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return userInfo;
    }
    
    public static void initialize(){
        if (initialized) {
            return;
        }
        Security.addProvider(new BouncyCastleProvider());
        initialized = true;
    }
    
    // 生成iv  
    public static AlgorithmParameters generateIV(byte[] iv) throws Exception {  
        AlgorithmParameters params = AlgorithmParameters.getInstance("AES");  
        params.init(new IvParameterSpec(iv));  
        return params;  
    }
    
}