以下是Java後臺生成一個小程式的完整程式碼。

import com.alibaba.fastjson.JSONObject;

import java.io.BufferedInputStream;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.PrintWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Base64;
import java.util.HashMap;
import java.util.Map;
public class WxQrCode {
    private static final String GETWXACODEUNLIMIT_URL = "https://api.weixin.qq.com/wxa/getwxacodeunlimit?access_token=%s";// 生成小程式碼地址
    private static final String URL_GET_TOKEN = "https://api.weixin.qq.com/cgi-bin/token";//獲取access_token地址
    private static final String APP_ID = "";// 小程式appid
    private static final String APP_SECRET = ""; // 小程式祕鑰

    public static final String PATH_HOME = "";

    /**
     * 獲取微信Access_Token
     * @param appid
     * @param secret
     * @return
     */
    public static String getWeixinAccessToken(String appid, String secret){
        String url = URL_GET_TOKEN;
        Map<String, String> paramMap = new HashMap<String, String>();
        paramMap.put("grant_type", "client_credential");
        paramMap.put("appid", appid);
        paramMap.put("secret", secret);
        String content = HttpClientUtils.executeHttpGet(url, paramMap);
        Map map = (Map) JsonMapper.fromJsonString(content, HashMap.class);
        return (String)map.get("access_token");
    }

    /**
     * 生成小程式碼返回base64字串
     * @param sceneStr 要攜帶的引數
     * @param accessToken 
     * @param page 要跳轉的小程式頁面（必須是已釋出的）
     * @return
     */
    public static String getminiqrQr(String sceneStr, String accessToken,String page) {
        HttpURLConnection httpURLConnection = null;
        try {
            URL url = new URL(String.format(GETWXACODEUNLIMIT_URL,accessToken));
            httpURLConnection = (HttpURLConnection) url.openConnection();
            httpURLConnection.setRequestMethod("POST");// 提交模式
            // 傳送POST請求必須設定如下兩行
            httpURLConnection.setDoOutput(true);
            httpURLConnection.setDoInput(true);
            // 獲取URLConnection物件對應的輸出流
            PrintWriter printWriter = new PrintWriter(httpURLConnection.getOutputStream());
            // 傳送請求引數
            JSONObject paramJson = new JSONObject();
            paramJson.put("scene", sceneStr);
            paramJson.put("page", page);
            printWriter.write(paramJson.toString());
            // flush輸出流的緩衝
            printWriter.flush();
            //開始獲取資料

            BufferedInputStream bis = new BufferedInputStream(httpURLConnection.getInputStream());
            try (InputStream is = httpURLConnection.getInputStream(); ByteArrayOutputStream baos = new ByteArrayOutputStream();){
                byte[] buffer = new byte[1024];
                int len = -1;
                while ((len = is.read(buffer)) != -1) {
                    baos.write(buffer, 0, len);
                }
                return Base64.getEncoder().encodeToString(baos.toByteArray());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (httpURLConnection != null){
                httpURLConnection.disconnect();
            }
        }
        return null;
    }

    /**
     * 模擬呼叫
     * @param args
     */
    public static void main(String[] args) {
        String accessToken = getWeixinAccessToken(APP_ID,APP_SECRET);
        String qrBase64  = getminiqrQr("1024",accessToken,PATH_HOME);
        System.out.println(qrBase64);
    }
 

寫在後面：
1.以上程式碼中HttpClientUtils用你自己專案中的請求工具類就好，無特殊要求；
2.以上生成程式碼均模擬正常情況下請求，未對一些異常進行處理，該部分自行處理；
3.正常業務中不會每次都呼叫微信介面獲取accessToken，一般是會放進快取，定時更新，具體根據各自業務進行修改。