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";// 生成小程序码地址
    private static final String URL_GET_TOKEN = "https://api.weixin.qq.com/cgi-bin/token";//获取access_token地址
    private static final String APP_ID = "";// 小程序appid
    private static final String APP_SECRET = ""; // 小程序秘钥

    public static final String PATH_HOME = "";
    private static final String BASE_PREFIX = "data:image/png;base64,"; // base64图片固定前缀

    /**
     * 获取微信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");
    }

    /**
     * 生成小程序码返回base64字符串
     * @param sceneStr 要携带的参数
     * @param accessToken 上面方法得到的token
     * @param page 要跳转的小程序页面（必须是已发布的）
     * @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
            // 发送POST请求必须设置如下两行
            httpURLConnection.setDoOutput(true);
            httpURLConnection.setDoInput(true);
            // 获取URLConnection对象对应的输出流
            PrintWriter printWriter = new PrintWriter(httpURLConnection.getOutputStream());
            // 发送请求参数
            JSONObject paramJson = new JSONObject();
            paramJson.put("scene", sceneStr);
            paramJson.put("page", page);
            printWriter.write(paramJson.toString());
            // flush输出流的缓冲
            printWriter.flush();
            //开始获取数据

            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 BASE_PREFIX+Base64.getEncoder().encodeToString(baos.toByteArray());
            }
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (httpURLConnection != null){
                httpURLConnection.disconnect();
            }
        }
        return null;
    }

    /**
     * 模拟调用
     * @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);
    }

写在后面：
1.以上代码中HttpClientUtils用你自己项目中的请求工具类就好，无特殊要求；
2.以上生成代码均模拟正常情况下请求，未对一些异常进行处理，该部分自行处理；
3.正常业务中不会每次都调用微信接口获取accessToken，一般是会放进缓存，定时更新，具体根据各自业务进行修改。