How to transport files between ROS and Android?

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I am an Android developer, and i am new on ROS. My goal is transporting files between ROS and Android. After some test code, i can transport a small PNG image from ROS to Android via ROSBridge. Here is the code:

ROS node code, this code is copy from image_transport/Tutorials/PublishingImages:

#include <ros/ros.h>
#include <image_transport/image_transport.h>
#include <opencv2/highgui/highgui.hpp>
#include <cv_bridge/cv_bridge.h>

int main(int argc, char** argv)
{
  ros::init(argc, argv, "image_publisher");
  ros::NodeHandle nh;
  image_transport::ImageTransport it(nh);
  image_transport::Publisher pub = it.advertise("camera/image", 1);
  cv::Mat image = cv::imread(argv[1], CV_LOAD_IMAGE_COLOR);
  cv::waitKey(30);
  sensor_msgs::ImagePtr msg = cv_bridge::CvImage(std_msgs::Header(), "bgr8", image).toImageMsg();

  ros::Rate loop_rate(5);
  while (nh.ok()) {
    pub.publish(msg);
    ros::spinOnce();
    loop_rate.sleep();
  }
}

Android code:

// Websocket was connected and the "/camera/image" topic was subscribed before.
public void onMessage(@NotNull WebSocket webSocket, @NotNull String text) {
    super.onMessage(webSocket, text);

    try {
        JSONObject jsonObj = new JSONObject(text);
        if ("/camera/image".equals(jsonObj.getString("topic"))) {
            decodeImage(jsonObj);
        }
    } catch (JSONException e) {
        e.printStackTrace();
        return;
    }
}

/**
 * Decoding image data from ROSBridge and save to SD card.
 */
private void decodeImage(JSONObject jsonObj) {
    try {
        JSONObject msgJsonObj = jsonObj.getJSONObject("msg");
        int width = msgJsonObj.getInt("width");
        int height = msgJsonObj.getInt("height");
        int step = msgJsonObj.getInt("step");
        String data = msgJsonObj.getString("data");
        byte[] bytes = Base64.decode(data, Base64.DEFAULT);
        Bitmap bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888);
        for (int y = 0; y < height; y++) {
            for (int x = 0; x < width; x++) {
                int bluePos = y * step + x * 3;
                byte blue = bytes[bluePos];
                byte green = bytes[bluePos + 1];
                byte red = bytes[bluePos + 2];
                bitmap.setPixel(x, y, Color.argb(255, red & 0xFF, green & 0xFF, blue & 0xFF));
            }
        }
        String filePath = getExternalCacheDir().getAbsolutePath() + File.separator + System.currentTimeMillis() + ".png";
        FileUtil.saveBitmapAsPng(filePath, bitmap);
    } catch (JSONException e) {
        e.printStackTrace();
    }
}

The code works fine when the ROS node publish a small PNG image. But when i try to transport a large PNG image, such as a 6816x4480 1.4MB PNG file, the Android websocket occurs OOM problem.
Is there any better ways to transport large files between ROS and Android?

Thanks in advance!