Mihai Todor Mihai Todor - 5 months ago 54
Android Question

Sending files using POST with HttpURLConnection

Since the Android developers recommend to use the

HttpURLConnection
class, I was wondering if anyone can provide me with a good example on how to send a bitmap "file" (actually an in-memory stream) via POST to an Apache HTTP server. I'm not interested in cookies or authentication or anything complicated, but I just want to have a reliable and logic implementation. All the examples that I've seen around here look more like "let's try this and maybe it works".

Right now, I have this code:

URL url;
HttpURLConnection urlConnection = null;
try {
url = new URL("http://example.com/server.cgi");

urlConnection = (HttpURLConnection) url.openConnection();

} catch (Exception e) {
this.showDialog(getApplicationContext(), e.getMessage());
}
finally {
if (urlConnection != null)
{
urlConnection.disconnect();
}
}


where showDialog should just display an
AlertDialog
(in case of an invalid URL?).

Now, let's say that I generate a bitmap like so:
Bitmap image = this.getBitmap()
inside a control derived from
View
and I want to send it via POST. What would be the proper procedure to achieve such a thing? What classes do I need to use? Can I use
HttpPost
like in this example? If so, how would I construct the
InputStreamEntity
for my bitmap? I would find it revolting to be required to first store the bitmap in a file on the device.




I should also mention that I really need to send every unaltered pixel of the original bitmap to the server, so I can't convert it to JPEG.

Answer

I have no idea why the HttpURLConnection class does not provide any means to send files without having to compose the file wrapper manually. Here's what I ended up doing, but if someone knows a better solution, please let me know.

Input data:

Bitmap bitmap = myView.getBitmap();

Static stuff:

String attachmentName = "bitmap";
String attachmentFileName = "bitmap.bmp";
String crlf = "\r\n";
String twoHyphens = "--";
String boundary =  "*****";

Setup the request:

HttpURLConnection httpUrlConnection = null;
URL url = new URL("http://example.com/server.cgi");
httpUrlConnection = (HttpURLConnection) url.openConnection();
httpUrlConnection.setUseCaches(false);
httpUrlConnection.setDoOutput(true);

httpUrlConnection.setRequestMethod("POST");
httpUrlConnection.setRequestProperty("Connection", "Keep-Alive");
httpUrlConnection.setRequestProperty("Cache-Control", "no-cache");
httpUrlConnection.setRequestProperty(
    "Content-Type", "multipart/form-data;boundary=" + this.boundary);

Start content wrapper:

DataOutputStream request = new DataOutputStream(
    httpUrlConnection.getOutputStream());

request.writeBytes(this.twoHyphens + this.boundary + this.crlf);
request.writeBytes("Content-Disposition: form-data; name=\"" +
    this.attachmentName + "\";filename=\"" + 
    this.attachmentFileName + "\"" + this.crlf);
request.writeBytes(this.crlf);

Convert Bitmap to ByteBuffer:

//I want to send only 8 bit black & white bitmaps
byte[] pixels = new byte[bitmap.getWidth() * bitmap.getHeight()];
for (int i = 0; i < bitmap.getWidth(); ++i) {
    for (int j = 0; j < bitmap.getHeight(); ++j) {
        //we're interested only in the MSB of the first byte, 
        //since the other 3 bytes are identical for B&W images
        pixels[i + j] = (byte) ((bitmap.getPixel(i, j) & 0x80) >> 7);
    }
}

request.write(pixels);

End content wrapper:

request.writeBytes(this.crlf);
request.writeBytes(this.twoHyphens + this.boundary + 
    this.twoHyphens + this.crlf);

Flush output buffer:

request.flush();
request.close();

Get response:

InputStream responseStream = new 
    BufferedInputStream(httpUrlConnection.getInputStream());

BufferedReader responseStreamReader = 
    new BufferedReader(new InputStreamReader(responseStream));

String line = "";
StringBuilder stringBuilder = new StringBuilder();

while ((line = responseStreamReader.readLine()) != null) {
    stringBuilder.append(line).append("\n");
}
responseStreamReader.close();

String response = stringBuilder.toString();

Close response stream:

responseStream.close();

Close the connection:

httpUrlConnection.disconnect();

PS: Of course I had to wrap the request in private class AsyncUploadBitmaps extends AsyncTask<Bitmap, Void, String>, in order to make the Android platform happy, because it doesn't like to have network requests on the main thread.