Android Development: Sending POST Requests with Parameters

After I had published a tutorial about how to request RESTful web services in a native Android App, I have been asked by some of you about how to also send data to web services. That is why I wanted to quickly demonstrate how you would achieve sending POST requests containing parameters to a web server.

Sending POST Requests

Though it is actually not required by any specification, RESTful web services usually expect a POST request when data shall be updated on the server. In order to tell our connection to submit a POST request, we need to add few lines to the method which is opening the HTTP connection:

URL urlToRequest = new URL(urlStr);
HttpURLConnection urlConnection = 
	(HttpURLConnection) urlToRequest.openConnection();
urlConnection.setDoOutput(true);
urlConnection.setRequestMethod("POST");
urlConnection.setRequestProperty("Content-Type", 
	"application/x-www-form-urlencoded");

Adding Request Parameters

Now we need to write our parameters, if any, to the request output stream.

urlConnection.setFixedLengthStreamingMode(
	postParameters.getBytes().length);
PrintWriter out = new PrintWriter(urlConnection.getOutputStream());
out.print(postParameters);
out.close();

Whereas postParameters is the query string, such as “param1=value1&param2=value2”.

Create Query Strings

For the sake of a nicer API, I always offer a little helper method which creates correctly encoded query strings.

private static final char PARAMETER_DELIMITER = '&';
private static final char PARAMETER_EQUALS_CHAR = '=';
public static Sring createQueryStringForParameters(Map<String, String> parameters) {
	StringBuilder parametersAsQueryString = new StringBuilder();
	if (parameters != null) {
		boolean firstParameter = true;
		
		for (String parameterName : parameters.keySet()) {
			if (!firstParameter) {
				parametersAsQueryString.append(PARAMETER_DELIMITER);
			} 
			
			parametersAsQueryString.append(parameterName)
				.append(PARAMETER_EQUALS_CHAR)
				.append(URLEncoder.encode(
					parameters.get(parameterName)));
			
			firstParameter = false;
		}
	}
	return parametersAsQueryString;
}

GET and POST combined

Combining the snippets, we can adapt the code from the previous tutorial in a way that the method can send both GET and POST requests.

public static String requestUrl(String url, String postParameters) 
	throws YourServiceRequestException {
	if (Log.isLoggable(LOGGER_TAG, Log.INFO)) {
		Log.i(LOGGER_TAG, "Requesting service: " + url);
	}
	
	disableConnectionReuseIfNecessary();

	HttpURLConnection urlConnection = null;
	try {
		// create connection
		URL urlToRequest = new URL(url);
		urlConnection = (HttpURLConnection) urlToRequest.openConnection();
		urlConnection.setConnectTimeout(CONNECTION_TIMEOUT);
		urlConnection.setReadTimeout(DATARETRIEVAL_TIMEOUT);
		
		// handle POST parameters
		if (postParameters != null) {
			
			if (Log.isLoggable(LOGGER_TAG, Log.INFO)) {
				Log.i(LOGGER_TAG, "POST parameters: " + postParameters);
			}
			
			urlConnection.setDoOutput(true);
			urlConnection.setRequestMethod("POST");
			urlConnection.setFixedLengthStreamingMode(
				postParameters.getBytes().length);
			urlConnection.setRequestProperty("Content-Type", 
				"application/x-www-form-urlencoded");
			
			//send the POST out
			PrintWriter out = new PrintWriter(urlConnection.getOutputStream());
			out.print(postParameters);
			out.close();
		}
		
		// handle issues
		int statusCode = urlConnection.getResponseCode();
		if (statusCode != HttpURLConnection.HTTP_OK) {
			// throw some exception
		}
		
		// read output (only for GET)
		if (postParameters != null) {
			return null;
		} else {
			InputStream in = 
				new BufferedInputStream(urlConnection.getInputStream());
			return getResponseText(in);
		}
		
		
	} catch (MalformedURLException e) {
		// handle invalid URL
	} catch (SocketTimeoutException e) {
		// hadle timeout	
	} catch (IOException e) {
		// handle I/0
	} finally {
		if (urlConnection != null) {
			urlConnection.disconnect();
		}
	}
	
	return null;
}