How to sign POST requests with OAuth 1.0 using oauth-signpost and HttpURLConnection

Situation

Signpost's documentation states that the library cannot be used along with HttpURLConnection to sign standard POST requests. This is not entirely true.

Method setAdditionalParameters() can be used to add a few parameters to the OAuth base string. The trick is to add them already percent-encoded.

Code sample

package test_oauth;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Iterator;
import java.util.Scanner;

import oauth.signpost.OAuth;
import oauth.signpost.OAuthConsumer;
import oauth.signpost.basic.DefaultOAuthConsumer;
import oauth.signpost.exception.OAuthCommunicationException;
import oauth.signpost.exception.OAuthExpectationFailedException;
import oauth.signpost.exception.OAuthMessageSignerException;
import oauth.signpost.http.HttpParameters;

public class TestOAuth {

	public static void main(String[] args) throws IOException, OAuthMessageSignerException, OAuthExpectationFailedException, OAuthCommunicationException {
		final String endpointUrl = "http://some-api.example.org/index.php/api/v1/end/point";

		//Create an HttpURLConnection and add some headers
		URL url = new URL(endpointUrl);
		HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
		urlConnection.setRequestProperty("Accept", "application/json");
		urlConnection.setRequestMethod("POST");
		urlConnection.setDoOutput(true);

		//Build the list of parameters
		HttpParameters params =  new HttpParameters();
		params.put("param1", "49607,49608,49609");
		params.put("param2", "1396430420");
		params.put("param3", "coucou");
		
		//Sign the request
		//The key to signing the POST fields is to add them as additional parameters, but already percent-encoded; and also to add the realm header.
		OAuthConsumer dealabsConsumer = new DefaultOAuthConsumer ("5314c6fbe7437", "5314c6fbe7bfd");
		HttpParameters doubleEncodedParams =  new HttpParameters();
        Iterator<String> iter = params.keySet().iterator();
        while (iter.hasNext()) {
            String key = iter.next();
            doubleEncodedParams.put(key, OAuth.percentEncode(params.getFirst(key)));
        }
        doubleEncodedParams.put("realm", endpointUrl);
		dealabsConsumer.setAdditionalParameters(doubleEncodedParams);
        dealabsConsumer.sign(urlConnection);
        
        //Create the POST payload
        StringBuilder sb = new StringBuilder();
        iter = params.keySet().iterator();
        for (int i = 0; iter.hasNext(); i++) {
            String param = iter.next();
            if (i > 0) {
                sb.append("&");
            }
            sb.append(params.getAsQueryString(param, false));  
        }
        
        //Send the payload to the connection
        String formEncoded = sb.toString();;
		OutputStreamWriter outputStreamWriter = null;
        try {
            outputStreamWriter = new OutputStreamWriter(urlConnection.getOutputStream(), "UTF-8");
            outputStreamWriter.write(formEncoded);
        } finally {
            if (outputStreamWriter != null) {
                outputStreamWriter.close();
            }
        }
        
        //Send the request and read the output
		try {
			System.out.println("Response: " + urlConnection.getResponseCode() + " " + urlConnection.getResponseMessage());
			InputStream in = new BufferedInputStream(urlConnection.getInputStream());
			String inputStreamString = new Scanner(in,"UTF-8").useDelimiter("\\A").next();
			System.out.println(inputStreamString);
		}
		finally {
			urlConnection.disconnect();
		}
	}

}

This code was tested on Java 7 on a PC, and on Android.