How to get Access Token for OAuth 2.0 using client_credentials using java code?

[avidCoder]

I have some API which requires access token to get the response. In postman we use OAuth 2.0 to get the access token by providing client username and password. In similar way, I want to fetch the new access token.

Here is the sample code which I have tried so far.

import java.io.*;
import java.net.URL;
import java.net.URLEncoder;
import java.util.Map;
import java.lang.reflect.Type;
import javax.net.ssl.HttpsURLConnection;

// Google Gson Libraries used for Json Parsing
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class AuthGoogle {

    /**
     * @param args
     * @throws IOException 
     */
    public static void main(String[] args) throws IOException {
        // TODO Auto-generated method stub
         String grantType = "client_credentials";
            String applicationID = "application";
            String username = "username";
            String password = "password";
            String url = "url_link";
            HttpsURLConnection httpConn = null;
            BufferedReader in = null;

            try {

                // Create the data to send
                StringBuilder data = new StringBuilder();
                data.append("grant_type=" + URLEncoder.encode(grantType, "UTF-8"));
                data.append("&client_id=" + URLEncoder.encode(applicationID, "UTF-8"));
                data.append("&username=" + URLEncoder.encode(username, "UTF-8"));
                data.append("&password=" + URLEncoder.encode(password, "UTF-8"));

                // Create a byte array of the data to be sent
                byte[] byteArray = data.toString().getBytes("UTF-8");

                // Setup the Request
                URL request = new URL(null, url,  new sun.net.www.protocol.https.Handler());
                httpConn = (HttpsURLConnection)request.openConnection();
                httpConn.setRequestMethod("POST");
                httpConn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
                httpConn.setRequestProperty("Content-Length", "" + byteArray.length);
                httpConn.setDoOutput(true);

                // Write data
                OutputStream postStream = httpConn.getOutputStream();
                postStream.write(byteArray, 0, byteArray.length);
                postStream.close();

                // Send Request & Get Response
                InputStreamReader reader = new InputStreamReader(httpConn.getInputStream());
                in = new BufferedReader(reader);

                // Get the Json reponse containing the Access Token
                String json = in.readLine();
                System.out.println("Json String = " + json);

                // Parse the Json response and retrieve the Access Token
                Gson gson = new Gson();
                Type mapType  = new TypeToken<Map<String,String>>(){}.getType();
                Map<String,String> ser = gson.fromJson(json, mapType);
                String accessToken = ser.get("access_token");
                System.out.println("Access Token = " + accessToken);

            } catch (java.io.IOException e) {

                // This exception will be raised if the server didn't return 200 - OK
                // Retrieve more information about the error
                System.out.println(e.getMessage());

            } finally {

                // Be sure to close out any resources or connections
                if (in != null) in.close();
                if (httpConn != null) httpConn.disconnect();
            }
        }

}

I am getting output as Connection refused: connect.

Another code I have tried is:-

import org.apache.oltu.oauth2.client.OAuthClient;
import org.apache.oltu.oauth2.client.URLConnectionClient;
import org.apache.oltu.oauth2.client.request.OAuthClientRequest;
import org.apache.oltu.oauth2.client.response.OAuthJSONAccessTokenResponse;
import org.apache.oltu.oauth2.common.OAuth;
import org.apache.oltu.oauth2.common.message.types.GrantType;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import org.apache.commons.codec.binary.Base64;

public class OltuJavaClient {

    public static final String TOKEN_REQUEST_URL = "url_link";
    public static final String CLIENT_ID = "client_id";
    public static final String CLIENT_SECRET = "client_pass";

    public static void main(String[] args) {
        try {
            OAuthClient client = new OAuthClient(new URLConnectionClient());

            OAuthClientRequest request =
                    OAuthClientRequest.tokenLocation(TOKEN_REQUEST_URL)
                    .setGrantType(GrantType.CLIENT_CREDENTIALS)
                    .setClientId(CLIENT_ID)
                    .setClientSecret(CLIENT_SECRET)
                    // .setScope() here if you want to set the token scope
                    .buildQueryMessage();
            request.addHeader("Accept", "application/json");
            request.addHeader("Content-Type", "application/json");
            request.addHeader("Authorization", base64EncodedBasicAuthentication());

            String token = client.accessToken(request, OAuth.HttpMethod.POST, OAuthJSONAccessTokenResponse.class).getAccessToken();
            System.out.println(token.toString());

        } catch (Exception exn) {
            exn.printStackTrace();
        }
    }

    private static String base64EncodedBasicAuthentication() {
        // TODO Auto-generated method stub
        return null;
    }
}

Here I am getting this error:- OAuthProblemException{error='unsupported_response_type', description='Invalid response! Response body is not application/json encoded', uri='null', state='null', scope='null', redirectUri='null', responseStatus=0, parameters={}}

Can we do this way? Any leads would be appreciated.

[avidCoder]

Use java_1.8.0_181 version.