Oauth2 token call failing with "request is missing a required parameter"

I'm trying to connect to the REST API using application/x-www-form-urlencoded,
using the following parameters:

 private String tokenEndpoint = "https://xxxxxxxxxxxxx/oauth2/token";
	private String client_id = "2xxxxxxxxxxxxxxxxx";
	private String client_secret = "xxxxxxxxxxxxxxxxxxxx";
	private String grant_type = "client_credentials";
	private String scope = "projects:read";

and finally get the following error:

>{"error":"invalid_request","error_description":"The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed. Client credentials missing or malformed in both HTTP Authorization header and HTTP POST body."}

My code is:

package testRestAPI;

import java.io.IOException;
import java.io.UnsupportedEncodingException;
import java.security.cert.X509Certificate;
import java.util.Base64;
import javax.net.ssl.SSLContext;
import org.apache.commons.io.IOUtils;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.ssl.TrustStrategy;


public class OAuthClient {

	private String tokenEndpoint = "https://xxxxxxxxxxxxx/oauth2/token";
	private String client_id = "2xxxxxxxxxxxxxxxxx";
	private String client_secret = "xxxxxxxxxxxxxxxxxxxx";
	private String grant_type = "client_credentials";
	private String scope = "projects:read";
	

	public String callService()  {


		String result = null;

		try {

                        //My site is set as insecure, so I set the following setting
			TrustStrategy acceptingTrustStrategy = (X509Certificate[] chain, String authType) -> true;

			SSLContext sslContext = org.apache.http.ssl.SSLContexts.custom()
					.loadTrustMaterial(null, acceptingTrustStrategy)
					.build();

			SSLConnectionSocketFactory csf = new SSLConnectionSocketFactory(sslContext);

			CloseableHttpClient httpClient = HttpClients.custom()
					.setSSLSocketFactory(csf)
					.build();


			/* HTTPCLIENT AND HTTPPOST OOBJECT */
			//		HttpClient httpClient = HttpClientBuilder.create().build();
			HttpPost httpPost = new HttpPost(tokenEndpoint);

			/* AUTHENTICATION CREDENTIALS ENCODING */
			String base64Credentials = Base64.getEncoder().encodeToString((client_id + ":" 
					+ client_secret).getBytes());

			/* HEADER INFO */
			httpPost.addHeader("Authorization","Bearer " +  base64Credentials);
			httpPost.addHeader("Content-Type", "application/x-www-form-urlencoded");

			//		/* PROXY CONFIG */
			//				HttpHost target = new HttpHost("proxy", 8080, "http");
			//				RequestConfig config = RequestConfig.custom().setProxy(target).build();
			//				httpPost.setConfig(config);

			/* OAUTH PARAMETERS ADDED TO BODY */
			StringEntity input = null;
			try {
				input = new StringEntity("grant_type=" + grant_type + "&scope=" + scope);
				httpPost.setEntity(input);
			} catch (UnsupportedEncodingException e) {
				e.printStackTrace();
			}

			/* SEND AND RETRIEVE RESPONSE */
			HttpResponse response = null;
			try {
				response = httpClient.execute(httpPost);
			} catch (IOException e) {
				e.printStackTrace();
			}

			/* RESPONSE AS STRING */
			//		String result = null;
			try {
				result = IOUtils.toString(response.getEntity().getContent(), "UTF-8");
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
		catch (Exception e) {
			// TODO: handle exception
		}
		return result;
	}

	public static void main(String[] args) {
		OAuthClient oauthClient = new OAuthClient();
		String res = oauthClient.callService();
		System.out.println(res);
	}
}

and finally get the following error:

> {"error":"invalid_request","error_description":"The request is missing a required parameter, includes an invalid parameter value, includes a parameter more than once, or is otherwise malformed. Client credentials missing or malformed in both HTTP Authorization header and HTTP POST body."}

In Postman the connection is made successfully.Connection with Postman

link to Oauth provider Api Doc

There are a couple of issues with your code.

Primarily, you are not including the client_id and client_secret in the form. I observed with Fiddlr that Postman sends all these fields on a submitted form.

Other changes to improve, but probably not required to fix the code are to:

• Remove the nonsense Authorization Bearer header. Postman does not
  send and doesn't make sense to send one until a token has been
  received.

• Replaced the home-made form emulation and replaced with the standard
  UrlEncodedFormEntity passing a List<NameValuePair>. It adds the "Content-Type", "application/x-www-form-urlencoded" header to the request.

Try this:

import java.io.IOException;
import java.security.cert.X509Certificate;
import java.util.ArrayList;
import java.util.List;
import javax.net.ssl.SSLContext;
import org.apache.commons.io.IOUtils;
import org.apache.http.Consts;
import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.ssl.TrustStrategy;
public class OAuthClient {
private String tokenEndpoint = "https://xxxxxxxxxxxxx/oauth2/token";
private String client_id = "2xxxxxxxxxxxxxxxxx";
private String client_secret = "xxxxxxxxxxxxxxxxxxxx";
private String grant_type = "client_credentials";
private String scope = "projects:read";
public String callService()  {
String result = null;
try {
//My site is set as insecure, so I set the following setting
TrustStrategy acceptingTrustStrategy = (X509Certificate[] chain, String authType) -> true;
SSLContext sslContext = org.apache.http.ssl.SSLContexts.custom()
.loadTrustMaterial(null, acceptingTrustStrategy)
.build();
SSLConnectionSocketFactory csf = new SSLConnectionSocketFactory(sslContext);
CloseableHttpClient httpClient = HttpClients.custom()
.setSSLSocketFactory(csf)
.build();
/* HTTPCLIENT AND HTTPPOST OOBJECT */
HttpPost httpPost = new HttpPost(tokenEndpoint);
/* OAUTH PARAMETERS ADDED TO FORM */
List<NameValuePair> form = new ArrayList<>();
form.add(new BasicNameValuePair("grant_type", grant_type));
form.add(new BasicNameValuePair("scope", scope));
form.add(new BasicNameValuePair("client_id", client_id)); // these two were missing
form.add(new BasicNameValuePair("client_secret", client_secret));
// UrlEncodedFormEntity is the standard way of building a firm submission
// and it removes the need to manually add the "Content-Type", "application/x-www-form-urlencoded" header
UrlEncodedFormEntity entity = new UrlEncodedFormEntity(form, Consts.UTF_8);
httpPost.setEntity(entity);
/* SEND AND RETRIEVE RESPONSE */
HttpResponse response = null;
try {
response = httpClient.execute(httpPost);
} catch (IOException e) {
e.printStackTrace();
}
/* RESPONSE AS STRING */
//      String result = null;
try {
result = IOUtils.toString(response.getEntity().getContent(), "UTF-8");
} catch (IOException e) {
e.printStackTrace();
}
}
catch (Exception e) {
// TODO: handle exception
}
return result;
}
public static void main(String[] args) {
OAuthClient oauthClient = new OAuthClient();
String res = oauthClient.callService();
System.out.println(res);
}
}