(Java) OAuth2 Token using IdentityServer4 with Client Credentials

Demonstrates how to get an OAuth2 access token using the client credential flow with IdentityServer4.

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import com.chilkatsoft.*;

public class ChilkatExample {

  static {
    try {
        System.loadLibrary("chilkat");
    } catch (UnsatisfiedLinkError e) {
      System.err.println("Native code library failed to load.\n" + e);
      System.exit(1);
    }
  }

  public static void main(String argv[])
  {
    // This example assumes the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    CkHttp http = new CkHttp();

    // The first step is to fetch your IdentityServer4's discovery document
    // (OpenID Connect defines a discovery mechanism, called OpenID Connect Discovery, where an OpenID server publishes its metadata at a well-known URL, 
    // typically https://server.com/.well-known/openid-configuration

    CkHttpResponse resp = http.QuickRequest("GET","https://localhost:5000/.well-known/openid-configuration");
    if (http.get_LastMethodSuccess() != true) {
        System.out.println(http.lastErrorText());
        return;
        }

    if (resp.get_StatusCode() != 200) {
        System.out.println("Received response status code " + resp.get_StatusCode());
        System.out.println("Response body containing error text or JSON:");
        System.out.println(resp.bodyStr());

        return;
        }

    CkJsonObject json = new CkJsonObject();
    boolean success = json.Load(resp.bodyStr());

    // We have the discovery document, which contains something like this:

    // You can use this online tool to generate parsing code from sample JSON: 
    // Generate Parsing Code from JSON

    // {
    //   "issuer": "https://localhost:5000",
    //   "jwks_uri": "https://localhost:5000/.well-known/openid-configuration/jwks",
    //   "authorization_endpoint": "https://localhost:5000/connect/authorize",
    //   "token_endpoint": "https://localhost:5000/connect/token",
    //   "userinfo_endpoint": "https://localhost:5000/connect/userinfo",
    //   "end_session_endpoint": "https://localhost:5000/connect/endsession",
    //   "check_session_iframe": "https://localhost:5000/connect/checksession",
    //   "revocation_endpoint": "https://localhost:5000/connect/revocation",
    //   "introspection_endpoint": "https://localhost:5000/connect/introspect",
    //   "frontchannel_logout_supported": true,
    //   "frontchannel_logout_session_supported": true,
    //   "backchannel_logout_supported": true,
    //   "backchannel_logout_session_supported": true,
    //   "scopes_supported": [
    //     "openid",
    //     "profile",
    //     "email",
    //     "MyCompany.profile",
    //     "MyCompany.Identity.WebApi",
    //     "MyCompany.TriHub.WebApi",
    //     "offline_access"
    //   ],
    //   "claims_supported": [
    //     "sub",
    //     "updated_at",
    //     "locale",
    //     "zoneinfo",
    //     "birthdate",
    //     "gender",
    //     "website",
    //     "profile",
    //     "preferred_username",
    //     "nickname",
    //     "middle_name",
    //     "given_name",
    //     "family_name",
    //     "name",
    //     "picture",
    //     "email_verified",
    //     "email",
    //     "userId",
    //     "groups",
    //     "fullname"
    //   ],
    //   "grant_types_supported": [
    //     "authorization_code",
    //     "client_credentials",
    //     "refresh_token",
    //     "implicit",
    //     "password"
    //   ],
    //   "response_types_supported": [
    //     "code",
    //     "token",
    //     "id_token",
    //     "id_token token",
    //     "code id_token",
    //     "code token",
    //     "code id_token token"
    //   ],
    //   "response_modes_supported": [
    //     "form_post",
    //     "query",
    //     "fragment"
    //   ],
    //   "token_endpoint_auth_methods_supported": [
    //     "client_secret_basic",
    //     "client_secret_post"
    //   ],
    //   "subject_types_supported": [
    //     "public"
    //   ],
    //   "id_token_signing_alg_values_supported": [
    //     "RS256"
    //   ],
    //   "code_challenge_methods_supported": [
    //     "plain",
    //     "S256"
    //   ]
    // }
    // 

    // The next steps are to (1) get the token_endpoint,
    // and (2) verify that the client_credentials grant type is supported.

    String tokenEndpoint = json.stringOf("token_endpoint");

    CkJsonArray grantTypes = json.ArrayOf("grant_types_supported");
    int clientCredentialsIdx = grantTypes.FindString("client_credentials",true);

    // If clientCredentialsIdx is less then zero (-1) then the "client_credentials" string was not found.
    if (clientCredentialsIdx < 0) {
        System.out.println("The client credentials grant type is not supported.");
        return;
        }

    // Request the access token using our Client ID and Client Secret.
    // We're going to duplicate this CURL statement:

    // curl --request POST \
    //   --url '<tokenEndpoint>' \
    //   --header 'content-type: application/x-www-form-urlencoded' \
    //   --data 'grant_type=client_credentials&client_id=CLIENT_ID&client_secret=CLIENT_SECRET'

    CkHttpRequest req = new CkHttpRequest();
    req.put_HttpVerb("POST");
    req.AddParam("grant_type","client_credentials");
    req.AddParam("client_id","CLIENT_ID");
    req.AddParam("client_secret","CLIENT_SECRET");

    resp = http.PostUrlEncoded(tokenEndpoint,req);
    if (http.get_LastMethodSuccess() == false) {
        System.out.println(http.lastErrorText());
        return;
        }

    // Make sure we got a 200 response status code, otherwise it's an error.
    if (resp.get_StatusCode() != 200) {
        System.out.println("POST to token endpoint failed.");
        System.out.println("Received response status code " + resp.get_StatusCode());
        System.out.println("Response body containing error text or JSON:");
        System.out.println(resp.bodyStr());

        return;
        }

    success = json.Load(resp.bodyStr());

    // Our JSON response should contain this:
    // {
    //   "access_token":"eyJz93a...k4laUWw",
    //   "token_type":"Bearer",
    //   "expires_in":86400
    // }

    // Get the access token:
    String accessToken = json.stringOf("access_token");

    // The access token is what gets added to "Authorization: Bearer <access_token>"
    // for the subsequent REST API calls..
  }
}