Java: Connect to a Rest API with OAuth2 throwing SSLHandshakeException: PKIX path building failed

Question

I am trying to connect to a Rest API, and the code is as below,

    public class TestingDevelop {
    
        public static String fetchAccessToken(){
            HttpClient httpclient = new DefaultHttpClient();
            String client_id ="clientID";
            String client_secret ="clientSecret";
            String grant_type="client_credentials";
            String accessTokenUrl = "tokenURL";
            String scope = "scope";
            HttpPost post = new HttpPost(accessTokenUrl);
            List<NameValuePair> params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("client_id",client_id ));
            params.add(new BasicNameValuePair("client_secret", client_secret));
            params.add(new BasicNameValuePair("grant_type", grant_type));
            params.add(new BasicNameValuePair("scope", scope));
            post.setEntity(new UrlEncodedFormEntity(params));
            System.out.println(post);
            HttpResponse response = httpclient.execute(post);
            String body = EntityUtils.toString(response.getEntity());
            //the response is a json response and I am fetching the token from this.    
            return token;
        }
    
    }

I have used the generated token in postman and have checked, it works fine as expected.

Now, when I try to call the GET api, it throws

javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Get API code:

private static void useBearerToken(String bearerToken) {
            BufferedReader reader = null;
            try {
                
                String restApiUrl ="https://api.byu.edu:443/echo/v1/status?test=testing";
                URL url = new URL(restApiUrl);
                HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
                connection.setRequestProperty("Authorization", "Bearer " + bearerToken);
                connection.setDoOutput(true);
                connection.setRequestMethod("GET");
                reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String line = null;
                StringWriter out = new StringWriter(connection.getContentLength() > 0 ? connection.getContentLength() : 2048);
                while ((line = reader.readLine()) != null) {
                    out.append(line);
                }
                String response = out.toString();
                System.out.println(response);
            } catch (Exception e) {

            }
    
    }

When, I look at the configuration at the postman side, the SSL is disabled.

I have seen similar question being asked but it did not work for me. Here are some of the links which I have gone through:

1. Resolving javax.net.ssl.SSLHandshakeException: sun.security.validator.ValidatorException: PKIX path building failed Error?
2. Tried disabling the cert validation: SSLHandshakeException: ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException:
3. PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException java

Updates

Postman console log:

tls: {…}
reused: false
authorized: false
authorizationError: "UNABLE_TO_VERIFY_LEAF_SIGNATURE"

Setting self signed certificate:

keytool -genkey -keyalg RSA -alias selfsigned -keystore keystore.jks -storepass password -validity 360 -keysize 2048

setting system properties:

System.setProperty("javax.net.ssl.trustStore", "keystore.jks");
System.setProperty("javax.net.ssl.trustStorePassword", "password");

and setting SSLContext

 SSLContext.setDefault(
 createSSLContext("/keystore.jks", "password"));
 

// call fetchAccessToken and useBearerToken()

The call fails before going to fetchAccessToken method.