Small fix

Author: emmyzhou-db

databricks-sdk-java/src/main/java/com/databricks/sdk/core/DefaultCredentialsProvider.java

@@ -16,7 +16,7 @@ public class DefaultCredentialsProvider implements CredentialsProvider {
   private static final Logger LOG = LoggerFactory.getLogger(DefaultCredentialsProvider.class);
 
   /* List of credential providers that will be tried in sequence */
-  private List<CredentialsProvider> providers;
+  private List<CredentialsProvider> providers = new ArrayList<>();
 
   /* The currently selected authentication type */
   private String authType = "default";
@@ -145,8 +145,11 @@ private void addOIDCCredentialsProviders(DatabricksConfig config) {
    *
    * @param config The Databricks configuration to use for provider initialization
    */
-  private void addDefaultCredentialsProviders(DatabricksConfig config) {
-    providers = new ArrayList<>();
+  private synchronized void addDefaultCredentialsProviders(DatabricksConfig config) {
+    if (!providers.isEmpty()) {
+      return;
+    }
+
     providers.add(new PatCredentialsProvider());
     providers.add(new BasicCredentialsProvider());
     providers.add(new OAuthM2MServicePrincipalCredentialsProvider());