suggestions from gemini review + lint fixes

Author: ldetmer

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/MutableCredentials.java

@@ -26,8 +26,8 @@
  * A mutable {@link Credentials} implementation that delegates authentication behavior to a scoped
  * {@link ServiceAccountCredentials} instance.
  *
- * <p>This class is intended for scenarios where an application needs to rotate or replace the
- * underlying service account credentials for a running Spanner Client.
+ * <p>This class is intended for scenarios where an application needs to replace the underlying
+ * service account credentials for a long running Spanner Client.
  *
  * <p>All operations inherited from {@link Credentials} are forwarded to the current delegate,
  * including request metadata retrieval and token refresh. Calling {@link
@@ -36,8 +36,8 @@
  * constructed.
  */
 public class MutableCredentials extends Credentials {
-  ServiceAccountCredentials delegate;
-  List<String> scopes;
+  private volatile ServiceAccountCredentials delegate;
+  private final List<String> scopes;
 
 
   public MutableCredentials(ServiceAccountCredentials credentials, List<String> scopes) {

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/ITMutableCredentialsTest.java

@@ -92,3 +92,4 @@ public void testMutableCredentialsUpdateAuthorizationForRunningClient() throws I
     }
   }
 }
+

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/MutableCredentialsTest.java

@@ -52,9 +52,8 @@ public class MutableCredentialsTest {
   String updatedAuthType = "auth-2";
 
   @Test
-  public void testCreateMutableCredentialsAndUpdate() throws IOException {
+  public void testCreateMutableCredentials() throws IOException {
     setupInitialCredentials();
-    setupUpdatedCredentials();
 
     MutableCredentials credentials = new MutableCredentials(initialCredentials, scopes);
 
@@ -68,6 +67,16 @@ public void testCreateMutableCredentialsAndUpdate() throws IOException {
     credentials.refresh();
 
     verify(initialScopedCredentials, times(1)).refresh();
+  }
+
+  @Test
+  public void testUpdateMutableCredentials() throws IOException {
+    setupInitialCredentials();
+    setupUpdatedCredentials();
+
+    MutableCredentials credentials = new MutableCredentials(initialCredentials, scopes);
+
+    assertEquals(initialAuthType, credentials.getAuthenticationType());
 
     credentials.updateCredentials(updatedCredentials);
 
@@ -97,3 +106,4 @@ private void setupUpdatedCredentials() throws IOException {
     when(updatedScopedCredentials.hasRequestMetadataOnly()).thenReturn(false);
   }
 }
+