feedback: add test for universedomain exception with mock.

Author: zhumin8

oauth2_http/javatests/com/google/auth/oauth2/ImpersonatedCredentialsTest.java

@@ -40,6 +40,8 @@
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
 
 import com.google.api.client.http.HttpStatusCodes;
 import com.google.api.client.json.GenericJson;
@@ -940,6 +942,32 @@ public void sign_sameAs_nonGDU() {
     assertArrayEquals(expectedSignature, targetCredentials.sign(expectedSignature));
   }
 
+  @Test
+  public void sign_universeDomainException() throws IOException {
+    // Currently, no credentials allowed as source credentials throws exception for
+    // getUniverseDomain(), mock this behavior for test only. ServiceAccountCredentials
+    // should not throw for getUniverseDomain() calls.
+    ServiceAccountCredentials sourceCredentialsMock = mock(ServiceAccountCredentials.class);
+    when(sourceCredentialsMock.getUniverseDomain()).thenThrow(IOException.class);
+
+    MockIAMCredentialsServiceTransportFactory transportFactory =
+        new MockIAMCredentialsServiceTransportFactory();
+    ImpersonatedCredentials targetCredentials =
+        ImpersonatedCredentials.create(
+            sourceCredentialsMock,
+            IMPERSONATED_CLIENT_EMAIL,
+            null,
+            IMMUTABLE_SCOPES_LIST,
+            VALID_LIFETIME,
+            transportFactory);
+
+    byte[] expectedSignature = {0xD, 0xE, 0xA, 0xD};
+
+    IllegalStateException exception =
+        assertThrows(IllegalStateException.class, () -> targetCredentials.sign(expectedSignature));
+    assertEquals("Failed to sign: Error obtaining universe domain", exception.getMessage());
+  }
+
   @Test
   public void idTokenWithAudience_sameAs() throws IOException {
     mockTransportFactory.getTransport().setTargetPrincipal(IMPERSONATED_CLIENT_EMAIL);