test(auth): verify ID token caching

Add explicit unit test coverage in IdTokenCredentialsTest to verify that IdTokenCredentials successfully caches fetched ID tokens and prevents subsequent network calls when the token is not expired.

Other language client libraries (such as Python and Go) verify that ID token caching behaves as expected, but the Java client library was missing explicit test coverage to assert that the token provider's retrieve method is called exactly once across multiple credential calls.

Author: westarle

google-auth-library-java/oauth2_http/javatests/com/google/auth/oauth2/IdTokenCredentialsTest.java

@@ -32,7 +32,14 @@
 package com.google.auth.oauth2;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.anyString;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
+import com.google.api.client.util.Clock;
 import java.io.IOException;
 import org.junit.jupiter.api.Test;
 
@@ -110,4 +117,29 @@ void serialize() throws IOException, ClassNotFoundException {
     IdTokenCredentials deserializedCredentials = serializeAndDeserialize(tokenCredential);
     assertEquals(tokenCredential, deserializedCredentials);
   }
+
+  @Test
+  void caching() throws IOException {
+    IdTokenProvider mockProvider = mock(IdTokenProvider.class);
+    IdToken idToken = IdToken.create(ComputeEngineCredentialsTest.STANDARD_ID_TOKEN);
+    when(mockProvider.idTokenWithAudience(anyString(), any())).thenReturn(idToken);
+
+    IdTokenCredentials credentials =
+        IdTokenCredentials.newBuilder()
+            .setIdTokenProvider(mockProvider)
+            .setTargetAudience("https://foo.bar")
+            .build();
+    credentials.clock =
+        new Clock() {
+          @Override
+          public long currentTimeMillis() {
+            return 1564471451000L; // 2019-07-30T08:24:11Z (STANDARD_ID_TOKEN iat)
+          }
+        };
+
+    credentials.refreshIfExpired();
+    credentials.refreshIfExpired();
+
+    verify(mockProvider, times(1)).idTokenWithAudience(anyString(), any());
+  }
 }