Update to use better error message when URI scheme is null in endpointOverride

Author: zoewangg

core/sdk-core/src/main/java/software/amazon/awssdk/core/client/builder/SdkDefaultClientBuilder.java

@@ -284,7 +284,8 @@ private Executor resolveAsyncFutureCompletionExecutor(SdkClientConfiguration con
     }
 
     /**
-     * Finalize which scheduled executor service will be used for retries in the created client.
+     * Finalize the internal SDK scheduled executor service that is used for scheduling tasks such
+     * as async retry attempts and timeout task.
      */
     private ScheduledExecutorService resolveScheduledExecutorService() {
         return Executors.newScheduledThreadPool(5, new ThreadFactoryBuilder()
@@ -301,6 +302,8 @@ private List<ExecutionInterceptor> resolveExecutionInterceptors(SdkClientConfigu
 
     @Override
     public final B endpointOverride(URI endpointOverride) {
+        Validate.paramNotNull(endpointOverride, "endpointOverride");
+        Validate.paramNotNull(endpointOverride.getScheme(), "The URI scheme of endpointOverride");
         clientConfiguration.option(SdkClientOption.ENDPOINT, endpointOverride);
         return thisBuilder();
     }

core/sdk-core/src/test/java/software/amazon/awssdk/core/client/builder/DefaultClientBuilderTest.java

@@ -16,6 +16,7 @@
 package software.amazon.awssdk.core.client.builder;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.Matchers.any;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
@@ -90,6 +91,13 @@ public void buildWithEndpointShouldHaveCorrectEndpointAndSigningRegion() {
         assertThat(client.clientConfiguration.option(SdkClientOption.ENDPOINT)).isEqualTo(ENDPOINT);
     }
 
+    @Test
+    public void buildWithEndpointWithoutScheme_shouldThrowException() {
+        assertThatThrownBy(() -> testClientBuilder().endpointOverride(URI.create("localhost")).build())
+            .hasMessageContaining("The URI scheme of endpointOverride must not be null");
+
+    }
+
     @Test
     public void noClientProvided_DefaultHttpClientIsManagedBySdk() {
         TestClient client = testClientBuilder().build();