If a GraphQLErrorHandler is present, do not override with ExceptionHa…

graphql-kickstart-spring-support/src/main/java/graphql/kickstart/spring/error/GraphQLErrorStartupListener.java

@@ -18,9 +18,11 @@ public GraphQLErrorStartupListener(ErrorHandlerSupplier errorHandlerSupplier, bo
 
   @Override
   public void onApplicationEvent(@NonNull ApplicationReadyEvent event) {
-    ConfigurableApplicationContext context = event.getApplicationContext();
-    GraphQLErrorHandler errorHandler = new GraphQLErrorHandlerFactory().create(context, exceptionHandlersEnabled);
-    context.getBeanFactory().registerSingleton(errorHandler.getClass().getCanonicalName(), errorHandler);
-    errorHandlerSupplier.setErrorHandler(errorHandler);
+    if (!errorHandlerSupplier.isPresent()) {
+      ConfigurableApplicationContext context = event.getApplicationContext();
+      GraphQLErrorHandler errorHandler = new GraphQLErrorHandlerFactory().create(context, exceptionHandlersEnabled);
+      context.getBeanFactory().registerSingleton(errorHandler.getClass().getCanonicalName(), errorHandler);
+      errorHandlerSupplier.setErrorHandler(errorHandler);
+    }
   }
 }

graphql-kickstart-spring-support/src/test/java/graphql/kickstart/spring/error/GraphQLErrorStartupListenerTest.java

@@ -0,0 +1,36 @@
+package graphql.kickstart.spring.error;
+
+import graphql.kickstart.execution.error.GraphQLErrorHandler;
+import org.assertj.core.api.Assertions;
+import org.junit.jupiter.api.Test;
+import org.mockito.Mockito;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.context.event.ApplicationReadyEvent;
+import org.springframework.web.context.support.AnnotationConfigWebApplicationContext;
+
+public class GraphQLErrorStartupListenerTest {
+
+  @Test
+  void error_handler_is_not_overridden_when_present() {
+    GraphQLErrorHandler expectedErrorHandler = Mockito.mock(GraphQLErrorHandler.class);
+    ErrorHandlerSupplier errorHandlerSupplier = new ErrorHandlerSupplier(expectedErrorHandler);
+    GraphQLErrorStartupListener graphQLErrorStartupListener = new GraphQLErrorStartupListener(errorHandlerSupplier, false);
+    graphQLErrorStartupListener.onApplicationEvent(getApplicationReadyEvent());
+    Assertions.assertThat(errorHandlerSupplier.get()).isEqualTo(expectedErrorHandler);
+  }
+
+  @Test
+  void error_handler_is_set_when_not_present() {
+    ErrorHandlerSupplier errorHandlerSupplier = new ErrorHandlerSupplier(null);
+    GraphQLErrorStartupListener graphQLErrorStartupListener = new GraphQLErrorStartupListener(errorHandlerSupplier, false);
+    graphQLErrorStartupListener.onApplicationEvent(getApplicationReadyEvent());
+    Assertions.assertThat(errorHandlerSupplier.get()).isNotNull();
+  }
+
+  private ApplicationReadyEvent getApplicationReadyEvent() {
+    AnnotationConfigWebApplicationContext annotationConfigWebApplicationContext = new AnnotationConfigWebApplicationContext();
+    annotationConfigWebApplicationContext.refresh();
+    return new ApplicationReadyEvent(new SpringApplication(), new String[0], annotationConfigWebApplicationContext);
+  }
+
+}