IntegrationFlow: optional @MessagingGateway

When we use `IntegrationFlows.from(Class<?>)`, we don't need to require
present for `@MessagingGateway`, we can just synthesize it with the
default properties

This way we can simply make gateway proxies for the `java.util.function`
interfaces which should be useful for Spring Cloud Function

spring-integration-core/src/main/java/org/springframework/integration/dsl/IntegrationFlows.java

@@ -274,7 +274,7 @@ public final class IntegrationFlows {
 	 * on the provided service interface.
 	 * <p>A gateway proxy bean for provided service interface is registered under a name of
 	 * the {@link IntegrationFlow} bean plus {@code .gateway} suffix.
-	 * @param serviceInterface the class with a
+	 * @param serviceInterface the service interface class with an optional
 	 * {@link org.springframework.integration.annotation.MessagingGateway} annotation.
 	 * @return new {@link IntegrationFlowBuilder}.
 	 */

spring-integration-core/src/main/java/org/springframework/integration/gateway/AnnotationGatewayProxyFactoryBean.java

@@ -23,6 +23,7 @@ import java.util.concurrent.Executor;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.core.annotation.AnnotationAttributes;
+import org.springframework.core.annotation.AnnotationUtils;
 import org.springframework.expression.Expression;
 import org.springframework.expression.common.LiteralExpression;
 import org.springframework.integration.annotation.AnnotationConstants;
@@ -32,8 +33,9 @@ import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 /**
- * A {@link GatewayProxyFactoryBean} extension for Java configuration,
- * when service interface is marked with the {@link MessagingGateway} annotation.
+ * A {@link GatewayProxyFactoryBean} extension for Java configuration.
+ * The service interface may be marked with the {@link MessagingGateway} annotation.
+ * Otherwise the default state is applied.
  *
  * @author Artem Bilan
  *
@@ -45,11 +47,13 @@ public class AnnotationGatewayProxyFactoryBean extends GatewayProxyFactoryBean {
 
 	public AnnotationGatewayProxyFactoryBean(Class<?> serviceInterface) {
 		super(serviceInterface);
-		this.gatewayAttributes = AnnotatedElementUtils.getMergedAnnotationAttributes(serviceInterface,
+		AnnotationAttributes gatewayAttributes = AnnotatedElementUtils.getMergedAnnotationAttributes(serviceInterface,
 				MessagingGateway.class.getName(), false, true);
-		Assert.notNull(this.gatewayAttributes,
-				"The Messaging Gateway interface must be annotated with the @MessagingGateway");
-
+		if (gatewayAttributes == null) {
+			gatewayAttributes = AnnotationUtils.getAnnotationAttributes(
+					AnnotationUtils.synthesizeAnnotation(MessagingGateway.class), false, true);
+		}
+		this.gatewayAttributes = gatewayAttributes;
 	}
 
 	@Override

spring-integration-core/src/test/java/org/springframework/integration/dsl/flows/IntegrationFlowTests.java

@@ -31,6 +31,7 @@ import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Function;
 
 import org.aopalliance.aop.Advice;
 import org.aopalliance.intercept.MethodInterceptor;
@@ -423,11 +424,11 @@ public class IntegrationFlowTests {
 	}
 
 	@Autowired
-	private ErrorRecovererFlowGateway errorRecovererFlowGateway;
+	private Function<String, String> errorRecovererFlowGateway;
 
 	@Test
 	public void testReplyChannelFromReplyMessage() {
-		assertEquals("foo", this.errorRecovererFlowGateway.testIt("foo"));
+		assertEquals("foo", this.errorRecovererFlowGateway.apply("foo"));
 	}
 
 	@Autowired
@@ -719,7 +720,7 @@ public class IntegrationFlowTests {
 
 		@Bean
 		public IntegrationFlow errorRecovererFlow() {
-			return IntegrationFlows.from(ErrorRecovererFlowGateway.class)
+			return IntegrationFlows.from(Function.class)
 					.handle((GenericHandler<?>) (p, h) -> {
 						throw new RuntimeException("intentional");
 					}, e -> e.advice(retryAdvice()))
@@ -764,13 +765,6 @@ public class IntegrationFlowTests {
 
 	}
 
-	@MessagingGateway
-	private interface ErrorRecovererFlowGateway {
-
-		String testIt(String payload);
-
-	}
-
 	@Service
 	public static class GreetingService extends AbstractReplyProducingMessageHandler {