Improve functions support

* Add `functions-support.adoc` chapter
* Add more tests
* Improve `InboundChannelAdapterAnnotationPostProcessor` to support
Kotlin `Function0`
* Add `FunctionsTests.kt`
* Reformat Kotlin classes to use tabs
* Upgrade to Kotlin `1.2.71`

* Add `What's New` bullet

Doc Polishing

build.gradle

@@ -1,5 +1,5 @@
 buildscript {
-	ext.kotlinVersion = '1.2.61'
+	ext.kotlinVersion = '1.2.71'
 	repositories {
 		maven { url 'https://repo.spring.io/plugins-release' }
 	}

spring-integration-core/src/main/java/org/springframework/integration/config/annotation/InboundChannelAdapterAnnotationPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2016 the original author or authors.
+ * Copyright 2014-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,6 +34,7 @@
 import org.springframework.integration.util.MessagingAnnotationUtils;
 import org.springframework.messaging.MessageHandler;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -42,11 +43,27 @@
  * @author Artem Bilan
  * @author Gary Russell
  * @author Oleg Zhurakousky
+ *
  * @since 4.0
  */
 public class InboundChannelAdapterAnnotationPostProcessor extends
 		AbstractMethodAnnotationPostProcessor<InboundChannelAdapter> {
 
+	private static final Class<?> kotlinFunction0Class;
+
+	static {
+		Class<?> kotlinClass = null;
+		try {
+			kotlinClass = ClassUtils.forName("kotlin.jvm.functions.Function0", ClassUtils.getDefaultClassLoader());
+		}
+		catch (ClassNotFoundException e) {
+			//Ignore: assume no Kotlin in classpath
+		}
+		finally {
+			kotlinFunction0Class = kotlinClass;
+		}
+	}
+
 	public InboundChannelAdapterAnnotationPostProcessor(ConfigurableListableBeanFactory beanFactory) {
 		super(beanFactory);
 	}
@@ -58,7 +75,8 @@ protected String getInputChannelAttribute() {
 
 	@Override
 	public Object postProcess(Object bean, String beanName, Method method, List<Annotation> annotations) {
-		String channelName = MessagingAnnotationUtils.resolveAttribute(annotations, AnnotationUtils.VALUE, String.class);
+		String channelName = MessagingAnnotationUtils
+				.resolveAttribute(annotations, AnnotationUtils.VALUE, String.class);
 		Assert.hasText(channelName, "The channel ('value' attribute of @InboundChannelAdapter) can't be empty.");
 
 		MessageSource<?> messageSource = null;
@@ -86,23 +104,33 @@ private MessageSource<?> createMessageSource(Object bean, String beanName, Metho
 		MessageSource<?> messageSource = null;
 		if (AnnotatedElementUtils.isAnnotated(method, Bean.class.getName())) {
 			Object target = this.resolveTargetBeanFromMethodWithBeanAnnotation(method);
-			Assert.isTrue(target instanceof MessageSource || target instanceof Supplier, "The '" + this.annotationType + "' on @Bean method " +
-					"level is allowed only for: " + MessageSource.class.getName() + " or " + Supplier.class.getName() + " beans");
+			Class<?> targetClass = target.getClass();
+			Assert.isTrue(MessageSource.class.isAssignableFrom(targetClass) ||
+							Supplier.class.isAssignableFrom(targetClass) ||
+							(kotlinFunction0Class == null || kotlinFunction0Class.isAssignableFrom(targetClass)),
+					"The '" + this.annotationType + "' on @Bean method " + "level is allowed only for: "
+							+ MessageSource.class.getName() + " or " + Supplier.class.getName()
+							+ (kotlinFunction0Class != null ? " or " + kotlinFunction0Class.getName() : "") + " beans");
 			if (target instanceof MessageSource<?>) {
 				messageSource = (MessageSource<?>) target;
 			}
-			else {
+			else if (target instanceof Supplier<?>) {
 				method = ReflectionUtils.findMethod(Supplier.class, "get");
 				bean = target;
 			}
+			else if (kotlinFunction0Class != null) {
+				method = ReflectionUtils.findMethod(kotlinFunction0Class, "invoke");
+				bean = target;
+			}
 		}
 		if (messageSource == null) {
 			MethodInvokingMessageSource methodInvokingMessageSource = new MethodInvokingMessageSource();
 			methodInvokingMessageSource.setObject(bean);
 			methodInvokingMessageSource.setMethod(method);
 			String messageSourceBeanName = this.generateHandlerBeanName(beanName, method);
 			this.beanFactory.registerSingleton(messageSourceBeanName, methodInvokingMessageSource);
-			messageSource =  (MessageSource<?>) this.beanFactory.initializeBean(methodInvokingMessageSource, messageSourceBeanName);
+			messageSource = (MessageSource<?>) this.beanFactory
+					.initializeBean(methodInvokingMessageSource, messageSourceBeanName);
 		}
 		return messageSource;
 	}

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

@@ -34,6 +34,7 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
+import java.util.function.Supplier;
 
 import org.aopalliance.aop.Advice;
 import org.aopalliance.intercept.MethodInterceptor;
@@ -526,10 +527,20 @@ public interface ControlBusGateway {
 	@EnableIntegration
 	public static class SupplierContextConfiguration1 {
 
+		@Bean
+		public Function<String, String> toUpperCaseFunction() {
+			return String::toUpperCase;
+		}
+
+		@Bean
+		public Supplier<String> stringSupplier() {
+			return () -> "foo";
+		}
+
 		@Bean
 		public IntegrationFlow supplierFlow() {
-			return IntegrationFlows.from(() -> "foo")
-					.<String, String>transform(String::toUpperCase)
+			return IntegrationFlows.from(stringSupplier())
+					.transform(toUpperCaseFunction())
 					.channel("suppliedChannel")
 					.get();
 		}

spring-integration-core/src/test/java/org/springframework/integration/handler/ServiceActivatorDefaultFrameworkMethodTests-context.xml

@@ -1,11 +1,17 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <beans:beans xmlns="http://www.springframework.org/schema/integration"
-	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-	xmlns:beans="http://www.springframework.org/schema/beans"
-	xsi:schemaLocation="http://www.springframework.org/schema/beans
+			 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+			 xmlns:beans="http://www.springframework.org/schema/beans"
+			 xmlns:context="http://www.springframework.org/schema/context"
+			 xsi:schemaLocation="http://www.springframework.org/schema/beans
 			http://www.springframework.org/schema/beans/spring-beans.xsd
 			http://www.springframework.org/schema/integration
-			http://www.springframework.org/schema/integration/spring-integration.xsd">
+			http://www.springframework.org/schema/integration/spring-integration.xsd
+			http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
+
+	<context:annotation-config/>
+
+	<beans:bean class="org.springframework.integration.handler.ServiceActivatorDefaultFrameworkMethodTests$FunctionConfiguration"/>
 
 	<message-history/>
 
@@ -76,4 +82,6 @@
 		<queue />
 	</channel>
 
+	<service-activator input-channel="processorViaFunctionChannel" ref="functionAsService"/>
+
 </beans:beans>

spring-integration-core/src/test/java/org/springframework/integration/handler/ServiceActivatorDefaultFrameworkMethodTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,13 +31,15 @@
 import static org.springframework.integration.test.matcher.HeaderMatcher.hasHeaderKey;
 
 import java.util.concurrent.atomic.AtomicReference;
+import java.util.function.Function;
 
 import org.hamcrest.Matchers;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.springframework.beans.factory.BeanCreationException;
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.context.annotation.Bean;
 import org.springframework.context.support.ClassPathXmlApplicationContext;
 import org.springframework.integration.channel.DirectChannel;
 import org.springframework.integration.channel.QueueChannel;
@@ -107,6 +109,9 @@ public class ServiceActivatorDefaultFrameworkMethodTests {
 	@Autowired
 	private PollableChannel errorChannel;
 
+	@Autowired
+	private MessageChannel processorViaFunctionChannel;
+
 	@Test
 	public void testGateway() {
 		QueueChannel replyChannel = new QueueChannel();
@@ -280,6 +285,16 @@ public void testAsyncErrorNoHeader() {
 		assertEquals("test", ((MessagingException) error.getPayload()).getFailedMessage().getPayload());
 	}
 
+	@Test
+	public void testFunctionFromXml() {
+		QueueChannel replyChannel = new QueueChannel();
+		Message<?> message = MessageBuilder.withPayload("test").setReplyChannel(replyChannel).build();
+		this.processorViaFunctionChannel.send(message);
+		Message<?> reply = replyChannel.receive(0);
+		assertNotNull(reply);
+		assertEquals("TEST", reply.getPayload());
+	}
+
 	public static void throwIllegalStateException(String message) {
 		throw new IllegalStateException(message);
 	}
@@ -316,6 +331,7 @@ public void handleMessage(Message<?> requestMessage) {
 			assertTrue(StackTraceUtils.isFrameContainingXBeforeFrameContainingY("AbstractSubscribableChannel",
 					"MethodInvokerHelper", st)); // close to the metal
 		}
+
 	}
 
 	private static class TestMessageProcessor implements MessageProcessor<String> {
@@ -331,6 +347,7 @@ public void setPrefix(String prefix) {
 		public String processMessage(Message<?> message) {
 			return prefix + ":" + message.getPayload();
 		}
+
 	}
 
 	private static class AsyncService {
@@ -341,11 +358,21 @@ private static class AsyncService {
 
 		@SuppressWarnings("unused")
 		public ListenableFuture<String> process(String payload) {
-			this.future = new SettableListenableFuture<String>();
+			this.future = new SettableListenableFuture<>();
 			this.payload = payload;
 			return this.future;
 		}
 
 	}
 
+	public static class FunctionConfiguration {
+
+		@Bean
+		public Function<String, String> functionAsService() {
+			return String::toUpperCase;
+		}
+
+	}
+
+
 }