Initial support for Reactive API

Fixes #520
Fixes #458 (for non-reactive binders)

Adds spring-cloud-stream-reactive module.
Introduces support for declarative @StreamListener and @Input and @Output annotated parameters.
Add StreamListenerArgumentAdapter and StreamListenerResultAdapter for wrapping bindable inputs
and outputs when passing arguments to declarative @StreamListener.
Adds support for using reactive types (Flux/Observable) with traditional binders.
Introduce FluxSender and ObservableSender for handling multiple streaming outputs per method.
Ensure that errors are caught and logged.

Fixing constructor assertions and Javadoc

Addressing PR comments

- rework @Input/@Output parameter validation
- renamed StreamListenerArgumentAdapter to StreamListenerParameterAdapter
- ensure that parameter direction is accounted for in the current adapters

spring-cloud-stream-integration-tests/src/test/java/org/springframework/cloud/stream/config/StreamListenerWithAnnotatedArgsTests.java

@@ -0,0 +1,79 @@
+/*
+ * Copyright 2016 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.stream.config;
+
+import java.util.concurrent.TimeUnit;
+
+import org.junit.Test;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.cloud.stream.annotation.EnableBinding;
+import org.springframework.cloud.stream.annotation.Input;
+import org.springframework.cloud.stream.annotation.Output;
+import org.springframework.cloud.stream.annotation.StreamListener;
+import org.springframework.cloud.stream.messaging.Processor;
+import org.springframework.cloud.stream.test.binder.MessageCollector;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
+import org.springframework.messaging.MessageHandler;
+import org.springframework.messaging.MessagingException;
+import org.springframework.messaging.SubscribableChannel;
+import org.springframework.messaging.support.MessageBuilder;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Marius Bogoevici
+ */
+public class StreamListenerWithAnnotatedArgsTests {
+
+	@Test
+	public void testInputOutputArgs() throws Exception {
+		ConfigurableApplicationContext context = SpringApplication.run(TestInputOutputArgs.class, "--server.port=0");
+		sendMessageAndValidate(context);
+	}
+
+	private void sendMessageAndValidate(ConfigurableApplicationContext context) throws InterruptedException {
+		@SuppressWarnings("unchecked")
+		Processor processor = context.getBean(Processor.class);
+		processor.input().send(MessageBuilder.withPayload("hello").setHeader("contentType", "text/plain").build());
+		MessageCollector messageCollector = context.getBean(MessageCollector.class);
+		Message<?> result = messageCollector.forChannel(processor.output()).poll(1000, TimeUnit.MILLISECONDS);
+		assertThat(result).isNotNull();
+		assertThat(result.getPayload()).isEqualTo("HELLO");
+		context.close();
+	}
+
+
+	@EnableBinding(Processor.class)
+	@EnableAutoConfiguration
+	public static class TestInputOutputArgs {
+
+		@StreamListener
+		public void receive(@Input(Processor.INPUT) SubscribableChannel input, @Output(Processor.OUTPUT) final MessageChannel output) {
+			input.subscribe(new MessageHandler() {
+				@Override
+				public void handleMessage(Message<?> message) throws MessagingException {
+					output.send(MessageBuilder.withPayload(message.getPayload().toString().toUpperCase()).build());
+				}
+			});
+		}
+	}
+
+}

spring-cloud-stream-integration-tests/src/test/java/org/springframework/cloud/stream/config/StreamListenerWithHandlerTests.java

@@ -51,7 +51,7 @@ import static org.junit.Assert.fail;
 /**
  * @author Marius Bogoevici
  */
-public class StreamListenerTests {
+public class StreamListenerWithHandlerTests {
 
 	@Test
 	public void testContentTypeConversion() throws Exception {
@@ -240,8 +240,8 @@ public class StreamListenerTests {
 
 		@StreamListener(Sink.INPUT)
 		public void receive(FooPojo fooPojo) {
-			receivedArguments.add(fooPojo);
-			latch.countDown();
+			this.receivedArguments.add(fooPojo);
+			this.latch.countDown();
 		}
 	}
 
@@ -254,7 +254,7 @@ public class StreamListenerTests {
 		@StreamListener(Processor.INPUT)
 		@SendTo(Processor.OUTPUT)
 		public String receive(FooPojo fooPojo) {
-			receivedPojos.add(fooPojo);
+			this.receivedPojos.add(fooPojo);
 			return fooPojo.getBar();
 		}
 	}
@@ -268,7 +268,7 @@ public class StreamListenerTests {
 		@StreamListener(Processor.INPUT)
 		@SendTo(Processor.OUTPUT)
 		public BazPojo receive(FooPojo fooPojo) {
-			receivedPojos.add(fooPojo);
+			this.receivedPojos.add(fooPojo);
 			BazPojo bazPojo = new BazPojo();
 			bazPojo.setQux(fooPojo.getBar());
 			return bazPojo;
@@ -285,9 +285,9 @@ public class StreamListenerTests {
 		public void receive(@Payload FooPojo fooPojo,
 				@Headers Map<String, Object> headers,
 				@Header(MessageHeaders.CONTENT_TYPE) String contentType) {
-			receivedArguments.add(fooPojo);
-			receivedArguments.add(headers);
-			receivedArguments.add(contentType);
+			this.receivedArguments.add(fooPojo);
+			this.receivedArguments.add(headers);
+			this.receivedArguments.add(contentType);
 		}
 	}
 
@@ -300,7 +300,7 @@ public class StreamListenerTests {
 		@StreamListener(Processor.INPUT)
 		@SendTo(Processor.OUTPUT)
 		public Message<?> receive(FooPojo fooPojo) {
-			receivedPojos.add(fooPojo);
+			this.receivedPojos.add(fooPojo);
 			BazPojo bazPojo = new BazPojo();
 			bazPojo.setQux(fooPojo.getBar());
 			return MessageBuilder.withPayload(bazPojo).setHeader("foo", "bar").build();
@@ -316,7 +316,7 @@ public class StreamListenerTests {
 		@StreamListener(Processor.INPUT)
 		@SendTo(Processor.OUTPUT)
 		public BazPojo receive(Message<String> fooMessage) {
-			receivedMessages.add(fooMessage);
+			this.receivedMessages.add(fooMessage);
 			BazPojo bazPojo = new BazPojo();
 			bazPojo.setQux(fooMessage.getPayload());
 			return bazPojo;
@@ -353,7 +353,7 @@ public class StreamListenerTests {
 		@StreamListener(Processor.INPUT)
 		@SendTo(Processor.OUTPUT)
 		public BazPojo receive(FooPojo fooMessage) {
-			receivedPojos.add(fooMessage);
+			this.receivedPojos.add(fooMessage);
 			BazPojo bazPojo = new BazPojo();
 			bazPojo.setQux(fooMessage.getBar());
 			return bazPojo;
@@ -365,7 +365,7 @@ public class StreamListenerTests {
 		private String bar;
 
 		public String getBar() {
-			return bar;
+			return this.bar;
 		}
 
 		public void setBar(String bar) {
@@ -378,7 +378,7 @@ public class StreamListenerTests {
 		private String qux;
 
 		public String getQux() {
-			return qux;
+			return this.qux;
 		}
 
 		public void setQux(String qux) {