Support flexible bound element types

Fixes #519

Introduces some internal changes to the framework allowing the use
of other types than MessageChannel/SubscribableChannel as bindable
types (e.g. Flux, Observable, KStream, etc.)

- Modify BinderFactory to allow the retrieval and lookup of a
  binder not only by name, but also by binding target type
- Subsequent changes to ChannelBindingService and tests to account
  for the modified signature
- Introduce BindingTargetFactory as the contract for creating bound
  elements
- Remove any references to chanels and bound elements and use
  'binding target' systematically across the board

Reinstate our own Checkstyle checks with a reduced set of rules so that header
  validation can be performed automatically at compile time.

Use ${project.version} for the checkstyle plugin configuration

Renaming some occurences of 'boundElement' to 'bindingTarget'

Renamed a stray occurence of 'channel'

Fix some occurences of String concatenation in the same line after reformatting

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

@@ -30,6 +30,7 @@ import org.springframework.cloud.stream.test.binder.TestSupportBinder;
 import org.springframework.context.annotation.PropertySource;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 
@@ -53,7 +54,7 @@ public class ContentTypeOutboundSourceTests {
 	@SuppressWarnings("unchecked")
 	public void testMessageHeaderWhenNoExplicitContentTypeOnMessage() throws Exception {
 		testSource.output().send(MessageBuilder.withPayload("{\"message\":\"Hi\"}").build());
-		Message<String> received = (Message<String>) ((TestSupportBinder) binderFactory.getBinder(null))
+		Message<String> received = (Message<String>) ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class))
 				.messageCollector().forChannel(testSource.output()).poll();
 		assertThat(received.getHeaders().get(MessageHeaders.CONTENT_TYPE).toString()).isEqualTo("application/json");
 		assertThat(received).hasFieldOrPropertyWithValue("payload", "{\"message\":\"Hi\"}");

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

@@ -37,6 +37,7 @@ import org.springframework.context.annotation.PropertySource;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.support.converter.DefaultDatatypeChannelMessageConverter;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.converter.AbstractMessageConverter;
 import org.springframework.messaging.converter.MessageConverter;
@@ -70,7 +71,7 @@ public class CustomMessageConverterTests {
 				BarConverter.class, DefaultDatatypeChannelMessageConverter.class);
 		testSource.output().send(MessageBuilder.withPayload(new Foo("hi")).build());
 		@SuppressWarnings("unchecked")
-		Message<String> received = (Message<String>) ((TestSupportBinder) binderFactory.getBinder(null))
+		Message<String> received = (Message<String>) ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class))
 				.messageCollector().forChannel(testSource.output()).poll(1, TimeUnit.SECONDS);
 		Assert.assertThat(received, notNullValue());
 		assertThat(received.getHeaders().get(MessageHeaders.CONTENT_TYPE)).isEqualTo(MimeType.valueOf("test/foo"));

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

@@ -34,6 +34,7 @@ import org.springframework.context.annotation.PropertySource;
 import org.springframework.integration.annotation.ServiceActivator;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
 
@@ -59,7 +60,7 @@ public class DeserializeJSONToJavaTypeTests {
 	public void testMessageDeserialized() throws Exception {
 		testProcessor.input().send(MessageBuilder.withPayload("{\"name\":\"Bar\"}").setHeader("contentType", "application/json").build());
 		@SuppressWarnings("unchecked")
-		Message<?> received = ((TestSupportBinder) binderFactory.getBinder(null))
+		Message<?> received = ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class))
 				.messageCollector().forChannel(testProcessor.output()).poll(1, TimeUnit.SECONDS);
 		assertThat(received).isNotNull();
 		assertThat(received.getPayload()).isInstanceOf(Foo.class);

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

@@ -35,6 +35,7 @@ import org.springframework.integration.annotation.Poller;
 import org.springframework.integration.channel.PublishSubscribeChannel;
 import org.springframework.integration.core.MessageSource;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessagingException;
 import org.springframework.messaging.support.ErrorMessage;
 import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
@@ -55,7 +56,7 @@ public class ErrorChannelTests {
 
 	@Test
 	public void testErrorChannelBinding() throws Exception {
-		Message<?> message = ((TestSupportBinder) binderFactory.getBinder(null)).messageCollector().forChannel(errorChannel).poll(10, TimeUnit.SECONDS);
+		Message<?> message = ((TestSupportBinder) binderFactory.getBinder(null, MessageChannel.class)).messageCollector().forChannel(errorChannel).poll(10, TimeUnit.SECONDS);
 		Assert.isTrue(message instanceof ErrorMessage, "Message should be an instance of ErrorMessage");
 		Assert.isTrue(message.getPayload() instanceof MessagingException, "Message payload should be an instance" +
 				"of MessagingException");

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import org.junit.Test;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -208,9 +208,9 @@ public class StreamListenerHandlerMethodTests {
 				latch.countDown();
 			}
 		});
-		processor.input().send(MessageBuilder.withPayload("{\"foo\":\"fooTESTing" + "\"}")
+		processor.input().send(MessageBuilder.withPayload("{\"foo\":\"fooTESTing\"}")
 				.setHeader("contentType", "application/json").build());
-		inboundChannel2.input().send(MessageBuilder.withPayload("{\"bar\":\"bartestING" + "\"}")
+		inboundChannel2.input().send(MessageBuilder.withPayload("{\"bar\":\"bartestING\"}")
 				.setHeader("contentType", "application/json").build());
 		assertThat(latch.await(1, TimeUnit.SECONDS));
 		context.close();

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import java.util.ArrayList;

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

@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.cloud.stream.config;
 
 import org.springframework.cloud.stream.annotation.Input;
@@ -53,7 +54,7 @@ public class StreamListenerTestUtils {
 
 	public interface FooInboundChannel1 {
 
-		public String INPUT = "foo1-input";
+		String INPUT = "foo1-input";
 
 		@Input(FooInboundChannel1.INPUT)
 		SubscribableChannel input();