GH-2265 Add support for creating explicit bindings

Resolves #2265

docs/src/main/asciidoc/_configprops.adoc

@@ -9,6 +9,7 @@
 |spring.cloud.stream.dynamic-destinations | `[]` | A list of destinations that can be bound dynamically. If set, only listed destinations can be bound.
 |spring.cloud.stream.function.batch-mode | `false` | 
 |spring.cloud.stream.function.bindings |  | 
+|spring.cloud.stream.input-bindings |  | A semi-colon delimited string to explicitly define input bindings (specifically for cases when there is no implicit trigger to create such bindings such as Function, Supplier or Consumer).
 |spring.cloud.stream.instance-count | `1` | The number of deployed instances of an application. Default: 1. NOTE: Could also be managed per individual binding "spring.cloud.stream.bindings.foo.consumer.instance-count" where 'foo' is the name of the binding.
 |spring.cloud.stream.instance-index | `0` | The instance id of the application: a number from 0 to instanceCount-1. Used for partitioning and with Kafka. NOTE: Could also be managed per individual binding "spring.cloud.stream.bindings.foo.consumer.instance-index" where 'foo' is the name of the binding.
 |spring.cloud.stream.instance-index-list |  | A list of instance id's from 0 to instanceCount-1. Used for partitioning and with Kafka. NOTE: Could also be managed per individual binding "spring.cloud.stream.bindings.foo.consumer.instance-index-list" where 'foo' is the name of the binding. This setting will override the one set in 'spring.cloud.stream.instance-index'
@@ -18,9 +19,10 @@
 |spring.cloud.stream.metrics.meter-filter |  | Pattern to control the 'meters' one wants to capture. By default all 'meters' will be captured. For example, 'spring.integration.*' will only capture metric information for meters whose name starts with 'spring.integration'.
 |spring.cloud.stream.metrics.properties |  | Application properties that should be added to the metrics payload For example: `spring.application**`.
 |spring.cloud.stream.metrics.schedule-interval | `60s` | Interval expressed as Duration for scheduling metrics snapshots publishing. Defaults to 60 seconds
+|spring.cloud.stream.output-bindings |  | A semi-colon delimited string to explicitly define output bindings (specifically for cases when there is no implicit trigger to create such bindings such as Function, Supplier or Consumer).
 |spring.cloud.stream.override-cloud-connectors | `false` | This property is only applicable when the cloud profile is active and Spring Cloud Connectors are provided with the application. If the property is false (the default), the binder detects a suitable bound service (for example, a RabbitMQ service bound in Cloud Foundry for the RabbitMQ binder) and uses it for creating connections (usually through Spring Cloud Connectors). When set to true, this property instructs binders to completely ignore the bound services and rely on Spring Boot properties (for example, relying on the spring.rabbitmq.* properties provided in the environment for the RabbitMQ binder). The typical usage of this property is to be nested in a customized environment when connecting to multiple systems.
 |spring.cloud.stream.pollable-source | `none` | A semi-colon delimited list of binding names of pollable sources. Binding names follow the same naming convention as functions. For example, name '...pollable-source=foobar' will be accessible as 'foobar-iin-0'' binding
 |spring.cloud.stream.sendto.destination | `none` | The name of the header used to determine the name of the output destination
-|spring.cloud.stream.source |  | A colon delimited string representing the names of the sources based on which source bindings will be created.  This is primarily to support cases where source binding may be required without providing a corresponding Supplier.  (e.g., for cases where the actual source of data is outside of scope of spring-cloud-stream - HTTP -> Stream)
+|spring.cloud.stream.source |  | A semi-colon delimited string representing the names of the sources based on which source bindings will be created.  This is primarily to support cases where source binding may be required without providing a corresponding Supplier.  (e.g., for cases where the actual source of data is outside of scope of spring-cloud-stream - HTTP -> Stream)  @deprecated use {@link #outputBindings}
 
 |===

docs/src/main/asciidoc/spring-cloud-stream.adoc

@@ -336,6 +336,45 @@ where you are clearly correlating the input of `uppercase` function to `sample-t
 For more on properties and other configuration options please see <<Configuration Options>> section.
 
 [[spring-cloud-stream-overview-producing-consuming-messages]]
+
+===== Explicit binding creation
+
+In the previous section we explained how bindings are created implicitly driven by Function, Supplier or Consumer provided by your application.
+However, there are times when you may need to create binding explicitly where bindings are not tied to any function. This is typically done to
+support integrations with other frameworks (e.g., Spring Integration framework) where you may need direct access to the underlying `MessageChannel`.
+
+Spring Cloud Stream allows you to define input and output bindings explicitly via `spring.cloud.stream.input-bindings` and  `spring.cloud.stream.output-bindings`
+properties. Noticed the plural in the property names allowing you to define multiple bindings by simply using `;` as a delimiter.
+Just look at the following test case as an example:
+
+----
+@Test
+public void testExplicitBindings() {
+	try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+		TestChannelBinderConfiguration.getCompleteConfiguration(EmptyConfiguration.class))
+				.web(WebApplicationType.NONE)
+				.run("--spring.jmx.enabled=false",
+					"--spring.cloud.stream.input-bindings=fooin;barin",
+					"--spring.cloud.stream.output-bindings=fooout;barout")) {
+
+	assertThat(context.getBean("fooin-in-0", MessageChannel.class)).isNotNull();
+	assertThat(context.getBean("barin-in-0", MessageChannel.class)).isNotNull();
+	assertThat(context.getBean("fooout-out-0", MessageChannel.class)).isNotNull();
+	assertThat(context.getBean("barout-out-0", MessageChannel.class)).isNotNull();
+	}
+}
+
+@EnableAutoConfiguration
+@Configuration
+public static class EmptyConfiguration {
+}
+----
+
+As you can see we have declared two input bindings and two output bindings while our configuration had no functions defined, yet we were able to successfully create these bindings and access their corresponding channels.
+
+The rest of the binding rules that apply to implicit bindings apply here as well (for example, you can see that `fooin` turned into `fooin-in-0` binding/channel etc). 
+
+
 === Producing and Consuming Messages
 
 You can write a Spring Cloud Stream application by simply writing functions and exposing them as `@Bean`s.