GH-2035 Add autodetect property to give user control over function autodetection

Resolves #2035

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

@@ -358,6 +358,9 @@ you must provide `spring.cloud.function.definition` property.
 NOTE: In the event you only have single bean of type `java.util.function.[Supplier/Function/Consumer]`, you can
 skip the `spring.cloud.function.definition` property, since such functional bean will be auto-discovered. However,
 it is considered best practice to use such property to avoid any confusion.
+Some time this auto-discovery can get in the way, since single bean of type `java.util.function.[Supplier/Function/Consumer]` 
+could be there for purposes other then handling messages, yet being single it is auto-discovered and auto-bound.
+For these rare scenarios you can disable auto-discovery by providing `spring.cloud.stream.function.autodetect` property with value set to `false`. 
 
 Here is the example of the application exposing message handler as `java.util.function.Function` effectively supporting
 _pass-thru_ semantics by acting as consumer and producer of data.

spring-cloud-stream/src/main/java/org/springframework/cloud/stream/function/FunctionConfiguration.java

@@ -760,7 +760,12 @@ public class FunctionConfiguration {
 			return outputCount;
 		}
 
+
 		private void determineFunctionName(FunctionCatalog catalog, Environment environment) {
+//=======
+//		private boolean determineFunctionName(FunctionCatalog catalog, Environment environment) {
+			boolean autodetect = environment.getProperty("spring.cloud.stream.function.autodetect", boolean.class, true);
+//>>>>>>> 9db08ec6... GH-2035 Add autodetect property to give user control over function autodetection
 			String definition = streamFunctionProperties.getDefinition();
 			if (!StringUtils.hasText(definition)) {
 				definition = environment.getProperty("spring.cloud.function.definition");
@@ -773,7 +778,7 @@ public class FunctionConfiguration {
 					|| environment.containsProperty("spring.cloud.function.routing-expression")) {
 				streamFunctionProperties.setDefinition(RoutingFunction.FUNCTION_NAME);
 			}
-			else {
+			else if (autodetect) {
 				streamFunctionProperties.setDefinition(((FunctionInspector) functionCatalog).getName(functionCatalog.lookup("")));
 			}
 		}

spring-cloud-stream/src/test/java/org/springframework/cloud/stream/function/ImplicitFunctionBindingTests.java

@@ -79,6 +79,23 @@ public class ImplicitFunctionBindingTests {
 		System.clearProperty("spring.cloud.function.definition");
 	}
 
+	@SuppressWarnings({"rawtypes" })
+	@Test
+	public void testDisableAutodetect() {
+		try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+				TestChannelBinderConfiguration.getCompleteConfiguration(SendToDestinationConfiguration.class))
+						.web(WebApplicationType.NONE)
+						.run("--spring.jmx.enabled=false", "--spring.cloud.stream.function.autodetect=false")) {
+
+			BindingsLifecycleController ctrl = context.getBean(BindingsLifecycleController.class);
+			Binding input = ctrl.queryState("echo-in-0");
+			Binding output = ctrl.queryState("echo-out-0");
+			assertThat(input).isNull();
+			assertThat(output).isNull();
+		}
+	}
+
+
 	@SuppressWarnings({"rawtypes" })
 	@Test
 	public void testBindingControl() {
@@ -214,7 +231,9 @@ public class ImplicitFunctionBindingTests {
 		try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
 				TestChannelBinderConfiguration.getCompleteConfiguration(NoEnableBindingConfiguration.class))
 						.web(WebApplicationType.NONE)
-						.run("--spring.jmx.enabled=false", "--spring.cloud.function.definition=func|addHeaders")) {
+						.run("--spring.jmx.enabled=false",
+								"--spring.cloud.stream.function.autodetect=false",
+								"--spring.cloud.function.definition=func|addHeaders")) {
 
 			InputDestination inputDestination = context.getBean(InputDestination.class);
 			OutputDestination outputDestination = context.getBean(OutputDestination.class);