GH-3203 Skip standalone bindings in supplierInitializer to avoid ClassCastException for output-only bindings

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

@@ -190,6 +190,14 @@ InitializingBean supplierInitializer(FunctionCatalog functionCatalog, StreamFunc
 
 		return () -> {
 			for (BindableFunctionProxyFactory proxyFactory : proxyFactories) {
+				// see https://github.com/spring-cloud/spring-cloud-stream/issues/3203
+				// A standalone output-/input-binding has no backing function, yet its
+				// function definition is now the binding name (since gh-3166 stopped
+				// null-ing it out in getFunctionDefinition()). Skip such bindings here so
+				// that a plain output channel is never resolved and treated as a Supplier.
+				if (!proxyFactory.isFunctionExist()) {
+					continue;
+				}
 				FunctionInvocationWrapper functionWrapper = functionCatalog.lookup(proxyFactory.getFunctionDefinition());
 				if (functionWrapper != null && functionWrapper.isSupplier()) {
 					// gather output content types

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

@@ -0,0 +1,68 @@
+/*
+ * Copyright 2024-present 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
+ *
+ *      https://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.function;
+
+import java.util.Map;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.endpoint.SourcePollingChannelAdapter;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Regression test for gh-3203: an output-only binding that has no backing
+ * {@code Supplier}/{@code Function} bean must not be treated as a {@code Supplier}.
+ * Prior to the fix the standalone output binding (whose function definition is the
+ * binding name) reached {@code supplierInitializer}, which built a
+ * {@link SourcePollingChannelAdapter} on top of the plain output channel and threw a
+ * {@code ClassCastException} (the channel cannot be cast to {@code Supplier}) on the
+ * first poll.
+ *
+ * @author Spring Cloud Stream contributors
+ */
+class OutputOnlyBindingSupplierTests {
+
+	@Test
+	void outputOnlyBindingIsNotTreatedAsSupplier() {
+		try (ConfigurableApplicationContext context = new SpringApplication(EmptyConfiguration.class).run(
+				"--spring.cloud.stream.output-bindings=testOutput",
+				"--spring.jmx.enabled=false",
+				"--spring.main.web-application-type=none",
+				"--spring.cloud.stream.default-binder=mock")) {
+
+			Map<String, SourcePollingChannelAdapter> pollingAdapters =
+					context.getBeansOfType(SourcePollingChannelAdapter.class);
+
+			assertThat(pollingAdapters)
+					.as("No SourcePollingChannelAdapter must be created for an output-only "
+							+ "binding without a backing Supplier")
+					.isEmpty();
+		}
+	}
+
+	@EnableAutoConfiguration
+	@Configuration
+	public static class EmptyConfiguration {
+
+	}
+
+}