GH-2107 Add test to validate no NPE for function returning null

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

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2021-2021 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.function.Function;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.boot.WebApplicationType;
+import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
+import org.springframework.boot.builder.SpringApplicationBuilder;
+import org.springframework.cloud.stream.binder.test.InputDestination;
+import org.springframework.cloud.stream.binder.test.OutputDestination;
+import org.springframework.cloud.stream.binder.test.TestChannelBinderConfiguration;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.messaging.support.GenericMessage;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ *
+ * @author Oleg Zhurakousky
+ *
+ */
+public class ScenarioTests {
+
+	@Test
+	public void test2107() {
+		try (ConfigurableApplicationContext context = new SpringApplicationBuilder(
+				TestChannelBinderConfiguration.getCompleteConfiguration(FunctionReturningNullConfiguration.class))
+						.web(WebApplicationType.NONE)
+						.run("--spring.jmx.enabled=false", "--spring.cloud.function.definition=uppercase")) {
+
+			InputDestination input = context.getBean(InputDestination.class);
+			input.send(new GenericMessage<byte[]>("a".getBytes()), "uppercase-in-0");
+			OutputDestination output = context.getBean(OutputDestination.class);
+			assertThat(new String(output.receive(2000, "uppercase-out-0").getPayload())).isEqualTo("a");
+			input.send(new GenericMessage<byte[]>("b".getBytes()), "uppercase-in-0");
+			assertThat(output.receive(2000, "uppercase-out-0")).isNull();
+		}
+	}
+
+	@EnableAutoConfiguration
+	@Configuration
+	public static class FunctionReturningNullConfiguration {
+		@Bean
+		public Function<String, String> uppercase() {
+			return v -> {
+				if ("a".equals(v)) {
+					return v;
+				}
+				else {
+					return null;
+				}
+			};
+		}
+	}
+}