INT-4432: Mock Handler: fix output channel

JIRA: https://jira.spring.io/browse/INT-4432

The `MockIntegrationContext.substituteMessageHandlerFor()` method
cannot get the correct output channel of target message handler
since the `outputChannel` property is not initialized.

* Use `getOutputChannel()` method instead of directly accessing
the `outputChannel` property. The `getOutputChannel()` method
guarantees the correct value is retrieved.

* Polishing Copyrights and author name
* Simplify `MockMessageHandlerTests.testHandlerSubstitutionWithOutputChannel()`
and its configuration

**Cherry-pick to 5.0.x**

spring-integration-test/src/main/java/org/springframework/integration/test/context/MockIntegrationContext.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2017-2018 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.
@@ -46,8 +46,10 @@ import org.springframework.util.ObjectUtils;
  * annotation and can be autowired into test class.
  *
  * @author Artem Bilan
+ * @author Yicheng Feng
  *
  * @since 5.0
+ *
  * @see SpringIntegrationTest
  */
 public class MockIntegrationContext implements BeanFactoryAware {
@@ -139,8 +141,7 @@ public class MockIntegrationContext implements BeanFactoryAware {
 
 		if (mockMessageHandler instanceof MessageProducer) {
 			if (targetMessageHandler instanceof MessageProducer) {
-				MessageChannel outputChannel = TestUtils.getPropertyValue(targetMessageHandler, "outputChannel",
-						MessageChannel.class);
+				MessageChannel outputChannel = ((MessageProducer) targetMessageHandler).getOutputChannel();
 				((MessageProducer) mockMessageHandler).setOutputChannel(outputChannel);
 			}
 			else {

spring-integration-test/src/test/java/org/springframework/integration/test/mock/MockMessageHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2017-2018 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.
@@ -49,6 +49,7 @@ import org.springframework.integration.config.EnableIntegration;
 import org.springframework.integration.endpoint.EventDrivenConsumer;
 import org.springframework.integration.expression.ValueExpression;
 import org.springframework.integration.handler.ExpressionEvaluatingMessageHandler;
+import org.springframework.integration.handler.ServiceActivatingHandler;
 import org.springframework.integration.support.MessageBuilder;
 import org.springframework.integration.test.context.MockIntegrationContext;
 import org.springframework.integration.test.context.SpringIntegrationTest;
@@ -65,6 +66,7 @@ import org.springframework.test.context.junit4.SpringRunner;
 
 /**
  * @author Artem Bilan
+ * @author Yicheng Feng
  *
  * @since 5.0
  */
@@ -85,12 +87,18 @@ public class MockMessageHandlerTests {
 	@Autowired
 	private MessageChannel pojoServiceChannel;
 
+	@Autowired
+	private MessageChannel startChannel;
+
 	@Autowired
 	private MessageChannel rawChannel;
 
 	@Autowired
 	private QueueChannel results;
 
+	@Autowired
+	private ArgumentCaptor<Message<?>> argumentCaptorForOutputTest;
+
 	@Autowired
 	private ArgumentCaptor<Message<?>> messageArgumentCaptor;
 
@@ -209,6 +217,22 @@ public class MockMessageHandlerTests {
 		}
 	}
 
+	/**
+	 * Test whether the output channel is working after the target service activator is substituted.
+	 */
+	@Test
+	public void testHandlerSubstitutionWithOutputChannel() {
+		this.mockIntegrationContext
+				.substituteMessageHandlerFor("mockMessageHandlerTests.Config.mockService.serviceActivator",
+						mockMessageHandler()
+								.handleNextAndReply(m -> m));
+		Message<String> message = MessageBuilder.withPayload("bar").build();
+		this.startChannel.send(message);
+		this.startChannel.send(message);
+		List<Message<?>> list = argumentCaptorForOutputTest.getAllValues();
+		assertEquals(2, list.size());
+	}
+
 	@Configuration
 	@EnableIntegration
 	public static class Config {
@@ -254,6 +278,23 @@ public class MockMessageHandlerTests {
 					new ExpressionEvaluatingMessageHandler(new ValueExpression<>("test")));
 		}
 
+		@ServiceActivator(inputChannel = "startChannel", outputChannel = "nextChannel")
+		public String mockService(String payload) {
+			return payload + "test";
+		}
+
+		@Bean
+		public ArgumentCaptor<Message<?>> argumentCaptorForOutputTest() {
+			return MockIntegration.messageArgumentCaptor();
+		}
+
+		@Bean
+		@ServiceActivator(inputChannel = "nextChannel")
+		public MessageHandler handleNextInput() {
+			return mockMessageHandler(argumentCaptorForOutputTest())
+					.handleNext(m -> {});
+		}
+
 	}
 
 }