Improve application aggregation and testability

Fixes #723

- Add support for registering an `AggregateApplication` bean for accessing the
  components from underlying subcontexts
- Testing support and samples

Signed-off-by: Marius Bogoevici <mbogoevici@pivotal.io>

Fix Javadoc

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/aggregate/AggregateTestWithBean.java

@@ -0,0 +1,78 @@
+package org.springframework.cloud.stream.test.aggregate;
+
+import java.util.concurrent.TimeUnit;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.cloud.stream.aggregate.AggregateApplication;
+import org.springframework.cloud.stream.aggregate.AggregateApplicationBuilder;
+import org.springframework.cloud.stream.annotation.EnableBinding;
+import org.springframework.cloud.stream.messaging.Processor;
+import org.springframework.cloud.stream.test.binder.MessageCollector;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.annotation.Transformer;
+import org.springframework.integration.support.MessageBuilder;
+import org.springframework.messaging.Message;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Marius Bogoevici
+ */
+@RunWith(SpringJUnit4ClassRunner.class)
+@SpringBootTest(classes = AggregateTestWithBean.ChainedProcessors.class, properties = {"server.port=-1"})
+public class AggregateTestWithBean {
+
+	@Autowired
+	public MessageCollector messageCollector;
+
+	@Autowired
+	public AggregateApplication aggregateApplication;
+
+	@Test
+	public void testAggregateApplication() throws InterruptedException {
+		Processor uppercaseProcessor = aggregateApplication.getBinding(Processor.class, "upper");
+		Processor suffixProcessor = aggregateApplication.getBinding(Processor.class, "suffix");
+		uppercaseProcessor.input().send(MessageBuilder.withPayload("Hello").build());
+		Message<?> receivedMessage = messageCollector.forChannel(suffixProcessor.output()).poll(1, TimeUnit.SECONDS);
+		assertThat(receivedMessage).isNotNull();
+		assertThat(receivedMessage.getPayload()).isEqualTo("HELLO WORLD!");
+	}
+
+	@SpringBootApplication
+	@EnableBinding
+	public static class ChainedProcessors {
+
+		@Bean
+		public AggregateApplication aggregateApplication() {
+			return new AggregateApplicationBuilder().from(UppercaseProcessor.class)
+					.namespace("upper").to(SuffixProcessor.class).namespace("suffix").build();
+		}
+	}
+
+	@Configuration
+	@EnableBinding(Processor.class)
+	public static class UppercaseProcessor {
+
+		@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
+		public String transform(String in) {
+			return in.toUpperCase();
+		}
+	}
+
+	@Configuration
+	@EnableBinding(Processor.class)
+	public static class SuffixProcessor {
+
+		@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
+		public String transform(String in) {
+			return in + " WORLD!";
+		}
+	}
+}

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/aggregate/AggregateTestWithMain.java

@@ -0,0 +1,61 @@
+package org.springframework.cloud.stream.test.aggregate;
+
+import java.util.concurrent.TimeUnit;
+
+import org.junit.Test;
+
+import org.springframework.cloud.stream.aggregate.AggregateApplication;
+import org.springframework.cloud.stream.aggregate.AggregateApplicationBuilder;
+import org.springframework.cloud.stream.annotation.EnableBinding;
+import org.springframework.cloud.stream.messaging.Processor;
+import org.springframework.cloud.stream.test.binder.MessageCollector;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.annotation.Transformer;
+import org.springframework.integration.support.MessageBuilder;
+import org.springframework.messaging.Message;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Marius Bogoevici
+ */
+public class AggregateTestWithMain {
+
+	@Test
+	public void testAggregateApplication() throws InterruptedException {
+		// emulate a main method
+		ConfigurableApplicationContext context = new AggregateApplicationBuilder().from(UppercaseProcessor.class)
+				.namespace("upper").to(SuffixProcessor.class).namespace("suffix").run();
+
+		AggregateApplication aggregateAccessor = context.getBean(AggregateApplication.class);
+		MessageCollector messageCollector = context.getBean(MessageCollector.class);
+		Processor uppercaseProcessor = aggregateAccessor.getBinding(Processor.class, "upper");
+		Processor suffixProcessor = aggregateAccessor.getBinding(Processor.class, "suffix");
+		uppercaseProcessor.input().send(MessageBuilder.withPayload("Hello").build());
+		Message<?> receivedMessage = messageCollector.forChannel(suffixProcessor.output()).poll(1, TimeUnit.SECONDS);
+		assertThat(receivedMessage).isNotNull();
+		assertThat(receivedMessage.getPayload()).isEqualTo("HELLO WORLD!");
+		context.close();
+	}
+
+	@Configuration
+	@EnableBinding(Processor.class)
+	public static class UppercaseProcessor {
+
+		@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
+		public String transform(String in) {
+			return in.toUpperCase();
+		}
+	}
+
+	@Configuration
+	@EnableBinding(Processor.class)
+	public static class SuffixProcessor {
+
+		@Transformer(inputChannel = Processor.INPUT, outputChannel = Processor.OUTPUT)
+		public String transform(String in) {
+			return in + " WORLD!";
+		}
+	}
+}

spring-cloud-stream-test-support/src/test/java/org/springframework/cloud/stream/test/example/ExampleTest.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.springframework.cloud.stream.test;
+package org.springframework.cloud.stream.test.example;
 
 import org.junit.Test;
 import org.junit.runner.RunWith;