INT-4563: Add JMX test for MQTT

JIRA: https://jira.spring.io/browse/INT-4563

* Add a `MqttDslTests` with the JMX configured to be sure that SI
managed components are registered in JMX properly.
* Also this test covers a Java DSL configuration for MQTT channel adapters
* Some polishing for `AbstractMqttMessageHandler`
* Document Java DSL configuration for the MQTT channel adapters

spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/MqttDslTests.java

@@ -0,0 +1,131 @@
+/*
+ * Copyright 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://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.integration.mqtt;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.util.Set;
+
+import javax.management.MBeanServer;
+import javax.management.MalformedObjectNameException;
+import javax.management.ObjectName;
+
+import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
+import org.junit.ClassRule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.beans.factory.annotation.Qualifier;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.integration.config.EnableIntegration;
+import org.springframework.integration.dsl.IntegrationFlow;
+import org.springframework.integration.dsl.IntegrationFlows;
+import org.springframework.integration.jmx.config.EnableIntegrationMBeanExport;
+import org.springframework.integration.mqtt.core.DefaultMqttPahoClientFactory;
+import org.springframework.integration.mqtt.inbound.MqttPahoMessageDrivenChannelAdapter;
+import org.springframework.integration.mqtt.outbound.MqttPahoMessageHandler;
+import org.springframework.integration.mqtt.support.MqttHeaders;
+import org.springframework.integration.support.MessageBuilder;
+import org.springframework.jmx.support.MBeanServerFactoryBean;
+import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageChannel;
+import org.springframework.messaging.PollableChannel;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.context.junit4.SpringRunner;
+
+/**
+ * @author Artem Bilan
+ *
+ * @since 5.1.2
+ */
+@RunWith(SpringRunner.class)
+@DirtiesContext
+public class MqttDslTests {
+
+	@ClassRule
+	public static final BrokerRunning brokerRunning = BrokerRunning.isRunning(1883);
+
+	@Autowired
+	@Qualifier("mqttOutFlow.input")
+	private MessageChannel mqttOutFlowInput;
+
+	@Autowired
+	private PollableChannel fromMqttChannel;
+
+	@Autowired
+	private MBeanServer server;
+
+	@Test
+	public void testMqttChannelAdaptersAndJmx() throws MalformedObjectNameException {
+		Set<ObjectName> mbeanNames = this.server.queryNames(
+				new ObjectName("org.springframework.integration:type=ManagedEndpoint,*"), null);
+
+		assertThat(mbeanNames.size()).isEqualTo(1);
+		ObjectName objectName = mbeanNames.iterator().next();
+		assertThat(objectName.toString()).contains("name=\"mqttInFlow.mqtt:inbound-channel-adapter#0\"");
+
+		String testPayload = "foo";
+
+		this.mqttOutFlowInput.send(
+				MessageBuilder.withPayload(testPayload)
+						.setHeader(MqttHeaders.TOPIC, "jmxTests")
+						.build());
+
+		Message<?> receive = this.fromMqttChannel.receive(10_000);
+
+		assertThat(receive).isNotNull();
+		assertThat(receive.getPayload()).isEqualTo(testPayload);
+	}
+
+	@Configuration
+	@EnableIntegration
+	@EnableIntegrationMBeanExport(server = "mbeanServer")
+	public static class Config {
+
+		@Bean
+		public static MBeanServerFactoryBean mbeanServer() {
+			return new MBeanServerFactoryBean();
+		}
+
+		@Bean
+		public DefaultMqttPahoClientFactory pahoClientFactory() {
+			DefaultMqttPahoClientFactory pahoClientFactory = new DefaultMqttPahoClientFactory();
+			MqttConnectOptions connectionOptions = new MqttConnectOptions();
+			connectionOptions.setServerURIs(new String[] { "tcp://localhost:1883" });
+			pahoClientFactory.setConnectionOptions(connectionOptions);
+			return pahoClientFactory;
+		}
+
+		@Bean
+		public IntegrationFlow mqttOutFlow() {
+			return f -> f.handle(new MqttPahoMessageHandler("jmxTestOut", pahoClientFactory()));
+		}
+
+		@Bean
+		public IntegrationFlow mqttInFlow() {
+			return IntegrationFlows.from(
+					new MqttPahoMessageDrivenChannelAdapter("jmxTestIn",
+							pahoClientFactory(), "jmxTests"))
+					.channel(c -> c.queue("fromMqttChannel"))
+					.get();
+		}
+
+	}
+
+}