GH-3959: MqttConFailedEvent for normal disconnect (#3961)

* GH-3959: MqttConFailedEvent for normal disconnect

Fixes #3959

The `MqttCallback.disconnected(MqttDisconnectResponse)` in Paho v5 client is also
called when server initiates a disconnection.
In this case that `MqttDisconnectResponse` does not have a `cause` value

* Modify `MqttConnectionFailedEvent` to make a `cause` property optional
* Fix `Mqttv5PahoMessageDrivenChannelAdapter` & `Mqttv5PahoMessageHandler`
to not check for `cause`, but emit an `MqttConnectionFailedEvent` for any `disconnected()` calls

Unfortunately current Paho v3 client does not call `connectionLost()` for normal disconnections
and we cannot react for this callback with an `MqttConnectionFailedEvent`

**Cherry-pick to `5.5.x`**

* Fix language in doc

Co-authored-by: Gary Russell <[email protected]>

Co-authored-by: Gary Russell <[email protected]>

Author: artembilan

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/event/MqttConnectionFailedEvent.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2019 the original author or authors.
+ * Copyright 2015-2022 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.
@@ -16,15 +16,26 @@
 
 package org.springframework.integration.mqtt.event;
 
+import org.springframework.lang.Nullable;
+
 /**
+ * The {@link MqttIntegrationEvent} to notify about lost connection to the server.
+ * When normal disconnection is happened (initiated by the server), the {@code cause} is null.
+ *
  * @author Gary Russell
+ * @author Artem Bilan
+ *
  * @since 4.2.2
  *
  */
 @SuppressWarnings("serial")
 public class MqttConnectionFailedEvent extends MqttIntegrationEvent {
 
-	public MqttConnectionFailedEvent(Object source, Throwable cause) {
+	public MqttConnectionFailedEvent(Object source) {
+		super(source);
+	}
+
+	public MqttConnectionFailedEvent(Object source, @Nullable Throwable cause) {
 		super(source, cause);
 	}
 

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/event/MqttIntegrationEvent.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014-2020 the original author or authors.
+ * Copyright 2014-2022 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.
@@ -17,12 +17,15 @@
 package org.springframework.integration.mqtt.event;
 
 import org.springframework.integration.events.IntegrationEvent;
+import org.springframework.lang.Nullable;
 
 /**
- * Base class for Mqtt Events. For {@link #getSourceAsType()}, you should use a sub type
+ * Base class for Mqtt Events. For {@link #getSourceAsType()}, you should use a subtype
  * of {@link org.springframework.integration.mqtt.core.MqttComponent} for the receiving
  * variable.
+ *
  * @author Gary Russell
+ * @author Artem Bilan
  *
  * @since 4.1
  */
@@ -33,7 +36,7 @@ public MqttIntegrationEvent(Object source) {
 		super(source);
 	}
 
-	public MqttIntegrationEvent(Object source, Throwable cause) {
+	public MqttIntegrationEvent(Object source, @Nullable Throwable cause) {
 		super(source, cause);
 	}
 

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/inbound/Mqttv5PahoMessageDrivenChannelAdapter.java

@@ -292,7 +292,7 @@ public void messageArrived(String topic, MqttMessage mqttMessage) {
 	public void disconnected(MqttDisconnectResponse disconnectResponse) {
 		MqttException cause = disconnectResponse.getException();
 		ApplicationEventPublisher applicationEventPublisher = getApplicationEventPublisher();
-		if (cause != null && applicationEventPublisher != null) {
+		if (applicationEventPublisher != null) {
 			applicationEventPublisher.publishEvent(new MqttConnectionFailedEvent(this, cause));
 		}
 	}

spring-integration-mqtt/src/main/java/org/springframework/integration/mqtt/outbound/Mqttv5PahoMessageHandler.java

@@ -274,7 +274,7 @@ public void deliveryComplete(IMqttToken token) {
 	public void disconnected(MqttDisconnectResponse disconnectResponse) {
 		MqttException cause = disconnectResponse.getException();
 		ApplicationEventPublisher applicationEventPublisher = getApplicationEventPublisher();
-		if (cause != null && applicationEventPublisher != null) {
+		if (applicationEventPublisher != null) {
 			applicationEventPublisher.publishEvent(new MqttConnectionFailedEvent(this, cause));
 		}
 	}

src/reference/asciidoc/mqtt.adoc

@@ -404,6 +404,7 @@ public class MqttJavaApplication {
 Certain application events are published by the adapters.
 
 * `MqttConnectionFailedEvent` - published by both adapters if we fail to connect or a connection is subsequently lost.
+For the MQTT v5 Paho client, this event is also emitted when the server performs a normal disconnection, in which case the `cause` of the lost connection is `null`.
 * `MqttMessageSentEvent` - published by the outbound adapter when a message has been sent, if running in asynchronous mode.
 * `MqttMessageDeliveredEvent` - published by the outbound adapter when the client indicates that a message has been delivered, if running in asynchronous mode.
 * `MqttSubscribedEvent` - published by the inbound adapter after subscribing to the topics.