GH-3732: Fix NPE in Mqttv5PahoMessageDrivenChA

Fixes #3732

The `Mqttv5PahoMessageDrivenChannelAdapter` unconditionally tries to (un)subscribe
to/from topics when the `mqqtClient` might not be initialized yet.

* Add `mqqtClient` initialization check before adding or removing topic.
Re-align logic with the `MqttPahoMessageDrivenChannelAdapter`

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

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -17,6 +17,7 @@
 package org.springframework.integration.mqtt.inbound;
 
 import java.util.LinkedHashSet;
+import java.util.Objects;
 import java.util.Set;
 import java.util.concurrent.locks.Lock;
 import java.util.concurrent.locks.ReentrantLock;
@@ -40,6 +41,7 @@
  * @author Gary Russell
  * @author Artem Bilan
  * @author Trung Pham
+ * @author Mikhail Polivakha
  *
  * @since 4.0
  *
@@ -331,15 +333,7 @@ public boolean equals(Object obj) {
 				return false;
 			}
 			Topic other = (Topic) obj;
-			if (this.topic == null) {
-				if (other.topic != null) {
-					return false;
-				}
-			}
-			else if (!this.topic.equals(other.topic)) {
-				return false;
-			}
-			return true;
+			return Objects.equals(this.topic, other.topic);
 		}
 
 		@Override

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 the original author or authors.
+ * Copyright 2021-2022 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.
@@ -65,6 +65,7 @@
  * See {@link #setPayloadType} for more information about type conversion.
  *
  * @author Artem Bilan
+ * @author Mikhail Polivakha
  *
  * @since 5.5.5
  *
@@ -141,13 +142,15 @@ public void setHeaderMapper(HeaderMapper<MqttProperties> headerMapper) {
 	@Override
 	protected void onInit() {
 		super.onInit();
-		try {
-			this.mqttClient = new MqttAsyncClient(getUrl(), getClientId(), this.persistence);
-			this.mqttClient.setCallback(this);
-			this.mqttClient.setManualAcks(isManualAcks());
-		}
-		catch (MqttException ex) {
-			throw new BeanCreationException("Cannot create 'MqttAsyncClient' for: " + getComponentName(), ex);
+		if (this.mqttClient == null) {
+			try {
+				this.mqttClient = new MqttAsyncClient(getUrl(), getClientId(), this.persistence);
+				this.mqttClient.setCallback(this);
+				this.mqttClient.setManualAcks(isManualAcks());
+			}
+			catch (MqttException ex) {
+				throw new BeanCreationException("Cannot create 'MqttAsyncClient' for: " + getComponentName(), ex);
+			}
 		}
 		if (this.messageConverter == null) {
 			setMessageConverter(getBeanFactory()
@@ -189,8 +192,10 @@ protected void doStop() {
 		this.topicLock.lock();
 		String[] topics = getTopic();
 		try {
-			this.mqttClient.unsubscribe(topics).waitForCompletion(getCompletionTimeout());
-			this.mqttClient.disconnect().waitForCompletion(getCompletionTimeout());
+			if (this.mqttClient != null && this.mqttClient.isConnected()) {
+				this.mqttClient.unsubscribe(topics).waitForCompletion(getCompletionTimeout());
+				this.mqttClient.disconnect().waitForCompletion(getCompletionTimeout());
+			}
 		}
 		catch (MqttException ex) {
 			logger.error(ex, () -> "Error unsubscribing from " + Arrays.toString(topics));
@@ -204,7 +209,9 @@ protected void doStop() {
 	public void destroy() {
 		super.destroy();
 		try {
-			this.mqttClient.close(true);
+			if (this.mqttClient != null) {
+				this.mqttClient.close(true);
+			}
 		}
 		catch (MqttException ex) {
 			logger.error(ex, "Failed to close 'MqttAsyncClient'");
@@ -215,8 +222,10 @@ public void destroy() {
 	public void addTopic(String topic, int qos) {
 		this.topicLock.lock();
 		try {
-			this.mqttClient.subscribe(topic, qos).waitForCompletion(getCompletionTimeout());
 			super.addTopic(topic, qos);
+			if (this.mqttClient != null && this.mqttClient.isConnected()) {
+				this.mqttClient.subscribe(topic, qos).waitForCompletion(getCompletionTimeout());
+			}
 		}
 		catch (MqttException ex) {
 			throw new MessagingException("Failed to subscribe to topic " + topic, ex);
@@ -230,7 +239,9 @@ public void addTopic(String topic, int qos) {
 	public void removeTopic(String... topic) {
 		this.topicLock.lock();
 		try {
-			this.mqttClient.unsubscribe(topic).waitForCompletion(getCompletionTimeout());
+			if (this.mqttClient != null && this.mqttClient.isConnected()) {
+				this.mqttClient.unsubscribe(topic).waitForCompletion(getCompletionTimeout());
+			}
 			super.removeTopic(topic);
 		}
 		catch (MqttException ex) {

spring-integration-mqtt/src/test/java/org/springframework/integration/mqtt/Mqttv5BackToBackTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2021 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -22,6 +22,7 @@
 import java.util.ArrayList;
 import java.util.List;
 
+import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.beans.factory.annotation.Autowired;
@@ -55,6 +56,7 @@
 /**
  * @author Gary Russell
  * @author Artem Bilan
+ * @author Mikhail Polivakha
  *
  * @since 5.5.5
  *
@@ -73,6 +75,18 @@ public class Mqttv5BackToBackTests implements MosquittoContainerTest {
 	@Autowired
 	private Config config;
 
+	@Test //GH-3732
+	public void testNoNpeIsNotThrownInCaseDoInitIsNotInvokedBeforeTopicAddition() {
+		Mqttv5PahoMessageDrivenChannelAdapter channelAdapter = new Mqttv5PahoMessageDrivenChannelAdapter("tcp://mock-url.com:8091", "mock-client-id", "123");
+		Assertions.assertDoesNotThrow(() -> channelAdapter.addTopic("abc", 1));
+	}
+
+	@Test //GH-3732
+	public void testNoNpeIsNotThrownInCaseDoInitIsNotInvokedBeforeTopicRemoval() {
+		Mqttv5PahoMessageDrivenChannelAdapter channelAdapter = new Mqttv5PahoMessageDrivenChannelAdapter("tcp://mock-url.com:8091", "mock-client-id", "123");
+		Assertions.assertDoesNotThrow(() -> channelAdapter.removeTopic("abc"));
+	}
+
 	@Test
 	public void testSimpleMqttv5Interaction() {
 		String testPayload = "foo";