1. Open ports
EMQ X Provided by default 5 individual Listener, They will take up the following ports ：
| port | explain |
|---|---|
| 1883 | MQTT/TCP Protocol port |
| 11883 | MQTT/TCP Protocol internal port , For native client connections only |
| 8883 | MQTT/SSL Protocol port |
| 8083 | MQTT/WS Protocol port |
| 8084 | MQTT/WSS Protocol port |
ECS can be released on the security group port 
Or enter the command to open the corresponding port number firewall-cmd --zone=public --add-port=1883-11883/tcp --permanent // permanent firewall-cmd --zone=public --add-port=1883-11883/tcp // temporary firewall-cmd --zone=public --add-port=1883/tcp --permanent // Permanent opening 1883 port firewall-cmd --zone=public --remove-port=1883/tcp --permanent // Permanent ban 1883 port firewall-cmd --reload // Reload the firewall without changing state -- Configuration takes effect immediately
notes ： If you use the pagoda, remember not to forget that the pagoda also needs port clearance
2. download MqttSDK
MqttSDK Download link ：MqttSDK
Download and unzip and import into idea in 
Core abstract classes ：MessageHandler, use final modification ServerMqttClient Improve code security .
3. Sign in EMQ X Administrative console
Use the browser to open the address http:// The server ip:18083 , To log in, enter the default user name admin With default password public , Log in Dashboard
You can see that there is no client connected at this time
4. Publish subscriptions to messages
perform main Method , Look again Dashboard client , At this point, you can see that a user is connected 【 notes ： because EMQ Login authentication is not configured , All users can connect to anonymous login 】
Click on the console WebSocket, Subscribe to topics emq,QoS(0: At most one transmission /1: At least one transmission /2: Only one transmission ), Here, in order to ensure that the news will reach , We choose QoS=2
Modify the code ： The subject of subscription is java, Release message The theme of emq, perform main After the method, at the same time Dashboard Click publish subscribe , You can see the log printed by the console , And the topic message received by the console 
thus java Connect EMQ, And simulate the publication and subscription of topic messages OK 了 ！
【 Reference documents 】EMQ The website links ：EMQ Official website Return to upper level [Centos8.0-EMQ The construction and deployment of ]：Centos8.0-EMQ The construction and deployment of Go back to the next level [EMQ Data persistence ]