我使用Amazon MQ创建了一个单实例代理，并且能够使用eclipse Paho MQTT Client仅使用用户名和密码来订阅代理。

代码：

//sample endpoint from Amazon MQ
final String WIRE_LEVEL_ENDPOINT = "ssl://b-1234a5b6-78cd-901e-2fgh-3i45j6k178l9-1.mq.us-east-2.amazonaws.com:8883";
final String actIVE_MQ_username = "user";
final String actIVE_MQ_PASSWORD = "password";

// Specify the topic name and the message text.
final String topic = "whatever";
final String text = "Hello from Amazon MQ!";

// Create the MQTT client and specify the connection options.
final String clientId = "abc123";
final MqttClient client = new MqttClient(WIRE_LEVEL_ENDPOINT,clientId);
final MqttConnectOptions connOpts = new MqttConnectOptions();

// Pass the username and password.
connOpts.setusername(actIVE_MQ_username);
connOpts.setPassword(actIVE_MQ_PASSWORD.toCharArray());

// Create a session and subscribe to a topic filter.
client.connect(connOpts);
client.setCallback(this);
client.subscribe(topic);

// Create a message.
final MqttMessage message = new MqttMessage(text.getBytes());

// Publish the message to a topic.
client.publish(topic,message);
System.out.println("Published message.");

// Wait for the message to be received.
Thread.sleep(3000L);

// Clean up the connection.
client.disconnect();

运行上面的代码表明，我可以订阅该主题，还可以接收我发送的消息。

但是，使用mosquitto_sub客户端执行相同操作，会给我协议错误：

mosquitto_sub -h host -p 8883 -u user -P password -t whatever -i abc123

错误：

  

错误：与主机通讯时发生网络协议错误   经纪人。

我搜索了如何使用SSL连接到MQTT Broker。我发现我需要为客户端设置证书。

但是在Java中它如何在没有任何证书集的情况下起作用？