使用 paho MQTT 代理订阅时出错答案

【问题标题】：Error while subscribing with paho MQTT broker使用 paho MQTT 代理订阅时出错
【发布时间】：2019-08-21 19:37:13
【问题描述】：

我在使用用户名和密码订阅 MQTT 代理 (org.eclipse.paho.client.mqttv3.MqttClient) 时遇到错误：

版本：

Paho MQTT Client: org.eclipse.paho.client.mqttv3-1.2.0.jar
Mosquitto Version: mosquitto-1.5
mosquitto-auth-plugin: compatible with 1.5

错误@经纪人

1558438891: New connection from 192.168.50.128 on port 1883.
1558438891: |-- mosquitto_auth_unpwd_check(testUser)
1558438891: |-- ** checking backend sqlite
1558438891: |-- getuser(testUser) AUTHENTICATED=1 by sqlite
1558438891: New client connected from 192.168.50.128 as TestApp (c1, k60, u'testUser').
1558438891: No will message specified.
1558438891: Sending CONNACK to TestApp (0, 0)
1558438891: Received SUBSCRIBE from TestApp
1558438891:     MyTopic/# (QoS 1)
1558438891: Sending SUBACK to TestApp
1558438891: Socket error on client TestApp, disconnecting.
1558438891: |-- mosquitto_auth_acl_check(..., client id not available, testUser, MyTopic/#, MOSQ_ACL_WRITE)
1558438891: |-- aclcheck(testUser, MyTopic/#, 4) CACHEDAUTH: 17

代码片段：

options = new MqttConnectOptions();
//...
//...
options.setCleanSession(true);
options.setUserName(username);
options.setPassword(password.toCharArray());
//...

client = new MqttClient(clientHostname.toString(), "TestApp"); ===> OK
//...
client.subscribe(lS.getSubscriptionTopic(), 1); ===> Error
    MqttException (128)
    at org.eclipse.paho.client.mqttv3.MqttClient.subscribe(MqttClient.java:438)
    at org.eclipse.paho.client.mqttv3.MqttClient.subscribe(MqttClient.java:424)

【问题讨论】：

编辑问题以包含您使用的是什么版本的 mosquitto 和 mosquitto_auth_plugin？
还包括 testUser 的 ACL 表中的条目。
更新 mosquitto 版本信息
还有问题用户的 ACL 信息？

标签： java mqtt paho

【解决方案1】：

我在使用 paho MQTT 代理时遇到了同样的异常。

MqttException (128)

我的问题是 MQTT paho 客户端一次只能订阅 50 个订阅主题。如果我尝试用同一个 MQTT 客户端订阅第 51 个订阅主题，我会得到上述异常。

解决方案是在我收到关于该主题的回复后取消订阅该主题。

org.eclipse.paho.client.mqttv3.MqttClient mqttClient;

mqttClient.subscribe(String subscribeTopic);
mqttClient.publish(String publishTopic, String payload);

//Read the message published back on the subscribe topic.
//Now unsubscribe from that subscribe topic.

mqttClient.unsubscribe(String subscribeTopic);

【讨论】：