连接到 Mqtt 在线代理时出现 NullPointerException

Question

我正在尝试使用下面的代码和 Java 中的 Paho 库连接到在线代理https://test.mosquitto.org/ ：private final String brokerURI = "test.mosquitto.org:1883"; //should be changed to 8883 with SSLtry { //tentativo di creazione del client        client = new MqttClient(brokerURI, idClient); <--NullPointerException here        client.setCallback(new ClientCallback(codaTopic, codaMessaggi, finestra)); //set delle callback        setConnectionOptions(); //set delle opzioni connessione        client.connect(opzioni); //connessione al server    } catch (MqttException e) {        System.err.println(e.getMessage());        System.err.println("Connessione fallita Client, riavviare il sistema.");    }连接选项在这里设置：    private void setConnectionOptions() {    opzioni = new MqttConnectOptions();    opzioni.setAutomaticReconnect(true);    opzioni.setCleanSession(false);    opzioni.setConnectionTimeout(30);    opzioni.setKeepAliveInterval(60);}但它NullPointerException在创建MqttClient. 特别是控制台显示：Exception in thread "Thread-3" java.lang.NullPointerExceptionat org.eclipse.paho.client.mqttv3.MqttConnectOptions.validateURI(MqttConnectOptions.java:489)at org.eclipse.paho.client.mqttv3.MqttAsyncClient.<init>(MqttAsyncClient.java:291)at org.eclipse.paho.client.mqttv3.MqttAsyncClient.<init>(MqttAsyncClient.java:185)at org.eclipse.paho.client.mqttv3.MqttClient.<init>(MqttClient.java:226)at org.eclipse.paho.client.mqttv3.MqttClient.<init>(MqttClient.java:138)at client.Client.run(Client.java:78)我如何设法连接和使用 SSL？网上冲浪没有任何教程或指南有用，我已经下载了mosquitto.org.crtSSL 连接文件，但我不知道在哪里使用它，也没有找到教程。编辑将 BrokerUri 更改为private final String brokerURI = "tcp://test.mosquitto.org:1883"; //indirizzo broker在尝试使用指令订阅主题时client.subscribe(topic, 1);主题参数是一个包含主题名称的字符串。

Answer 1

Mosquitto 的 URI 需要协议。看看它的源代码，这是你的异常被抛出的地方，类MqttConnectOpts.java：

protected static int validateURI(String srvURI) {

    try {

        URI vURI = new URI(srvURI);

        if (!vURI.getPath().equals("")) {

            throw new IllegalArgumentException(srvURI);

        }

        if (vURI.getScheme().equals("tcp")) {

            return URI_TYPE_TCP;

        }

        else if (vURI.getScheme().equals("ssl")) {

            return URI_TYPE_SSL;

        }

        else if (vURI.getScheme().equals("local")) {

            return URI_TYPE_LOCAL;

        }

        else {

            throw new IllegalArgumentException(srvURI);

        }

    } catch (URISyntaxException ex) {

        throw new IllegalArgumentException(srvURI);

    }

}

因此，它接受 3 种类型的协议前缀：tcp, ssl, local. 关于您的示例，您可以这样尝试：

TCP

 private final String brokerURI = "tcp://test.mosquitto.org:1883";

SSL

private final String brokerURI = "ssl://test.mosquitto.org:8883";