java mqtt连接丢失重连后无法订阅之前的主题

在MQTT连接丢失重连后，需要重新订阅之前的主题。这是因为MQTT客户端在连接丢失后会自动断开之前的订阅，所以需要在重连后重新订阅。

以下是一个示例代码，展示了Java中如何重新订阅之前的主题：

import org.eclipse.paho.client.mqttv3.IMqttClient;
import org.eclipse.paho.client.mqttv3.IMqttMessageListener;
import org.eclipse.paho.client.mqttv3.IMqttToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class MqttReconnectExample {

    private static final String BROKER = "tcp://mqtt.eclipse.org:1883";
    private static final String CLIENT_ID = "java-mqtt-client";

    private static IMqttClient mqttClient;

    public static void main(String[] args) throws MqttException {
        MemoryPersistence persistence = new MemoryPersistence();
        mqttClient = new MqttClient(BROKER, CLIENT_ID, persistence);

        MqttConnectOptions connOpts = new MqttConnectOptions();
        connOpts.setCleanSession(true);

        mqttClient.setCallback(new MqttCallback() {
            @Override
            public void connectionLost(Throwable cause) {
                // 处理连接丢失的逻辑
                System.out.println("Connection lost! Reconnecting...");
                try {
                    mqttClient.reconnect();
                } catch (MqttException e) {
                    e.printStackTrace();
                }
            }

            @Override
            public void messageArrived(String topic, MqttMessage message) throws Exception {
                // 处理收到消息的逻辑
                System.out.println("Received message: " + new String(message.getPayload()));
            }

            @Override
            public void deliveryComplete(IMqttDeliveryToken token) {
                // 处理消息发送完成的逻辑
            }
        });

        mqttClient.connect(connOpts);
        System.out.println("Connected to MQTT broker");

        // 在订阅之前的主题之前，先重新连接
        IMqttToken token = mqttClient.connectWithResult(connOpts);
        token.waitForCompletion();

        // 重新订阅之前的主题
        String[] previousTopics = {"topic1", "topic2"};
        int[] qos = {0, 1};
        mqttClient.subscribe(previousTopics, qos);

        // 阻塞主线程，保持连接
        while (true) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

在上述代码中，当连接丢失时，会调用connectionLost()方法进行重连。在重连完成后，通过mqttClient.subscribe()方法重新订阅之前的主题