在Java中實現MQTT客戶端時，處理重連是一個重要的任務，因為網絡不穩定或其他原因可能導致連接中斷。以下是一個簡單的示例，展示了如何使用MQTT客戶端庫（如Eclipse Paho）實現重連功能：

1. 首先，添加Eclipse Paho MQTT客戶端庫的依賴。如果你使用Maven，可以在pom.xml文件中添加以下依賴：

<dependency>
    <groupId>org.eclipse.paho</groupId>
    <artifactId>org.eclipse.paho.client.mqttv3</artifactId>
    <version>1.2.5</version>
</dependency>

2. 創建一個MQTT客戶端類，實現重連邏輯：

import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;

public class MQTTClient {
    private static final String BROKER_URL = "tcp://broker.hivemq.com:1883";
    private static final String CLIENT_ID = "JavaSampleClient";
    private static final String TOPIC = "test/topic";

    private MqttClient mqttClient;
    private MqttConnectOptions connectOptions;

    public MQTTClient() {
        connectOptions = new MqttConnectOptions();
        connectOptions.setCleanSession(true);
        connectOptions.setAutomaticReconnect(true);
        connectOptions.setConnectionTimeout(30);
        connectOptions.setKeepAliveInterval(60);
    }

    public void connect() throws MqttException {
        mqttClient = new MqttClient(BROKER_URL, CLIENT_ID, new MemoryPersistence());
        mqttClient.setCallback(new MqttCallback() {
            @Override
            public void connectionLost(MqttException cause) {
                System.out.println("Connection lost: " + cause.getMessage());
                reconnect();
            }

            @Override
            public void messageArrived(String topic, MqttMessage message) {
                System.out.println("Message arrived: " + new String(message.getPayload()));
            }

            @Override
            public void deliveryComplete(IMqttDeliveryToken token) {
            }
        });
        mqttClient.connect(connectOptions);
    }

    public void publishMessage(String message) throws MqttException {
        if (mqttClient != null && mqttClient.isConnected()) {
            mqttClient.publish(TOPIC, message.getBytes());
        } else {
            System.out.println("Client not connected, cannot publish message.");
        }
    }

    public void disconnect() throws MqttException {
        if (mqttClient != null && mqttClient.isConnected()) {
            mqttClient.disconnect();
        }
    }

    private void reconnect() {
        int retryCount = 0;
        boolean connected = false;

        while (!connected && retryCount < 5) {
            try {
                System.out.println("Reconnecting... (" + (retryCount + 1) + "/5)");
                Thread.sleep(2000); // Wait for 2 seconds before reconnecting
                connect();
                connected = true;
            } catch (MqttException | InterruptedException e) {
                System.out.println("Reconnection failed: " + e.getMessage());
                retryCount++;
            }
        }

        if (!connected) {
            System.out.println("Failed to reconnect after multiple attempts.");
        }
    }

    public static void main(String[] args) {
        MQTTClient mqttClient = new MQTTClient();
        try {
            mqttClient.connect();
            mqttClient.publishMessage("Hello, MQTT!");
            Thread.sleep(5000); // Wait for 5 seconds before disconnecting
            mqttClient.disconnect();
        } catch (MqttException | InterruptedException e) {
            System.out.println("Error: " + e.getMessage());
        }
    }
}

在這個示例中，我們創建了一個名為MQTTClient的類，它包含了連接、發布消息、斷開連接和重連的方法。connect()方法用于連接到MQTT代理，publishMessage()方法用于發布消息，disconnect()方法用于斷開連接，reconnect()方法用于在連接丟失時嘗試重新連接。

在main()方法中，我們創建了一個MQTTClient實例，連接到代理，發布一條消息，然后斷開連接。如果連接在發布消息過程中丟失，reconnect()方法會自動嘗試重新連接。