在Java中實現MQTT會話管理，你需要使用一個MQTT客戶端庫。一個流行的庫是Eclipse Paho。以下是一個簡單的示例，展示了如何使用Eclipse Paho MQTT客戶端庫來管理MQTT會話：

1. 首先，確保你已經將Eclipse Paho MQTT客戶端庫添加到項目的依賴中。如果你使用的是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客戶端實例，并指定MQTT代理服務器地址、客戶端ID和主題：

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";

    public static void main(String[] args) {
        MqttClient mqttClient = new MqttClient(BROKER_URL, CLIENT_ID, new MemoryPersistence());
    }
}

3. 實現MqttCallback接口，以便在連接、斷開連接、收到消息等事件發生時執行相應的操作：

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

public class MQTTClient {
    // ... 其他代碼 ...

    public static void main(String[] args) {
        MqttClient mqttClient = new MqttClient(BROKER_URL, CLIENT_ID, new MemoryPersistence());
        mqttClient.setCallback(new MqttCallback() {
            @Override
            public void connectionLost(Throwable cause) {
                System.out.println("Connection lost: " + cause.getMessage());
            }

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

            @Override
            public void deliveryComplete(IMqttDeliveryToken token) {
                System.out.println("Delivery complete");
            }
        });
    }
}

4. 連接到MQTT代理服務器：

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

public class MQTTClient {
    // ... 其他代碼 ...

    public static void main(String[] args) {
        MqttClient mqttClient = new MqttClient(BROKER_URL, CLIENT_ID, new MemoryPersistence());
        mqttClient.setCallback(new MqttCallback() {
            // ... 其他回調方法 ...
        });

        try {
            mqttClient.connect();
            System.out.println("Connected to MQTT broker");
        } catch (MqttException e) {
            System.out.println("Failed to connect to MQTT broker: " + e.getMessage());
        }
    }
}

5. 訂閱指定的主題：

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

public class MQTTClient {
    // ... 其他代碼 ...

    public static void main(String[] args) {
        MqttClient mqttClient = new MqttClient(BROKER_URL, CLIENT_ID, new MemoryPersistence());
        mqttClient.setCallback(new MqttCallback() {
            // ... 其他回調方法 ...
        });

        try {
            mqttClient.connect();
            System.out.println("Connected to MQTT broker");
            mqttClient.subscribe(TOPIC);
            System.out.println("Subscribed to topic: " + TOPIC);
        } catch (MqttException e) {
            System.out.println("Failed to connect to MQTT broker: " + e.getMessage());
        }
    }
}

6. 發布消息到指定的主題：

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

public class MQTTClient {
    // ... 其他代碼 ...

    public static void main(String[] args) {
        MqttClient mqttClient = new MqttClient(BROKER_URL, CLIENT_ID, new MemoryPersistence());
        mqttClient.setCallback(new MqttCallback() {
            // ... 其他回調方法 ...
        });

        try {
            mqttClient.connect();
            System.out.println("Connected to MQTT broker");
            mqttClient.subscribe(TOPIC);
            System.out.println("Subscribed to topic: " + TOPIC);

            MqttMessage message = new MqttMessage(TOPIC.getBytes());
            message.setQos(1);
            mqttClient.publish(TOPIC, message);
            System.out.println("Published message to topic: " + TOPIC);
        } catch (MqttException e) {
            System.out.println("Failed to connect to MQTT broker: " + e.getMessage());
        }
    }
}

這個示例展示了如何使用Eclipse Paho MQTT客戶端庫在Java中實現基本的MQTT會話管理。你可以根據實際需求對這個示例進行擴展，例如使用連接池、處理重連邏輯等。