在Java中實現MQTT并處理保留消息，你需要使用一個MQTT客戶端庫，例如Eclipse Paho。以下是一個簡單的示例，展示了如何使用Eclipse Paho MQTT客戶端庫連接到MQTT代理，訂閱主題并處理保留消息。

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，并連接到MQTT代理：

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

public class MQTTClient {
    public static void main(String[] args) {
        String brokerUrl = "tcp://broker.hivemq.com:1883";
        String clientId = "JavaSampleClient";
        String topic = "test/topic";

        MqttClient mqttClient = new MqttClient(brokerUrl, clientId);
        MemoryPersistence persistence = new MemoryPersistence();
        mqttClient.setPersistence(persistence);

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

3. 訂閱主題并處理保留消息：

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) {
        // ... 連接到MQTT代理的代碼 ...

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

            // 訂閱主題
            mqttClient.subscribe(topic);
            System.out.println("Subscribed to topic: " + topic);

            // 處理保留消息
            mqttClient.setCallback(new MqttCallback() {
                @Override
                public void messageArrived(String topic, MqttMessage message) {
                    System.out.println("Message arrived: " + new String(message.getPayload()));
                }

                @Override
                public void connectionLost(Throwable cause) {
                    System.out.println("Connection lost");
                }

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

            // 保持客戶端活躍
            Thread.sleep(60000);
        } catch (MqttException | InterruptedException e) {
            System.out.println("Failed to subscribe to topic or handle retained message");
            e.printStackTrace();
        } finally {
            try {
                mqttClient.disconnect();
                System.out.println("Disconnected from MQTT broker");
            } catch (MqttException e) {
                System.out.println("Failed to disconnect from MQTT broker");
                e.printStackTrace();
            }
        }
    }
}

在這個示例中，我們創建了一個MQTT客戶端實例，連接到MQTT代理，訂閱了test/topic主題，并設置了一個回調來處理保留消息。當其他客戶端向該主題發布消息時，我們的客戶端將收到這些消息，并在控制臺中打印出消息內容。