要在AWS IoT Java SDK中实现在消息发布后断开连接的代理，可以按照以下步骤进行操作：

1. 首先，确保您已经安装了AWS IoT Java SDK。您可以在项目的pom.xml文件中添加以下依赖项：

    com.amazonaws
    aws-iot-device-sdk-java
    1.4.2

2. 创建一个Java类来实现代理。在该类中，您可以设置AWS IoT连接，发布消息，并在发布后断开连接。

import com.amazonaws.services.iot.client.AWSIotConnectionStatusCallback;
import com.amazonaws.services.iot.client.AWSIotDevice;
import com.amazonaws.services.iot.client.AWSIotDeviceProperty;
import com.amazonaws.services.iot.client.AWSIotMqttClient;
import com.amazonaws.services.iot.client.AWSIotQos;
import com.amazonaws.services.iot.client.AWSIotTimeoutException;
import com.amazonaws.services.iot.client.AWSIotTopic;
import com.amazonaws.services.iot.client.AWSIotException;
import com.amazonaws.services.iot.client.AWSIotConnectionStatus;

public class DisconnectAfterPublishProxy {

    private static final String ENDPOINT = ""; // AWS IoT端点
    private static final String CLIENT_ID = ""; // 客户端ID
    private static final String KEY_STORE_PATH = ""; // 密钥库路径
    private static final String KEY_STORE_PASSWORD = ""; // 密钥库密码

    private static final String TOPIC = ""; // 发布的主题
    private static final AWSIotQos QOS = AWSIotQos.QOS0; // 消息质量

    public static void main(String[] args) {
        AWSIotMqttClient client = new AWSIotMqttClient(ENDPOINT, CLIENT_ID, KEY_STORE_PATH, KEY_STORE_PASSWORD);

        try {
            client.connect();

            // 创建代理设备
            DisconnectAfterPublishDevice device = new DisconnectAfterPublishDevice(CLIENT_ID);
            device.setClient(client);
            device.connect();

            // 发布消息
            String message = "Hello, AWS IoT!";
            device.publish(TOPIC, QOS, message.getBytes());

            // 断开连接
            device.disconnect();

            System.out.println("Message published and connection disconnected successfully.");
        } catch (AWSIotException | AWSIotTimeoutException e) {
            e.printStackTrace();
        }
    }

    // 自定义设备类
    private static class DisconnectAfterPublishDevice extends AWSIotDevice {
        public DisconnectAfterPublishDevice(String thingName) {
            super(thingName);
        }

        @AWSIotDeviceProperty(topic = "status", read = true, write = true)
        private AWSIotConnectionStatus connectionStatus;

        // 连接状态回调
        public void onConnectionStatusChanged(final AWSIotConnectionStatus status, final Throwable throwable) {
            if (status == AWSIotConnectionStatus.DISCONNECTED) {
                System.out.println("Connection is disconnected.");
            }
        }
    }
}

在上述代码中，您需要将，，，和替换为您自己的信息。

3. 运行代理代码，它将连接到AWS IoT并发布一条消息后断开连接。您可以在onConnectionStatusChanged方法中执行其他操作，以响应连接状态的更改。

请注意，上述示例中的代码是基于AWS IoT Java SDK 1.4.2版本编写的，如果您使用的是其他版本，请相应地调整代码。