Question

我正在为订阅主题的MQTT客户端寻找一个java代码，在该主题上发布的每条消息都应该只到达客户端一次。我写了很多代码，在所有情况下，消息都正确地传递给客户端当它连接到代理但是如果订阅的客户端与代理断开连接一段时间然后再次连接回来，则它不会收到在它未连接期间发送的消息并且我已设置了干净的会话标志也是假的，但仍然不起作用，我使用的代码如下所示

import org.fusesource.hawtbuf.*;
import org.fusesource.mqtt.client.*;

/**
 * Uses an callback based interface to MQTT.  Callback based interfaces
 * are harder to use but are slightly more efficient.
 */
class Listener {

    public static void main(String []args) throws Exception {

        String user = env("APOLLO_USER", "admin");
        String password = env("APOLLO_PASSWORD", "password");
        String host = env("APOLLO_HOST", "localhost");
        int port = Integer.parseInt(env("APOLLO_PORT", "61613"));
        final String destination = arg(args, 1, "subject");


        MQTT mqtt = new MQTT();
        mqtt.setHost(host, port);
        mqtt.setUserName(user);
        mqtt.setPassword(password);
    mqtt.setCleanSession(false);
    mqtt.setClientId("newclient");

        final CallbackConnection connection = mqtt.callbackConnection();
        connection.listener(new org.fusesource.mqtt.client.Listener() {
            long count = 0;
            long start = System.currentTimeMillis();

            public void onConnected() {
            }
            public void onDisconnected() {
            }
            public void onFailure(Throwable value) {
                value.printStackTrace();
                System.exit(-2);
            }
            public void onPublish(UTF8Buffer topic, Buffer msg, Runnable ack) {
                System.out.println("Nisha Messages : " + msg);
                System.out.println("Nisha topic" + topic);
                System.out.println("Nisha Receive acknowledgement : " + ack);
                String body = msg.utf8().toString();
                if("SHUTDOWN".equals(body)) {
                    long diff = System.currentTimeMillis() - start;
                    System.out.println(String.format("Received %d in %.2f seconds", count, (1.0*diff/1000.0)));
                    connection.disconnect(new Callback<Void>() {
                        @Override
                        public void onSuccess(Void value) {
                            System.exit(0);
                        }
                        @Override
                        public void onFailure(Throwable value) {
                            value.printStackTrace();
                            System.exit(-2);
                        }
                    });
                } else {
                    if( count == 0 ) {
                        start = System.currentTimeMillis();
                    }
                    if( count % 1000 == 0 ) {
                        System.out.println(String.format("Received %d messages.", count));
                    }
                    count ++;
                }
            }
        });
        connection.connect(new Callback<Void>() {
            @Override
            public void onSuccess(Void value) {
                System.out.println("connected in :::: ");
                Topic[] topics = {new Topic(destination, QoS.AT_MOST_ONCE)};
                connection.subscribe(topics, new Callback<byte[]>() {
                    public void onSuccess(byte[] qoses) {
                    }
                    public void onFailure(Throwable value) {
                        value.printStackTrace();
                        System.exit(-2);
                    }
                });
            }
            @Override
            public void onFailure(Throwable value) {
                value.printStackTrace();
                System.exit(-2);
            }
        });

        // Wait forever..
        synchronized (Listener.class) {
            while(true)
                Listener.class.wait();
        }
    }

    private static String env(String key, String defaultValue) {
        String rc = System.getenv(key);
        if( rc== null )
            return defaultValue;
        return rc;
    }

    private static String arg(String []args, int index, String defaultValue) {
        if( index < args.length )
            return args[index];
        else
            return defaultValue;
    }
}

我在这里做错了吗？

Answer 1

它没有收到在未连接期间发送的消息

MQTT 不保留所有邮件。如果客户端脱机，则未传递的消息将丢失。保留机制仅保留发布到主题的最后消息。

您可以read more in the specs指向 3.3.1.3 RETAIN

正确的MQTT订阅代码，用于维护持久性

1 个答案: