smack无法通过OfflineMessageManager检索脱机消息

Question

我在通过smack库从ejabberd服务器18.01获取脱机消息时遇到问题。 我有一个连接器方法

private static AbstractXMPPConnection connectToServer(String serverHost, Integer port, String userName, String userPassword) {

        DomainBareJid xmppDomain = null;
        try {
            xmppDomain = JidCreate.domainBareFrom(serverHost);
        } catch (XmppStringprepException e) {
            System.out.println("XMPP Domain initialization exception " + e.getMessage());
            return null;
        }

        XMPPTCPConnectionConfiguration config = XMPPTCPConnectionConfiguration.builder()
                .setXmppDomain(xmppDomain)
                .setHost(serverHost)
                .setPort(port)
                .setDebuggerEnabled(true)
                .setSendPresence(false)
                .setSecurityMode(ConnectionConfiguration.SecurityMode.disabled) 
                .build();


        AbstractXMPPConnection connection = new XMPPTCPConnection(config);

        try {
            connection.connect().login(userName, userPassword);
            return connection;
        } catch (XMPPException | SmackException | IOException | InterruptedException e) {
            System.out.println("Something went wrong while connection to server: " + e.getMessage());
            connection.disconnect();
            return null;
        }
    }

和离线消息接收器

private XMPPConnection connection;

    @Override
    public void run() {
        connection = Connector.getInstance();

        if (connection != null) {
            System.out.println("Start listening messages in the background");

            String offlineMessageOffline = "Offline message is %s from %s";

            OfflineMessageManager offlineMessageManager = new OfflineMessageManager(connection);
            try {
                if (offlineMessageManager.supportsFlexibleRetrieval()) {
                    offlineMessageManager.getMessages()
                            .forEach(message -> {
                                System.out.println(String.format(offlineMessageOffline, message.getBody(), message.getFrom()));
                            });
//                    offlineMessageManager.deleteMessages();
                }
            } catch (SmackException.NoResponseException | XMPPException.XMPPErrorException |
                    SmackException.NotConnectedException | InterruptedException e) {
                System.out.println(String.format("Error while getting offline messages from server: by the reason %s", e.getMessage()));
            }
            try {
                connection.sendStanza(new Presence(Presence.Type.available));
            } catch (SmackException.NotConnectedException e) {
                e.printStackTrace();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            ChatManager chatManager = ChatManager.getInstanceFor(connection);

            IncomingChatMessageListener listener = (entityBareJid, message, chat) -> {
                System.out.println(String.format("Incoming message is %s from %s", message.getBody(), message.getFrom()));
            };
            chatManager.addIncomingListener(listener);
        } else {
            System.out.println("Connection has not been established. Please try to choose option 1");
        }
    }

我开始在后台运行消息监听器。我看到该数据库有一个离线消息，在dubug节点中，我看到offlineMessageManager.getMessageCount（）返回正确数量的离线消息，但方法offlineMessageManager.getMessages（）返回注释。 smack版本是4.2.3但早期版本有同样的问题

更新
我找到了一种解决方法，因为它看起来不是一个明显的解决方案

public class ListenMessagesFromServerCommand implements Runnable {

    private XMPPConnection connection;

    @Override
    public void run() {
        connection = Connector.getInstance();

        if (connection != null) {
            System.out.println("Start listening messages in the background");

            String offlineMessageOffline = "Offline message is %s from %s";

            OfflineMessageManager offlineMessageManager = new OfflineMessageManager(connection);
            try {
                if (offlineMessageManager.supportsFlexibleRetrieval() && offlineMessageManager.getMessageCount() > 0) {
                    List<String> messagesNodes = offlineMessageManager.getHeaders().stream()
                            .map(OfflineMessageHeader::getStamp)
                            .collect(Collectors.toList());
                    offlineMessageManager.getMessages(messagesNodes)
                            .forEach(message -> {
                                System.out.println(String.format(offlineMessageOffline, message.getBody(), message.getFrom()));
                            });
                    offlineMessageManager.deleteMessages();
                }
            } catch (SmackException.NoResponseException | XMPPException.XMPPErrorException |
                    SmackException.NotConnectedException | InterruptedException e) {
                System.out.println(String.format("Error while getting offline messages from server: by the reason %s", e.getMessage()));
            }
            try {
                connection.sendStanza(new Presence(Presence.Type.available));
            } catch (SmackException.NotConnectedException | InterruptedException e) {
                System.out.println("Error sending presence available type " + e.getMessage());
            }

            ChatManager chatManager = ChatManager.getInstanceFor(connection);

            IncomingChatMessageListener listener = (entityBareJid, message, chat) -> {
                System.out.println(String.format("Incoming message is %s from %s", message.getBody(), message.getFrom()));
            };
            chatManager.addIncomingListener(listener);
        } else {
            System.out.println("Connection has not been established. Please try to choose option 1");
        }
    }
}