文章只是選取了其中一段，無XMPP基礎的人可能看起來有點復雜;

假設我們註冊了一個用戶，用戶名叫shimiso，那麽我們如何讓shimiso這個用戶一登陸就取到離線消息呢？

PPConnection.DEBUG_ENABLED = false;
    AccountManager accountManager;
    final ConnectionConfiguration connectionConfig = new ConnectionConfiguration(
        "192.168.1.78", Integer.parseInt("5222"), "csdn.shimiso.com");
 
    
 
// 允許自動連接
    connectionConfig.setReconnectionAllowed(true);
    connectionConfig.setSendPresence(false);//不要告訴服務器自己的狀態
    Connection connection = new XMPPConnection(connectionConfig);
    try {
      connection.connect();// 開啟連接
      accountManager = connection.getAccountManager();// 獲取賬戶管理類
    } catch
 
 (XMPPException e) {
      throw new IllegalStateException(e);
    } 
    connection.login("shimiso", "123","SmackTest"); 
    OfflineMessageManager offlineManager = new OfflineMessageManager(
        connection);
    try {
      Iterator<org.jivesoftware.smack.packet.Message> it = offlineManager
          .getMessages();
 
      System.out.println(offlineManager.supportsFlexibleRetrieval());
      System.out.println(
 
"離線消息數量: " + offlineManager.getMessageCount());
 
      Map<String, ArrayList<Message>> offlineMsgs = new HashMap<String, ArrayList<Message>>();
 
      while (it.hasNext()) {
        org.jivesoftware.smack.packet.Message message = it.next();
        System.out
            .println("收到離線消息, Received from 【" + message.getFrom()
                + "】 message: " + message.getBody());
        String fromUser = message.getFrom().split("/")[0];
 
        if (offlineMsgs.containsKey(fromUser)) {
          offlineMsgs.get(fromUser).add(message);
        } else {
          ArrayList<Message> temp = new ArrayList<Message>();
          temp.add(message);
          offlineMsgs.put(fromUser, temp);
        }
      }
 
      // 在這裏進行處理離線消息集合......
      Set<String> keys = offlineMsgs.keySet();
      Iterator<String> offIt = keys.iterator();
      while (offIt.hasNext()) {
        String key = offIt.next();
        ArrayList<Message> ms = offlineMsgs.get(key);
 
        for (int i = 0; i < ms.size(); i++) {
          System.out.println("-->" + ms.get(i));
        }
      }
 
      offlineManager.deleteMessages();
    } catch (Exception e) {
      e.printStackTrace();
    }
    offlineManager.deleteMessages();//刪除所有離線消息
    Presence presence = new Presence(Presence.Type.available);
                nnection.sendPacket(presence);//上線了
                nnection.disconnect();//關閉連接

運行結果：

這裏我們需要特別當心的是先不要告訴openfire服務器你上線了，否則永遠也拿不到離線消息，用下面老外的話將就是在你上線之前去獲取離線消息，這麽設計是很有道理的。

The OfflineMessageManager helps manage offline messages even before the user has sent an available presence. When a user asks for his offline messages before sending an available presence then the server will not send a flood with all the offline messages when the user becomes online. The server will not send a flood with all the offline messages to the session that made the offline messages request or to any other session used by the user that becomes online.

同時拿到離線消息以後刪除離線消息offlineManager.deleteMessages();，同是通知服務器自己上線了。

https://blog.csdn.net/kazeik/article/details/44344609

Xmpp獲取離線消息