要维护和监控用户的在线状态，您可以使用以下方法：

1.通过使用 Firebase 实时数据库或其他实时数据库，在线状态数据存储在服务器上并在每个客户端上保持实时同步。

下面是一个使用 Firebase 实时数据库的示例代码：

FirebaseDatabase database = FirebaseDatabase.getInstance(); DatabaseReference myConnectionsRef = database.getReference("users/123/connections");

// Stores the timestamp of my last disconnect DatabaseReference lastOnlineRef = database.getReference("users/123/lastOnline"); FirebaseAuth auth = FirebaseAuth.getInstance();

// Get UID String uid = auth.getCurrentUser().getUid();

// Stores the timestamp of my last disconnect DatabaseReference lastOnlineRef = FirebaseDatabase.getInstance().getReference().child("users").child(uid).child("lastOnline");

// Stores my status DatabaseReference connectedRef = FirebaseDatabase.getInstance().getReference().child(".info/connected");

// Set status on connect/disconnect connectedRef.addValueEventListener(new ValueEventListener() { @Override public void onDataChange(DataSnapshot snapshot) { boolean connected = snapshot.getValue(Boolean.class); if (connected) { DatabaseReference con = FirebaseDatabase.getInstance().getReference("users/" + uid).child("online"); con.setValue(true); con.onDisconnect().setValue(false); lastOnlineRef.onDisconnect().setValue(ServerValue.TIMESTAMP); } }

@Override
public void onCancelled(DatabaseError error) {
    System.err.println("Listener was cancelled");
}

});

2.使用 WebSocket 协议或其他客户端-服务器通信协议建立长连接。这样做可以确保您的应用程序始终了解当前用户的在线状态。

下面是一个使用 WebSocket 的示例代码：

// Create a WebSocket connection WebSocket ws = new WebSocket("ws://localhost:8080");

// Send a message to the server ws.send("Hello, server!");

// Listen for messages from the server ws.onmessage = function(event) { console.log("Received message: " + event.data); };

// Listen for the WebSocket to close ws.onclose = function(event) { console.log("WebSocket closed with code: " + event.code); };

//