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I'm developing and android application that communicates with a server through sockets. I have one thread reading the socket, while the Main thread executes an Asynk Task that sends the messages through the same socket. The problem here is, there are some times (lets say 50% of times) that I call the OutputStream.write() method of the socket connection, the servers log shows that the client closed connection. I read that the java sockets are full duplex, which should allow me to send and receive messages through the same socket with no problems. So, why is this happening? Is the problem from the application? Or is it the server side code (to which I dont have access) the one closing the connection??
EDIT: The server is a raspberry Pi with a raspbian distribution, running only one java application which is the one communicating with my application. I have no access to the java code running on the server, or any of the logs the server shows, I only can acces to the screen connected to the server, which prints the messages the code "tells" it to print. When I execute the OutputStram.Write(), the server shows on screen: "Connection to client closed".
Many Thanks!
Why does OutputStream.write() closes socket connection?
It doesn't. It writes data to an OutputStream.
some times (lets say 50% of times) that I call the OutputStream.write() method of the socket connection, the servers log shows that the client closed connection.
No it doesn't. This is what it really shows:
When I execute the OutputStram.Write(), the server shows on screen: "Connection to client closed".
This says that the server closed the connection to the client. Not the other way around.
Probably you sent something invalid and the server barfed.
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I have a very simple RMI server/client that works fine on the same machine. I'm trying to now execute the server on one machine and client on another both are on the same network. I've set the property to network ip using:
System.getProperty("java.rmi.server.hostname", "192.168.x.x");
& disabled firewall on both machines but connection still fails.
Client code uses this to connect to server (the server uses createRegistry on 8888):
Registry registry = LocateRegistry.getRegistry("192.168.x.x",8888);
ITest stub = (ITest) registry.lookup("ITest");
However the execption being caught seems to show an IP not the same as the one given eg 192.168.x.x vs 169.254.x.x:
Client exception: java.rmi.ConnectionException: Connection refused to host: 169.254.x.x; nested exception is ...
As dave_thompson_085 said I was calling System.getProperty which was a typo that I completely missed. Calling System.setProperty corrects this problem and now works.
There could be couple of reasons for this exception:
You have not started your rmiregistry in background.
You are trying to connect to the wrong port number.
Your firewall maybe blocking the connections.
Also make sure to check and disable any routers firewall restriction if any.
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So I previously asked a question if you can send a client that's on a ServerSocket to a different Server. But now, I'm asking how are the players for minecraft sent from server to server using Bungeecord and/or Lilypad. I know that LilyPad uses proxies and that stuff, but how are the players, when connected to the Bungeecord/Lilypad server, sent to a hub/spawn and/or to other servers, for example a survival server and a creative server. You can find servers like this on servers that have networks. I know that Treasure wars uses lilypad, if that helps.
You answered your own question. Both Bungeecord and LilyPad are proxy server networks.
Though a server cannot redirect the client to another server without permission, the server can connect to another server (meaning the first server is a proxy) and send back that server's messages to the client. As the first server is, in essence, also a client to the second server, the first server can switch connections at any desired point.
In other words, the Minecraft client does not switch connections at any point. The IP of the server to which the Minecraft client is connected never changes, and it is that of the proxy. However, what does change is the server to which the proxy is connected. If the client sends some message to the proxy stating that it wishes to go to the hub, the proxy then disconnects from whatever server it is currently connected to, connects to the hub, and sends back to the Minecraft client whatever messages the hub responds with.
This is my understanding based on a quick search.
However, if the Minecraft client was modded as well, it would be possible to have a similar network without the need for a proxy like so:
Client interacts with the server in such a way (maybe the player walks into a portal) as to trigger some server-switching action.
The server recognizes this interaction and, as both it and the client are modded, knows how to respond. The response is simply the sending of a message to the client, saying, "Switch to a different server. Here is the IP: (Insert IP here)"
The client receives the message from the server and, by its own will, disconnects from the current server and reconnects to the new one.
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I am working on Lan Messenger, how can I check if somebody else's system has LAN plugged in?
Means, is that person Online?
Also, our hostel has LAN, and I've tried running the client server program a lot many times, but it runs fine on my system (2 clients on the same machine as server) but it doesn't run when server and client are on different machines.
The code is perfectly fine.
What could be the reason? Any special Firewall settings to be changed for allowing packets?
I'm creating a chat server using sockets right now and the way I'm doing it is I have every user query the server about every 20-30 seconds. The server keeps track of the last time a user "refreshed" itself. If a user's gone a certain time period or more without doing so, then the server tells anyone trying to contact this user that they are offline.
Here is a VERY good reference to work off of. Take a look at the Server folder for the server-side and the src folder for the client-side:
https://code.google.com/p/simple-android-instant-messaging-application/source/browse/trunk/#trunk%2FServer%253Fstate%253Dclosed
If you only want to communicate within a LAN, then the socket implementatation in that link is defininitely what you want. If you want to communicate globally( a user in 1 LAN to a user in some other LAN ) then you'll want to redesign it a little so that your server socket is actually on some server accepting client connections. The current implementation creates a server socket within each client and accepts connections from other clients trying to communicate with it. This design breaks due to NAT routers (for reasons I'd rather not explain unless you really want to know).
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I have a server written in Java which basically awaits requests from different clients and serves their requests. I am running this server from Eclipse. This server is accessible on the local network but I want to be able to access this service from outside the local network. Is there any way to do this please?
P.S. I am a real beginner in these things
you can open a port in the router that leads to the one that the server is listening on. You then connect to your public IP. This ip can be found on
As #Java Player said, the problem is that your router(Nat) denied any incoming packet to your local network...briefly, there is many solutions for this:
Third party server: you must have a dedicated server that plays the role of intermediary, between your client/server programs.
Pros:
Solve completely the problem related to the Nat.
Cons:
In addition of your client, you must code another third party that forward packet to the desired destination.
BTW it gets a little heavy(waste of bandwidth).
Reversed connexion: the server and the client program are reversed, that's mean the client become a server and the server become a client, 'used by most of trojan...
Pros:
Very easy to implement this approach.
cons:
You must at least has an opened port.
Udp hole punching: this approach is used by perhaps all peer2peer solutions(eg: skype, utorrent...).
Pros:
You don't need to any router configuration.
Direct connection between peers.
Cons:
You need also a third party server called STUN server to get informations about your router.
Not all router that works with udp hole punching, you must consider also the first solution.
Writing a hole punching solution is not easy task.
You could also download something like Hamachi and then download Hamachi and sign into your network on the other PCs and Macs (and Linux...which is currently in beta). Then you'll want to be able to access your server on.
Main Hamachi product page
Linux Beta
This question already has answers here:
Java socket API: How to tell if a connection has been closed?
(9 answers)
Closed 9 years ago.
I'm trying to find out whether a Java TCP Socket is currently connected, the following just seems to tell me whether the socket has been connected at some point - not whether it is currently still connected.
socket.isConnected();
Any help appreciated, thanks.
Assuming you have some level of control over the protocol, I'm a big fan of sending heartbeats to verify that a connection is active. It's proven to be the most fail proof method and will often give you the quickest notification when a connection has been broken.
TCP keepalives will work, but what if the remote host is suddenly powered off? TCP can take a long time to timeout. On the other hand, if you have logic in your app that expects a heartbeat reply every x seconds, the first time you don't get them you know the connection no longer works, either by a network or a server issue on the remote side.
See Do I need to heartbeat to keep a TCP connection open? for more discussion.