Java NIO Server Example

more stuff on nio is always welcome in my book :)

i am very interested to know exactly what the “few major flaws and a few unfixed bugs” are – as much as i dont really like the api, my game im making seems to be working ok so far – i mostly used online tutorials to help me learn nio. Only thing i can thing of is maybe a lack of built in timeout handling?

Hi there,
was interested by your post and project…
I am in the analysis phase of a big project to build a Internet Chess Server from Scratch that will eventually support 5.000 simult. users
could you please reply (using my email) so that we can talk a little about this framework and its suitability for such a project.
also interested in your skills

thanks

Hello,

I am (with educational purposes) making a server for a MMORPG-game, that is supposed to be able to hold up to around 2000 players at once, so I got a few questions to ask:

1. Does the RAM-requirement (either if it is 30.000 kB or above 100.000 kB) have any visible impact on the servers efficiency?

2. What would be the best method to handle static game-data (XML files using XStream, or something else)?

3. What algorithm should preferrably be used for area-searching, to see which entities overlap the view-field of the current entity?

4. Do you know of any very good books about the subject MMORPG-game development for Java, or something covering other heavy server-frameworks?

First of all, I want to thank you for the work you put into this. It looks quite nice. Also wanted to tell you that I found a bug in the code. If a client sends to the server a partial message the server spins trying to read the message even do the client has disconnected. It does not look like a bug in your code, it may be an issue in windows. However this is what I did to reproduce. I connected to the server using telnet and typed the following “1234567890”, the server read the first 4 bytes set it self to wait for a large message. On the telnet side, I pressed ctrl+] and quited out of telnet. The server stayed in a loop trying to read from client.

I read somewhere that selectors can only handle 64 sockets, I can’t recall were I read this and as you said the doc on nio is spotty at best. If it is so, what would you think is the best approach on your code to handle this. 1) create a thread for each selector or to process all the selectors using the same thread iterating threw the selectors ?

Thank You Again
Diego Cassinera

Adam, I’m running into another issue, and again I don’t think its your code.
During ProcessStringMessage I’m attaching my session object to the key. On following calls to ProcessStringMessage for the same client, the attachment is gone.
I read the documentation and it does not say anything about when or for how long the attachment will be there so I’m going with the assumption that is for the life of the key.

I know that its being set because I can retrieve it with a call attachment() after I have set it. Originally I was doing the attachment from a different thread, but I have changed my code so its done in ProcessStringMessage.

I also looked into your code and you are not using the attachment, so the only conclusion I have is that java lost it.
The attachment its not crucial since I can store my session object in a hashtable, however it would be nice to get to my session without the need of a lookup.

Also I have run the code under the debugger with a watch on the attachment, the watch does not trigger, but attachment still returns null.

Thank You
Again Diego

Thanks.

In regards to the spinning bug that I mentioned before. The simple fix is to add the following code to readIncomingMessageFromKey
if (bytesRead == -1)
{
throw new IOException(“Channel Closed”);
}
after
if( bufferIsEmpty )
{
bytesRead = ((ReadableByteChannel) key.channel()).read( bb );
}

Another question
In StringBasedServer.run after

Set keys = selector.selectedKeys();
Iterator iterator = keys.iterator();
while( iterator.hasNext() )

Hi Diego,

I need to write a chat and a voice chat server, which should support around 1000 users, Could you please guide me in this?

~Sandy

Sandy,

I’m not quite sure if this is the place to discuss a specific implementation of a chat server using this code, so I just will give you a gist.

1)Created a class that extends the StringBasedServer class.
In the overloaded processStringMessage member function I enqueue the received message along with its SelectionKey into a LinkedBlockingQueue.
2)Created a class called MessagePump that implements Runnable and dequeues messages from the LinkedBlockingQueue. This class implements most of the chat server protocol that I’m using. For example when a message its dequeued from the queue and the SelectionKey has never been seen by this class, a session object its created. In the session object I store the SelectionKey, and all other chat protocol information. The session object is stored in a Hashtable that uses the SelectionKey as its key.

3)To support 1000’s of users I did some modifications to Adam’s code. The key modifications that I did is to be able to extend StringBasedServer in a non listening mode. I did this mainly so I can have multiple threads that extend StringBasedServer each one processing all IO for a subset of the open connections. For my server the order at which messages are sent and received is not important, so I have multiple instances of StringBasedServer and MessagePump. Overall its more important to have multiple instances of message pump since one incoming message can become 1000 of outgoing messages. Also since the message pump implements the protocol its a bit more cpu intense than the network worker thread.

Since this is Adam’s page I’m not quite sure if he is interested on the details of a full blown description of my implementation. Theres are a few other modifications that I did, but this is just a gist of what I had to do. If you need to contact me directly, I can be reached at http://www.uiija.com, use the contact us link at the bottom of the page.

Hi Adam,

I opted to let the client connect, but then I send them a ‘please disconnect’ command. My client sidegets the message then politely closes the connection from its side. That seems to work great. The only thing I’m worried about is – the server right now can’t hard kick a user if it wants to, it needs to ask the client to do it! I’ll give your recommendation a try though, it sounds like another correct way of doing it.

The code you posted is great by the way, I was previously using a classic threaded server model, this architecture is much easier to work with in my opinion,

Thanks!

I just wanted to let everybody know that I’m running into an issue where the selector stops responding after running for 6 hours. I verified that the thread still running, however, selectedKeys() stops returning any keys. This is most probable a VM bug, I’m running 1.6.0.2 from sun on linux suse linux.

To work around the issue I added code to recreate the selector after 60 seconds of inactivity and no connected clients.
The code of the change I made follows:

Renamed the run function to runEx.
In the runEx function after while( iterator.hasNext() ) added
lastMessage = System.currentTimeMillis();

After lastPostProcessBegan = currentPostProcessBegan; added the following code to break out of the function if no messages were received in the las 60 seconds and I have no open connections.

if (currentPostProcessBegan – lastMessage > 60000 &&
connectedChannels.isEmpty())
{
System.out.println (Thread.currentThread().getName()+”::runEx “+
“No messages in the last 60 secs (rerun)”);
return;
}

created a new run function that calls runEx the function looks like this

public void run ()
{
while (status != STOPPING)
{
lastMessage = System.currentTimeMillis();
status = STARTED;
runEx();
if (listener)
{
try
{
ssc.close();
}
catch (Exception e)
{
e.printStackTrace();
}
ssc = null;
}
}
}

Note that the member variable listener is something I added to the code to support multiple threads working in parallel to support 1000’s of users where only one of the threads acts as a listener.

This statement is incorrect
This is done to really test if the socket its really open.
This is not done to test, is done to force an error if the connection is stale

Thanks Diego, I’ll give it a try.

If I’m reading the source right, there’s just a single thread handling the IO stuff – so everything is synchronized already? For example, If I have clients connecting, sending messages, and clients dropping their connection all at the same time, in the server class I’ll see everything serialized, something like:

processStringMessage();
processStringMessage();
keyCancelled();
processStringMessage();
keyCancelled();
processStringMessage();
etc…

I ask because I’m making a small player manager class which keeps a hash map of all connected clients etc. I guess I don’t need to synchronize anything in it because all these notifications are coming in serialized, as far as I can tell.

Thanks

Hi Adam,

I ‘am in the process of researching comet in general. I just wanted to prototype comet for one of my application where I am currently polling the server ( especially a queue) to download the messages if any.It would be gr8 if you could provide some hands on for this.

Thanks

Hello all,
is anybody still using this server?
I noticed there is a FIXME in the ResponseListener.java class.
Has anybody fixed it? if yes, how?
It says the bug existed on the Server too, so does it mean it’s fixed already on the server side?
thanks a lot.

This is idea of improve the performance of server. How about this ?

Single thread can’t accept the many connection at the same time,
Over 500 connection makes error and can’t handle.

Separate the accept(include bind) from the single thread, and make new thread.
This thread will be start by main thread. And accepted key stored queue for main thread.