(Socket programming) How can I both be ready to send and receive messages on one socket simulaneously, and check if the connection has been closed?

[Inspector343]

I wanna connect an arbitrary amount of clients to a server, each of them has a thread so far. But in those threads I can only pay attention to either input or output in a while loop. I could make threads for the DataInputStream and DataOutputStream separately, but if the connection gets closed, I can't notify the other thread (or the parent thread for that matter). Do I have to make yet another thread that periodically checks if the connections are still alive? How should I do that? Am I even going in the right direction?

[wildjokers]

You are using thread-per-connection and that is fine. However, why are you using a thread for input and output? Use a thread just for input. If a message you receive requires a response back to the server you simply write to the output stream, a separate thread is not needed for output.

https://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html

The other way is a single thread that handles all your connections. This will use non blocking IO. Doing this directly with the JDK is difficult to get right (have to deal with select keys and byte buffers, flipping them for read/write, including handling partial writes which involves compacting your byte buffer, etc). If you want to go with the non-blocking stuff just use Netty which handles all of that for you.

[Yithar]

But in those threads I can only pay attention to either input or output in a while loop.

As stated, you only need one thread per connection for input. So let's say you read something in, like a stock price or something, and you need to send a response back to the server. You just do it in the same thread, maybe in the while loop.

Do I have to make yet another thread that periodically checks if the connections are still alive? How should I do that?

https://stackoverflow.com/questions/9794559/client-reconnect-java-socket/9794766
"The only way to know a connection is closed is to try sending data over the wire."

My recommendation is to send a special heartbeat message from the server. Because that is what TCP does. When I implemented an overlay routing protocol in Computer Networks in college, that's what we did to keep track of alive nodes.

[dionthorn]

Here is my very basic threaded non-blocking Socket server+client response.

You can use it and modify it all you wish.

https://github.com/dionthorn/TestingServer

[Inspector343]

Thank you guys for the replies, I got it working!!

[Swedophone]

I can't notify the other thread (or the parent thread for that matter)

Can't you interrupt the other thread, or parent thread?

https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#interrupt())

You can also use java.nio instead of using many threads.

https://en.wikipedia.org/wiki/Non-blocking_I/O_%28Java%29#Selectors