CODEWITHSUNDEEP
javasocketstcp
giorgiosironi
giorgiosironi

Reputation: 1104

Closing a TCP connection from the server-side while letting the client know

I have a thread for each connection on the server-side. When the client is not sending commands, the server thread is blocking:

while ((commandHeader = fromNode.readLine()) != null) {

which internally calls readLine() on an OutputStream obtained from the TCP socket. When I call socket.close() from another thread, this calls wakes up with a SocketException and the thread can be terminated.

However, if a client than wakes up and decide to issue a command, it executes

stream.writeBytes("something\n");

which blocks indefinitely. I understand this is probably fine for TCP (it's just an half-close.) I should probably send something to the client upon quitting, like "QUIT\n"; it could also just read an EOF. But if I call readLine() or other read operations on the client before sending the command, they block waiting for data when the connection is not closed. How can the client detect that the connection has been half-closed before trying to write to it?

Upvotes: 2

Views: 3704

Answers (2)

Johnydep
Johnydep

Reputation: 6277

First i think your application logic should be such that to avoid Half Open TCP connection. You can think of adding timer on client side so that if nothing received it starts polling the server again.

From server point of view, another option is to set timer on the readLine. Make another method for readLine where you set a timer and if it excedes certain time, simply return some default value to the while loop.

EDIT:

You might want to read this article specially the section: What about threads blocked on IO?

Upvotes: 2

Fernando Miguélez
Fernando Miguélez

Reputation: 11316

When socket.close() is called on server the underlying TCP connection is closed with the typical FIN/FIN-ACK sequence plus RST packets, so the client will know. When the client calls stream.writeBytes() afterwards it should fail. If it doesn't it means there has been some missing packets and the connection eventually will fail anyhow.

Upvotes: 3

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