How to tell if a server is not available when connecting via TCP/IP

[Rick]

Hi,

I'm trying to connect to a server application listening on a particular
port. Soemtimes what happens is that the server crashes for some reason
and when my client application tries to create a Socket object from the
server's IP and PORT, an exception is thrown: java.net.ConnectException:
Connection refused: connect.

How can I tell that a server is not available before creating a socket?
And how do I cleanly "continue" from the point where the exception is
being thrown? Thanks!

-Rick

 

[Dobromir Gaydarov]

Just catch the exception and do whatever you want to do when the server is
not available.
You do not have to prevent checked exceptions from happening (you cannot),
you just have to write code to handle them.

I do not understand "cleanly", but seems you look at the exceptions as being
errors.
If that is the case you are confused - the exceptions are good thing:
- they are there to force you to thing about exceptional cases
- they allow you to write code for the normal case first, then consider the
"other" cases.

Dobromir

 

[Gordon Beaton]

when my client application tries to create a Socket object from the
server's IP and PORT, an exception is thrown:
java.net.ConnectException: Connection refused: connect.

How can I tell that a server is not available before creating a
socket?

You can't. The only way to tell whether it's available is to try to
connect to it.

Even if there were another way, there would be no guarantee that the
server is still up when you actually try to connect (even a fraction
of a second later), so you've still got to handle that possibility
anyway.

And how do I cleanly "continue" from the point where the exception
is being thrown? Thanks!

That really depends what you want to do when the connection attempt
fails. What can your application do without the connection?

Here's one possibility, retry the attempt a few times before giving
up:

int attempts = 5;
Socket s = null;

while (s == null) {
try {
s = new Socket(...);
}
catch (IOException e) {
if (--attempts == 0) {
throw e;
else {
try {
Thread.sleep(5000);
}
catch (InterruptedException f) {
// ignore
}
}
}
}

// here, s is connected

/gordon