Java for(;;) loop?

Question

I've been tearing into the Netty networking library trying to learn how to write some basic NIO networking code when I ran into something that didn't look right to me, a for-loop without anything inside of it, here's the code:

for (;;) {
    SocketChannel acceptedSocket = channel.socket.accept();
    if (acceptedSocket == null) {
        break;
    }
    registerAcceptedChannel(channel, acceptedSocket, thread);
}

I immediately checked the documentation tutorials of loops, located here and couldn't find anything relating to this statement.

The commentation directly above this code states the following:

 // accept connections in a for loop until no new connection is ready

But this doesn't really explain to me how it works, or why, it just says what it's doing.

If you need the entire method, here it is:

 @Override
protected void process(Selector selector) {
    Set<SelectionKey> selectedKeys = selector.selectedKeys();
    if (selectedKeys.isEmpty()) {
        return;
    }
    for (Iterator<SelectionKey> i = selectedKeys.iterator(); i.hasNext();) {
        SelectionKey k = i.next();
        i.remove();
        NioServerSocketChannel channel = (NioServerSocketChannel) k.attachment();

        try {
            // accept connections in a for loop until no new connection is ready
            for (;;) {
                SocketChannel acceptedSocket = channel.socket.accept();
                if (acceptedSocket == null) {
                    break;
                }
                registerAcceptedChannel(channel, acceptedSocket, thread);
            }
        } catch (CancelledKeyException e) {
            // Raised by accept() when the server socket was closed.
            k.cancel();
            channel.close();
        } catch (SocketTimeoutException e) {
            // Thrown every second to get ClosedChannelException
            // raised.
        } catch (ClosedChannelException e) {
            // Closed as requested.
        } catch (Throwable t) {
            if (logger.isWarnEnabled()) {
                logger.warn(
                        "Failed to accept a connection.", t);
            }

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e1) {
                // Ignore
            }
        }
    }
}

Every part of the `for` loop is optional. That could be seen as a for-ever loop, since there's no condition to check, and no increment step to run. — Makoto, Nov 29 '14 at 03:03
@SotiriosDelimanolis: I don't think that this question is a direct duplicate. It's most certainly *related*, but I don't think that this question is a duplicate. — Makoto, Nov 29 '14 at 03:08
http://stackoverflow.com/questions/8880870/java-for-vs-whiletrue — Sotirios Delimanolis, Nov 29 '14 at 03:13
http://stackoverflow.com/questions/11923128/infinite-loop-application-for and http://stackoverflow.com/questions/26653968/what-is-the-condition-for-this-for-loop — Sotirios Delimanolis, Nov 29 '14 at 03:13
@SotiriosDelimanolis: None of them. They don't ask the same question, "What does it mean?" — Makoto, Nov 29 '14 at 03:19
@Makoto http://stackoverflow.com/questions/7081339/what-does-for-mean-in-java — Sotirios Delimanolis, Nov 29 '14 at 03:21

score 5 · Answer 1 · answered Nov 29 '14 at 03:02

5

for (;;) is an infinite loop, which is equivalent to while (true). Because the for-loop has no termination statement, it will never terminate.

All three components in a for-loop are optional: for (initialization; termination; increment)

answered Nov 29 '14 at 03:02

August

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Ah! This makes sense! I had no idea you were able to void out statements like this, so basically `for(int i =0; ;i++)` would increment i forever, correct? – Hobbyist Nov 29 '14 at 03:06
@Christian.tucker Exactly. – August Nov 29 '14 at 03:06
Very handy, I'll mark this as correct when the timer expires. – Hobbyist Nov 29 '14 at 03:07
I believe `for(;;)` is even more efficient than `while(true)` because it doesn't have to check anything – Shadow Nov 29 '14 at 03:13
1

@user3189142 It compiles to identical bytecode, so they have same performance. – August Nov 29 '14 at 03:15

Java for(;;) loop?

1 Answers1