non-blocking channel Infinite loop in java.util.Scanner (original) (raw)

Alan Bateman Alan.Bateman at oracle.com
Tue Jun 5 10:40:05 UTC 2012

On 05/06/2012 11:00, Rémi Forax wrote:

One of my student find a bug in the implementation of Scanner, that allows you to use a non blocking channel as input of a Scanner.

The Scanner uses Channels.newReader() to create a Reader from a channel which itself create a StreamDecoder. In that case, StreamDecoder.impReader() goes into an infinite loop because impReader() calls readBytes() that does nothing if channel.read() returns zero. The javadoc of Channels.newReader() clearly states that it should throw a IllegalBlockingModeException but there is no code that checks that. I think a way to solve the problem is to insert a code that check the blocking state in Channels.newWriter(). if (ch instanceof SelectableChannel) { SelectableChannel sc = (SelectableChannel)ch; if (!sc.isBlocking()) throw new IllegalBlockingModeException(); } } This could be fixed in Channels.newReader or in StreamDecoder, the former would be consistent with Channels.newWriter. I guess you know this already, but you will need to synchronize on the selectable channel's blockingLock to ensure that the blocking mode doesn't change.

I've created a bug for this:

7174305: (ch) Channels.newReader doesn't throw IllegalBlockingMode if channel is configured non-blocking

-Alan.

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