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

Rémi Forax forax at univ-mlv.fr
Tue Jun 5 10:00:36 UTC 2012

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(); } }

and to document the exception in the constructor of Scanner that takes a channel.

If someone provide me a bug id, it will provide a patch :)

cheers, Rémi

PS: The code below is a simple test to reproduce the infinite loop.

import java.io.IOException; import java.net.InetSocketAddress; import java.nio.ByteBuffer; import java.nio.channels.ServerSocketChannel; import java.nio.channels.SocketChannel; import java.util.Scanner;

public class Main { public static void main(String[] args) throws IOException { ServerSocketChannel serverSocketChannel = ServerSocketChannel.open(); serverSocketChannel.bind(new InetSocketAddress(2332));

 new Thread(new Runnable() {
   @Override
   public void run() {
     try {
       SocketChannel channel = SocketChannel.open(new

InetSocketAddress("localhost", 2332)); do { channel.write(ByteBuffer.wrap(new byte[] {'A'})); Thread.sleep(1000); } while(true); } catch (IOException | InterruptedException e) { throw new AssertionError(e); }

   }
 }).start();

 SocketChannel socketChannel = serverSocketChannel.accept();
 socketChannel.configureBlocking(false);

 Scanner scanner = new Scanner(socketChannel);
 while(scanner.hasNextLine()) {
   System.out.println(scanner.nextLine());
 }

} }

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