Has anyone played with NIO pipes to filter / intercept System.out?

Question

As suggested here I would like to do that inside the selector loop. What I would really want is to read contents written to system out inside my selector loop.

E

Answer 1

One way to do it would be as follows:

• create a subclass of OutputStream that redirects its output to a Pipe's sink channel
• redirect System.out using this class: System.setOut(new PrintStream(new MyOutputStream(pipe));
• register the pipe's source channel with a selector and get whatever was written to System.out in the selector loop, i.e. the source channel's correpsonding SelectionKey is selected as readable()

The following immplementation is a naive but working implementation, which simply redirects to System.err everything that is written to System.out:

import java.io.*;
import java.nio.ByteBuffer;
import java.nio.channels.*;
import java.util.Iterator;

public class SystemOutPipe extends Thread {

  public static void main(String[] args)
  {
    try {
      SystemOutPipe sop = new SystemOutPipe();
      sop.start();
      System.out.println("This message should be redirected to System.err\nNow waiting 5 seconds ...");
      Thread.sleep(5000L);
      sop.setStopped(true);
      sop.join();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }

  private Selector selector;
  private Pipe pipe;
  private boolean stopped = false;

  public SystemOutPipe() throws IOException {
    super("SystemOutPipe");
    pipe = Pipe.open();
    System.setOut(new PrintStream(new PipeOutputStream(pipe)));
    selector = Selector.open();
    pipe.source().configureBlocking(false);
    pipe.source().register(selector, SelectionKey.OP_READ, ByteBuffer.allocate(1024));
  }

  @Override
  public void run() {
    try {
      while (!isStopped()) {
        int n = selector.select(1L);
        if (n > 0) {
          Iterator<SelectionKey> it = selector.selectedKeys().iterator();
          while (it.hasNext()) {
            SelectionKey key = it.next();
            it.remove();
            if (key.isReadable()) {
              new ReadHandler(key).run();
            }
          }
        }
      }
    } catch (Exception e) {
      e.printStackTrace(); // writes to System.err !
    }
  }

  public synchronized boolean isStopped() {
    return stopped;
  }

  public synchronized void setStopped(final boolean stopped) {
    this.stopped = stopped;
  }

  public class ReadHandler implements Runnable {
    private final SelectionKey key;

    public ReadHandler(final SelectionKey key) {
      this.key = key;
    }

    @Override
    public void run() {
      ByteBuffer bbuf = (ByteBuffer) key.attachment();
      ReadableByteChannel channel = (ReadableByteChannel) key.channel();
      try
      {
        int count = 0;
        do {
          bbuf.clear();
          count = channel.read(bbuf);
          if (count > 0) System.err.write(bbuf.array(), 0, count);
        } while(count > 0);
      } catch (IOException e) {
        e.printStackTrace();
        key.cancel();
      }
    }
  }

  public class PipeOutputStream extends OutputStream {
    private final Pipe pipe;

    public PipeOutputStream(final Pipe pipe) {
      this.pipe = pipe;
    }

    @Override
    public void write(final int b) throws IOException {
      write(new byte[] { (byte) b });
    }

    @Override
    public void write(final byte[] b) throws IOException {
      write(b, 0, b.length);
    }

    @Override
    public void write(final byte[] b, final int off, final int len) throws IOException {
      ByteBuffer bbuf = ByteBuffer.wrap(b, off, len);
      bbuf.position(len);
      bbuf.flip();
      int count = 0;
      while (count < len) {
        int n = pipe.sink().write(bbuf);
        if (n == 0) {
          // let's wait a bit and not consume cpu
          try {
            Thread.sleep(1L);
          } catch (InterruptedException e) {
            throw new IOException(e);
          }
        }
        else count += n;
      }
    }
  }
}