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copied!<p>One way to do it would be as follows:</p> <ul> <li>create a subclass of OutputStream that redirects its output to a Pipe's sink channel</li> <li>redirect System.out using this class: <code>System.setOut(new PrintStream(new MyOutputStream(pipe));</code></li> <li>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()</li> </ul> <p>The following immplementation is a naive but working implementation, which simply redirects to System.err everything that is written to System.out:</p> <pre><code>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; } } } } </code></pre>

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