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    <p>I have read tons of documentation, forums and, obviously each of your responses. I have developed and tested a solution. It is not the best, but it is my best approximation.</p> <p>I use <a href="http://docs.oracle.com/javase/7/docs/api/java/nio/channels/Selector.html#selectNow%28%29" rel="nofollow">selectNow()</a> method of <a href="http://docs.oracle.com/javase/7/docs/api/index.html?java/nio/channels/Selector.html" rel="nofollow">Selector</a>. Given a <code>Selector</code> named <code>selector</code> that have a <a href="http://docs.oracle.com/javase/7/docs/api/index.html?java/nio/channels/SocketChannel.html" rel="nofollow">SocketChannel</a> registered, I can use this method to know if the associated <a href="http://docs.oracle.com/javase/7/docs/api/java/net/Socket.html" rel="nofollow">Socket</a> has readable bytes:</p> <pre><code>public boolean isReadable() { try { selector.selectNow(); Set&lt;SelectionKey&gt; selectedKeys = selector.selectedKeys(); Iterator&lt;SelectionKey&gt; keyIterator = selectedKeys.iterator(); while(keyIterator.hasNext()) { SelectionKey selectionKey = keyIterator.next(); if(!selectionKey.isValid()) { continue; } if(selectionKey.isReadable()) { return true; } keyIterator.remove(); } } catch(IOException e) { // error } return false; } </code></pre> <p>It is the only way I have found to know if a Socket has readable bytes without reading the bytes. Do you know a better way to do it or disadvantages of doing it by this way?</p> <p><strong>EDIT:</strong></p> <p>This solution works the first time, but it has a drawback: it always returns true even if 0 bytes are readable. A socket can be readable even if no data is available. This is not the behaviour I want, because I don't want to read when there is no data in the read buffer.</p> <p><strong>EDIT2 BIS:</strong></p> <p>Test this code:</p> <pre><code>public static boolean isReadable(Selector selector) { try { selector.selectNow(); Set&lt;SelectionKey&gt; selectedKeys = selector.selectedKeys(); Iterator&lt;SelectionKey&gt; keyIterator = selectedKeys.iterator(); while (keyIterator.hasNext()) { SelectionKey selectionKey = keyIterator.next(); if (!selectionKey.isValid()) { continue; } if (selectionKey.isReadable()) { keyIterator.remove(); return true; } keyIterator.remove(); } } catch (IOException e) { System.err.println("Error!"); e.printStackTrace(); } return false; } public static void read() { try { Selector selector = Selector.open(); SocketChannel socketChannel = SocketChannel.open(); Socket socket = socketChannel.socket(); System.out.println("Connecting to server..."); socket.connect(new InetSocketAddress("localhost", 9999)); System.out.println("Connected to server."); socketChannel.configureBlocking(false); socketChannel.register(selector, SelectionKey.OP_READ); ByteBuffer bb = ByteBuffer.allocate(1024); while (true) { boolean readable = isReadable(selector); if (readable) { System.out.println("Readable data found! Let's read..."); bb.clear(); int readBytes = socketChannel.read(bb); if (readBytes &lt; 0) { System.out.println("End of Stream found"); break; } byte[] readArray = new byte[readBytes]; System.arraycopy(bb.array(), 0, readArray, 0, readBytes); System.out.println("Read (" + (readBytes) + " bytes): " + new String(readArray, Charset.forName("UTF-8"))); bb.flip(); socketChannel.write(bb); Thread.sleep(1000); } else { System.out.println("No data to read, sleeping"); Thread.sleep(1000); } } } catch (Exception e) { e.printStackTrace(); } } public static void main(String[] args) { read(); } </code></pre> <p>First of all, I execute an application that listens for connections and writes data: I used <a href="http://netcat.sourceforge.net/" rel="nofollow">netcat</a> with this command: <code>nc -l 9999</code>. Then I execute my Java program. Finally I can write text lines in <code>netcat</code> and my Java program works perfectly.</p> <p>Remember it is a proof of concept, not a way to real programming solution. In fact, it is a very bad practise. Thank you @EJP and @Markus!</p>
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    1. POcan I get available bytes to read from a Socket using Java NIO?
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    1. CORe: EDIT: Really? That should definitely not be the case. Usually the READABLE flag should only get reset and returned again if you actually do a read. And it should only get returned again from selectNow if there is data available. This is probably some other issue in your code, although from a first glance at your code snippet I can't see any glaring problems.
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    2. CO@Markus I have tested my snippet and selectNow() returns always after the first positive. If I put the method in a loop, it loops without stop even if the sender doesn't send any data.
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    3. COif you actually read the received data off the connection, it should stop returning immediately. The read request resets the READABLE flag. But if you simply call selectNow() again without reading then, yes, it will return again immediately since the data is still there waiting.
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