NIO sockets - cant get it to block!
Hi,
I'm trying to use NIO sockets in a program which sends data to a socket on another machine. I want it to block so that it must wait for a response before it continues. However this doesnt happen! I used Ethereal to see the packets being sent and recieved and the socket clearly doesnt block. Am I correct in thinking that NIO sockets are configured to block by default? I even tried using the configureBlocking(true) method to make sure but it still didnt block. Can anyone shed some light onto why this could be happening? Has this happened to anyone else?
Cheers
thomfost wrote:
I've tried using regular I/O but its a bit too slow for what I need, im trying to see if using NIO improves this.Well you can stop now because it won't. NIO, as mentioned, is non-blocking IO, so a thread that is titled "NIO sockets - cant get it to block!" is not ultra promising. More importantly NIO will not make your IO "faster" by some magic. That's not what it does. It helps you write programs that scale because you don't need to have a thread dedicated to every client who connects to your system.
At any rate you have multiple mistakes here which suggest the other slowness problem you refer to is a logical problem in your code. So why don't you tell us about that instead?
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Hello,
Im having difficulty understanding the WRITE issue of nio sockets. Ive read most of the nio-related posts on this forum but I still cant figure it out. Ive also looked at some examples by PkWooster but they seems really complicated.
I have a server which accepts connections and echos messages to all clients. Everything works fine except my client cannot write to the server. I know I need to switch back and forth between Op interests since they can block the cpu if set incorrectly but I dont really understand what to set when. Since my server seems to be working fine Ill post my Clients code:
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.net.InetSocketAddress;
import java.util.*;
import java.nio.charset.*;
public class GameClient implements Runnable {
Game game;
SocketChannel sc;
ByteBuffer buffer;
Selector selector;
ByteBuffer sendBuffer=null;
String host = "192.168.1.102";
int port = 8888;
private CharsetDecoder asciiDecoder;
private CharsetEncoder encoder;
private LinkedList sendQ = new LinkedList();
SelectionKey sk;
LinkedList invocations;
public GameClient (Game g) {
game = g;
buffer = ByteBuffer.allocateDirect(1024);
Charset charset = Charset.forName("ISO-8859-1");
asciiDecoder = charset.newDecoder();
encoder = charset.newEncoder();
game.CharMSGArrival("Starting..."); // transfers data to the game thread
public void run(){
try{
// Create a nonblocking socket channel and connect to server.
sc = SocketChannel.open();
sc.configureBlocking(false);
game.CharMSGArrival("Connecting to server..."+"\n");
InetSocketAddress addr = new InetSocketAddress(host, port);
sc.connect(addr); // Nonblocking
while (!sc.finishConnect()) {
game.CharMSGArrival("I am waiting ..."+"\n" );
game.CharMSGArrival("Connection acqired"+ "\n" );
// Send initial message to server.
buffer.put((new Date().toString()+ "\n").getBytes());
buffer.flip();
sc.write(buffer);
// Create a new selector for use.
selector = Selector.open();
// Register the socket channel with the selector.
sk = sc.register(selector, SelectionKey.OP_READ);
while (true) {
//System.out.println("Listening for server on port " +
// remotePort);
// Monitor the registered channel.
int n = selector.select();
if (n == 0) {
continue; // Continue to loop if no I/O activity.
// Get an iterator over the set of selected keys.
Iterator it = selector.selectedKeys().iterator();
// Look at each key in the selected set.
while (it.hasNext()) {
// Get key from the selected set.
SelectionKey key = (SelectionKey) it.next();
// Remove key from selected set.
it.remove();
// Get the socket channel from the key.
SocketChannel keyChannel = (SocketChannel) key.channel();
// Is there data to read on this channel?
if (key.isReadable()) {
replyServer(keyChannel);
if (key.isWritable()) {
doSend();
} catch (IOException ioe) {
private void replyServer(SocketChannel socketChannel)
throws IOException {
// Read from server.
buffer.flip( );
int count = socketChannel.read(buffer);
if (count <= 0) {
// Close channel on EOF or if there is no data,
// which also invalidates the key.
socketChannel.close();
return;
buffer.flip( );
String str = asciiDecoder.decode(buffer).toString( );
game.CharMSGArrival(str);
public void doSend(){
sk.interestOps(SelectionKey.OP_WRITE);
if(sendBuffer != null)write(sendBuffer);
writeQueued();
public void trySend(String text){ // this method is invoked when a send buttong is pressed in the 'game' thread
sendQ.add(text); // first put it on the queue
writeQueued(); // write all we can from the queue
private void writeQueued()
while(sendQ.size() > 0) // now process the queue
String msg = (String)sendQ.remove(0);
write(msg); // write the string
private void write(String text)
try
ByteBuffer buf = encoder.encode(CharBuffer.wrap(text));
write(buf);
catch(Exception e){e.printStackTrace();}
* write out a byte buffer
private void write(ByteBuffer data)
SocketChannel scc = (SocketChannel)sk.channel();
if(scc.isOpen())
int len=0;
if(data.hasRemaining())
try{len = sc.write(data);}
catch(IOException e){e.printStackTrace(); //closeComplete();
if(data.hasRemaining()) // write would have blocked
game.CharMSGArrival("write blocked"+"/n");
//writeReady = false;
sk.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE); // select OP_WRITE
sendBuffer = data; // save the partial buffer
else {sendBuffer = null;
sk.interestOps(SelectionKey.OP_READ); }
else game.CharMSGArrival("write closed"+"/n");
}public void doSend(){
sk.interestOps(SelectionKey.OP_WRITE);
if(sendBuffer != null)write(sendBuffer);
writeQueued();
}This method makes no sense. You only get into it if the key is writable, which means it is already registered for OP_WRITE, so why would you register it again?
Also you're frequently ignoring the result of many write calls, of which you have too many.
See the thread 'Taming the NIO circus': http://forum.java.sun.com/thread.jspa?threadID=459338,
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I think I'm being really dense here but for the life of me, I can't get this code to work properly. I am trying to build a NIO socket server using JDK 1.4.1-b21. I know about how the selector OP_WRITE functionality has changed and that isn't my problem. My problem is that when I run this code:
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}the isWritable if statement will always return (which is fine) and the isReadable if statement will NEVER return (which is most certainly NOT FINE!!). The readBuffer code is there just to clear out the read buffer so isReadable is only called once per data sent.
This SEEMS to be a bug in how the selector works? I would expect to see isReadable return true whenever data is sent, but that is not the case. Now here is the real kicker ;) Go ahead and change this line:
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http://www.electrotank.com/lab/personal/mike/NioTest.java
Please forgive the code, it's just the smallest test harness I could make and much of it is lifted from other posts on this forum. You can test this by using Telnet and connecting to 127.0.0.1:8080. You can test the isReadable piece by just typing in the Telnet window.
Someone else has listed something as a bug in the Bug Parade, but the test case is flawed:
http://developer.java.sun.com/developer/bugParade/bugs/4755720.html
If this does prove to be a bug, has someone listed this already? Is there a nice clean workaround? I'm getting really desperate here. This bug makes the NIO socket stuff pretty unusable. Thanks in advance for the help!!
Mike Grundvig
[email protected]
Electrotank, Inc.Yeah, isReadable crashed for me too.
My solution was to not call it. I set up two selectors for the two operations I wanted notifying (accept and read) and used them independently in different threads. The accept thread passes them over to the read thread when they're accepted.
This way I don't need to call isReadable since it is bound to be readable otherwise it wouldn't have returned, as read is the only operation I'm being notified about.
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