Java.nio selector non-blocking IO question
Hi,
I am designing an interactive server where multiple clients can log on and communicate with the server. I designed a protocol that the client/server use to talk to each other. My server runs a Selector to monitor a ServerSocket, accepting connections and reading continuously from clients.
Now my question is, since read() on ServerChannel are non-blocking using selector, how can I be sure that my entire protocol message will be read each time selector wakes up? For example, a slow client sends me a 5kb message, in one write() command, can I be sure that I will be able to read the entire message in one non-blocking read() command as well? If not, then the design becomes much more complicated, as I have to pipe each client's input into a handler thread that performs blocking i/o to read a protocol message one at a time. If I do that, then I might as well not use select() at all.
I did some preliminary tests, and it seems that for my purpose (message of size <= 50kb), a read() command will always be able to read the entire message. But I can't find any documentation on this subject. My guess is that I cannot trust non-blocking I/O as well, which means it does not fit my purpose.
Any help will be much appreciated.
Thanks,
Frank
You can't be sure a read() will read in all the data from a client in one call.
For example, say your message from the client to the server is of the following format. <start>message here<end>, where <start> indicates the start of a message and <end> the end of the message. In one read() call you might get "<start>message he". Your server would recognize this is partially correct but it needs the rest of the message. The server would store this and on the second read() you might get "re<end>" for the complete message.
The purpose of non-blocking I/O is so you don't have to wait around for the second part of the message, you can process other client messages while the first client finishes sending their message. This way other clients aren't waiting around while you(the server) sit and wait for client 1 to finish sending it's data.
So basically there is no gaurantee you will get a whole message intact. Your protocol will have to deal with partial messages, recognize them, store the partial message in a buffer, and on subsequent reads get the rest of the message.
Nick
Similar Messages
-
NIO SocketChannel non-blocking read
Hello.
I'm not sure a resembling message has already been posted in a forum. In such a case, thanks for redirecting to it.
Goals :
A selector is used by a main server thread to make accept and read operations non-blocking.
When a connection is accepted, the newly created socket channel is configured as non-blocking, and a key is added to the selector with the OP_READ flag.
When data are available on sockets, a new task (runnable) is created and submitted to a thread pool.
When a thread is ready to process the request, a pre-allocated bytebuffer is used to read arriving data.
The problem :
When the bytebuffer capacity is less than the received bytes count, the read method on the socket channel interrupts the selector on the same selection key. In response to this event, a new task is initiated, interferring with the previous one.
As expected according to my concurrency policy (ie. with a pool of threads), several requests are processed by parallel threads. To avoid unsafe accesses, i added a synchronized statement on the channel blocking lock, and it seems to works fine. But a better way should exist...
Questions :
Is this the expected behavior ?
Is there any other way to read received data with a small buffer ?
The full copy of the source code :
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.channels.spi.*;
import java.nio.charset.*;
import java.net.*;
import java.util.*;
import net.moon.threads.*;
public class Nio1 {
static class Request {
boolean isCompleted = false;
int inputs = 0;
Set workers = new HashSet();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte p = 0;
boolean isCompleted() {
return isCompleted;
void countInput() {
inputs++;
void append(final ByteBuffer byteBuffer) {
if (isCompleted)
throw new IllegalStateException("Request is already completed");
workers.add(Thread.currentThread());
while (byteBuffer.hasRemaining()) {
byte b = byteBuffer.get();
baos.write(b);
if ((b == '\r') && (p == '\n'))
isCompleted = true;
p = b;
int inputs() {
return inputs;
Thread[] workers() {
return (Thread[]) workers.toArray(new Thread[0]);
int size() {
return baos.size();
byte[] getData() {
return baos.toByteArray();
void reset() {
isCompleted = false;
inputs = 0;
baos.reset();
workers.clear();
static private class RequestTask implements Runnable {
private final static Charset charset = Charset.forName("US-ASCII");
private final Server server;
private final SelectionKey selectionKey;
RequestTask(final Server server, final SelectionKey selectionKey) {
this.server = server;
this.selectionKey = selectionKey;
public void run() {
log("*** Processing input...");
try {
SocketChannel channel = (SocketChannel) selectionKey.channel();
synchronized(channel.blockingLock()) {
Request request = (Request) selectionKey.attachment();
request.countInput();
State state = getState();
log("Reading first...");
int c = channel.read(state.byteBuffer);
log("... Read first : " + c);
if (c > 0) {
for(;;) {
state.byteBuffer.flip();
request.append(state.byteBuffer);
state.byteBuffer.clear();
if (c < state.byteBuffer.capacity()) break;
log("Reading next...");
c = channel.read(state.byteBuffer);
log("... Read next : " + c);
if (c <= 0) break;
if (request.isCompleted()) {
log("Request completed : " + request.inputs());
StringBuffer bodyBuffer = new StringBuffer();
bodyBuffer.append("-----------------------------\r\n");
bodyBuffer.append("Request processed in " + request.inputs() + " inputs\r\n");
bodyBuffer.append("Request size is " + request.size() + " bytes\r\n");
bodyBuffer.append("Participating workers :\r\n");
Thread[] workers = request.workers();
for (int i = 0; i < workers.length; i++)
bodyBuffer.append(" * " + workers[i] + "\r\n");
bodyBuffer.append("-----------------------------\r\n");
StringBuffer headerBuffer = new StringBuffer();
headerBuffer.append("HTTP/1.1 200 OK\r\n");
headerBuffer.append("Server: NIO Server 1\r\n");
headerBuffer.append("Content-Type: text/plain\r\n");
headerBuffer.append("Content-Length: ").append(request.size() + bodyBuffer.length()).append("\r\n");
headerBuffer.append("\r\n");
CharsetEncoder encoder = charset.newEncoder();
channel.write(encoder.encode(CharBuffer.wrap(headerBuffer)));
channel.write(encoder.encode(CharBuffer.wrap(bodyBuffer)));
channel.write(ByteBuffer.wrap(request.getData()));
request.reset();
if (c < 0) {
selectionKey.attach(null);
selectionKey.cancel();
log("!!! Connection terminated for channel " + channel);
catch(final Exception x) {
x.printStackTrace();
log("*** Request processed...");
private State getState() {
State state = (State) server.taskManager.getCurrentWorkerState();
if (state == null) {
state = new State();
server.taskManager.setCurrentWorkerState(state);
else {
state.byteBuffer.clear();
return state;
private void log(final String text) {
System.out.println(Thread.currentThread() + " : " + text);
static class State {
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(32);
static private class Server implements Runnable {
private final int port;
private Thread worker;
private FIFOTaskManager taskManager;
Server(final int port) {
this.port = port;
worker = null;
synchronized void start() throws Exception {
if (worker == null) {
log("Starting the server...");
taskManager = new FIFOTaskManager("Nio1Workers", 24);
worker = new Thread(this);
worker.start();
synchronized(worker) {
try {
worker.wait();
catch(InterruptedException x) {
log("Server started !");
public void run() {
try {
log("Server is starting...");
Selector selector = SelectorProvider.provider().openSelector();
log("Creating listener on port " + port);
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
InetSocketAddress inetSocketAddress = new InetSocketAddress(port);
serverSocketChannel.socket().bind(inetSocketAddress);
SelectionKey selectionKey = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
synchronized(worker) {
worker.notify();
while (selector.select() >= 0) {
Set readyKeys = selector.selectedKeys();
log("Keys are ready : " + readyKeys.size());
for (Iterator i = readyKeys.iterator(); i.hasNext(); ) {
SelectionKey selectedKey = (SelectionKey) i.next();
if (selectedKey.isAcceptable()) {
ServerSocketChannel ssc = (ServerSocketChannel) selectedKey.channel();
SocketChannel sc = ssc.accept();
sc.configureBlocking(false);
SelectionKey sk = sc.register(selector, SelectionKey.OP_READ);
sk.attach(new Request());
log("Connection accepted for channel " + sc);
else if (selectedKey.isReadable()) {
log("Key ready for input : " + selectedKey);
taskManager.execute(new RequestTask(this, selectedKey));
i.remove();
readyKeys = null;
log("Server loop interrupted !");
catch(Exception x) {
x.printStackTrace();
private void log(final String text) {
System.out.println("SERVER: " + text);
public static void main(final String[] args) throws Exception {
Server server = new Server(9001);
server.start();Thanks for the trick. I hope the code will be more readable than my sockets !
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.channels.spi.*;
import java.nio.charset.*;
import java.net.*;
import java.util.*;
import net.moon.threads.*;
public class Nio1 {
static class Request {
boolean isCompleted = false;
int inputs = 0;
Set workers = new HashSet();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte p = 0;
boolean isCompleted() {
return isCompleted;
void countInput() {
inputs++;
void append(final ByteBuffer byteBuffer) {
if (isCompleted)
throw new IllegalStateException("Request is already completed");
workers.add(Thread.currentThread());
while (byteBuffer.hasRemaining()) {
byte b = byteBuffer.get();
baos.write(b);
if ((b == '\r') && (p == '\n'))
isCompleted = true;
p = b;
int inputs() {
return inputs;
Thread[] workers() {
return (Thread[]) workers.toArray(new Thread[0]);
int size() {
return baos.size();
byte[] getData() {
return baos.toByteArray();
void reset() {
isCompleted = false;
inputs = 0;
baos.reset();
workers.clear();
static private class RequestTask implements Runnable {
private final static Charset charset = Charset.forName("US-ASCII");
private final Server server;
private final SelectionKey selectionKey;
RequestTask(final Server server, final SelectionKey selectionKey) {
this.server = server;
this.selectionKey = selectionKey;
public void run() {
log("*** Processing input...");
try {
SocketChannel channel = (SocketChannel) selectionKey.channel();
synchronized(channel.blockingLock()) {
Request request = (Request) selectionKey.attachment();
request.countInput();
State state = getState();
log("Reading first...");
int c = channel.read(state.byteBuffer);
log("... Read first : " + c);
if (c > 0) {
for(;;) {
state.byteBuffer.flip();
request.append(state.byteBuffer);
state.byteBuffer.clear();
if (c < state.byteBuffer.capacity()) break;
log("Reading next...");
c = channel.read(state.byteBuffer);
log("... Read next : " + c);
if (c <= 0) break;
if (request.isCompleted()) {
log("Request completed : " + request.inputs());
StringBuffer bodyBuffer = new StringBuffer();
bodyBuffer.append("-----------------------------\r\n");
bodyBuffer.append("Request processed in " + request.inputs() + " inputs\r\n");
bodyBuffer.append("Request size is " + request.size() + " bytes\r\n");
bodyBuffer.append("Participating workers :\r\n");
Thread[] workers = request.workers();
for (int i = 0; i < workers.length; i++)
bodyBuffer.append(" * " + workers[i] + "\r\n");
bodyBuffer.append("-----------------------------\r\n");
StringBuffer headerBuffer = new StringBuffer();
headerBuffer.append("HTTP/1.1 200 OK\r\n");
headerBuffer.append("Server: NIO Server 1\r\n");
headerBuffer.append("Content-Type: text/plain\r\n");
headerBuffer.append("Content-Length: ").append(request.size() + bodyBuffer.length()).append("\r\n");
headerBuffer.append("\r\n");
CharsetEncoder encoder = charset.newEncoder();
channel.write(encoder.encode(CharBuffer.wrap(headerBuffer)));
channel.write(encoder.encode(CharBuffer.wrap(bodyBuffer)));
channel.write(ByteBuffer.wrap(request.getData()));
request.reset();
if (c < 0) {
selectionKey.attach(null);
selectionKey.cancel();
log("!!! Connection terminated for channel " + channel);
catch(final Exception x) {
x.printStackTrace();
log("*** Request processed...");
private State getState() {
State state = (State) server.taskManager.getCurrentWorkerState();
if (state == null) {
state = new State();
server.taskManager.setCurrentWorkerState(state);
else {
state.byteBuffer.clear();
return state;
private void log(final String text) {
System.out.println(Thread.currentThread() + " : " + text);
static class State {
ByteBuffer byteBuffer = ByteBuffer.allocateDirect(32);
static private class Server implements Runnable {
private final int port;
private Thread worker;
private FIFOTaskManager taskManager;
Server(final int port) {
this.port = port;
worker = null;
synchronized void start() throws Exception {
if (worker == null) {
log("Starting the server...");
taskManager = new FIFOTaskManager("Nio1Workers", 24);
worker = new Thread(this);
worker.start();
synchronized(worker) {
try {
worker.wait();
catch(InterruptedException x) {
log("Server started !");
public void run() {
try {
log("Server is starting...");
Selector selector = SelectorProvider.provider().openSelector();
log("Creating listener on port " + port);
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.configureBlocking(false);
InetSocketAddress inetSocketAddress = new InetSocketAddress(port);
serverSocketChannel.socket().bind(inetSocketAddress);
SelectionKey selectionKey = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
synchronized(worker) {
worker.notify();
while (selector.select() >= 0) {
Set readyKeys = selector.selectedKeys();
log("Keys are ready : " + readyKeys.size());
for (Iterator i = readyKeys.iterator(); i.hasNext(); ) {
SelectionKey selectedKey = (SelectionKey) i.next();
if (selectedKey.isAcceptable()) {
ServerSocketChannel ssc = (ServerSocketChannel) selectedKey.channel();
SocketChannel sc = ssc.accept();
sc.configureBlocking(false);
SelectionKey sk = sc.register(selector, SelectionKey.OP_READ);
sk.attach(new Request());
log("Connection accepted for channel " + sc);
else if (selectedKey.isReadable()) {
log("Key ready for input : " + selectedKey);
taskManager.execute(new RequestTask(this, selectedKey));
i.remove();
readyKeys = null;
log("Server loop interrupted !");
catch(Exception x) {
x.printStackTrace();
private void log(final String text) {
System.out.println("SERVER: " + text);
public static void main(final String[] args) throws Exception {
Server server = new Server(9001);
server.start();
} -
Java.nio Selector registration
Dear everybody, can you help me to figure out this?
Case:
- open new Selector object
Selector selector = Selector.open();
- registering channels to the selector:
ServerSocketChannel chn1, chn2;
chn1.register(selector, SelectionKey.OP_ACCEPT);
chn2.register(selector, SelectionKey.OP_ACCEPT);
Question:
- How to unregister channel from the selector?
- If i'm invoke chn2.close(); is it have the same effect with unregistering chn2 from the selector?
Thanks & regards,
Loekitohi everybody sorry to bother all of you all....
btw, I already found the answer for my question... (thanks GOD).. and it's on J2SE docs... hee..
regards,
Loekito -
Troubles with timeout using java.nio.channels and non-blocking sockets
Hello.
I have a server application that employs java.nio.channels with non-blocking sockets.
The server waits for connections. The client should connect and be first in sending data.
Timeouts are significant! If client exceeds the allowed time to send data, the server should break the connection.
The huge trouble I've discovered that I cannot control the timeout when client connects but remains silent.
My code looks as follows:
<pre>
Selector oSel;
SocketChannel oSockChan;
Socket oSock;
SelectionKey oSelKey;
Iterator<SelectionKey> oItSelKeys;
int iCurrState, iMask, iCount;
iCurrState = INT_SERVER_WORKING;
iMask = SelectionKey.OP_ACCEPT | SelectionKey.OP_CONNECT | SelectionKey.OP_READ | SelectionKey.OP_WRITE;
while ( iCurrState == INT_SERVER_WORKING )
try
*// retrieving next action*
iCount = oSel.select();
if ( iCount > 0 )
oItSelKeys = oSel.selectedKeys().iterator();
while ( oItSelKeys.hasNext() )
oSelKey = oItSelKeys.next();
oItSelKeys.remove();
if ( oSelKey.isValid() )
switch ( oSelKey.readyOps() & iMask ) {
case SelectionKey.OP_ACCEPT :
oSockChan = oSSockChan.accept();
oSockChan.configureBlocking(false);
oSock = oSockChan.socket();
oSock.setKeepAlive(true);
oSockChan.register(oSel,SelectionKey.OP_READ,new MyPacket(oSock.getInetAddress(),oSock.getPort()));
break;
case SelectionKey.OP_READ :
oSelKey.interestOps(0);
((MyPacket) oSelKey.attachment()).inRequest(); *// preparing request*
this.getReader().add(oSelKey); *// sending key to reading thread*
break;
case SelectionKey.OP_WRITE :
oSelKey.interestOps(0);
((MyRequest) oSelKey.attachment()).inResponse(); *// preparing response*
this.getWriter().add(oSelKey); *// sending key to writing thread*
break;
case SelectionKey.OP_CONNECT :
default :
*// nothing to do*
catch ( IOException oExcept )
*// do some actions*
</pre>
Timeouts are easily controlled by reading and writing threads (see OP_READ and OP_WRITE ).
But when a client just connects without consequent data send, the state of this connection remains as OP_ACCEPT. The connection remains open for arbitrarily large time and I cannot control it!
Please help with idea how can I terminate such connections!How can I process the keys that weren't selected at the bottom of the loop? Should I use the method keys() ?Yes. Form a new set from keys() and removeAll(selectedKeys()). Do that before you process selectedKeys().
And the second moment: as I understood a single key may contain several operations simultaneously? Thus I should use several if's (but not if/else 'cause it's the equivalent of switch ... case ).If there is anything unclear about 'your switch statement is invalid. You need an if/else chain' I fail to see what it is. Try reading it again. And if several ifs were really the equivalent of "switch ... case", there wouldn't be a problem in the first place. They're not, and there is. -
Java.io.IOException: Connection timed out while using non-blocking NIO
Hi,
I am using non-blocking NIO on jdk 1.6 to transfer files between two processes. The problem is that the sender thread times out on the write call even though it's in non-blocking mode. It doesn't make sense. The receiver on the other end is running fine, and even tries to read from the socket after the sender drops the connection, and throws a Connection reset by peer exception. I should mention that the receiver is sometimes busy doing other stuff, and may be a second or two late in issuing the read call. But if I understand correctly, default TCP timeout is about 2 minutes, so there's no reason why it should timeout in a second or two. I'd appreciate it if anyone has any insight.
Thanks!No I have definitely completed the connection since I it happens a while after the transfer has actually started and part of the file has already been transferred. Oh btw, I should mention that it doesn't always happen. It's intermittent. And it's not a network issue since iperf tests between the two nodes show nothing unusual.
-
NIO Non-Blocking Server not Reading from Key
I have created a NIO non blocking server (below) and it will not pick up any input from the client.... My log doesnt even show that it enters the readKey() method, so it must be something before. Any help would be appreciated.
Scott
package jamb.server;
import java.io.IOException;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.ClosedChannelException;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.channels.spi.SelectorProvider;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.util.Iterator;
import java.util.Set;
import java.util.logging.Logger;
import java.util.prefs.Preferences;
import jamb.server.client.Client;
public class Server {
private Selector selector;
private ServerSocketChannel serverChannel;
private static Logger logger = Logger.getLogger("jamb.server");
private static Preferences prefs = Preferences.systemRoot().node("/jamb/server");
public void init() {
logger.entering("jamb.server.Server", "init");
//Get a selector...
try {
selector = SelectorProvider.provider().openSelector();
//Open the SocketChannel and make it non-blocking...
serverChannel = ServerSocketChannel.open();
serverChannel.configureBlocking(false);
//Bind the server to the port....
int port = prefs.getInt("Port", 4000);
logger.config("Server configured on port " + port + " (default: 4000)");
InetSocketAddress isa = new InetSocketAddress(
InetAddress.getLocalHost(), port);
serverChannel.socket().bind(isa);
} catch (IOException ioe) {
logger.severe ("IOException during server initialization!");
logger.exiting("jamb.server.Server", "init");
public void run() {
logger.entering("jamb.server.Server", "run");
int bufferSize = prefs.getInt("BufferSize", 8);
logger.config("Buffer size set to " + bufferSize + " (default: 8)");
SelectionKey acceptKey = null;
try {
acceptKey = serverChannel.register(
selector, SelectionKey.OP_ACCEPT);
} catch (ClosedChannelException cce) {
try {
while (acceptKey.selector().select() > 0) {
Set readyKeys = selector.selectedKeys();
Iterator i = readyKeys.iterator();
while (i.hasNext()) {
//logger.finest("Processing keys...");
//Get the key from the set and remove it
SelectionKey currentKey = (SelectionKey) i.next();
i.remove();
if (currentKey.isAcceptable()) {
logger.finest("Accepting key...");
acceptKey(currentKey);
} else if (currentKey.isReadable()) {
logger.finest("Reading key...");
readKey(currentKey, bufferSize);
} else if (currentKey.isWritable()) {
//logger.finest("Writing key...");
writeKey(currentKey);
} catch (IOException ioe) {
logger.warning("IOException during key handling!");
logger.exiting("jamb.server.Server", "run");
public void flushClient (Client client) {
try {
ByteBuffer buf = ByteBuffer.wrap( client.getOutputBuffer().toString().getBytes());
client.getChannel().write(buf);
} catch (IOException ioe) {
System.out.println ("Error writing to player");
client.setOutputBuffer(new StringBuffer());
private void acceptKey (SelectionKey acceptKey) {
logger.entering("jamb.server.Server", "acceptKey");
//Retrieve a SocketChannel for the new client, and register a new selector with
//read/write interests, and then register
try {
SocketChannel channel = ((ServerSocketChannel) acceptKey.channel()).accept();
channel.configureBlocking(false);
SelectionKey readKey = channel.register(
selector, SelectionKey.OP_READ|SelectionKey.OP_WRITE );
readKey.attach(new Client(this, channel));
} catch (IOException ioe) {
System.out.println ("Error accepting key");
logger.exiting("jamb.server.Server", "acceptKey");
private void readKey (SelectionKey readKey, int bufSize) {
logger.entering("jamb.server.Server", "readKey");
Client client = (Client) readKey.attachment();
try {
ByteBuffer byteBuffer = ByteBuffer.allocate(bufSize);
int nbytes = client.getChannel().read( byteBuffer );
byteBuffer.flip();
Charset charset = Charset.forName( "us-ascii" );
CharsetDecoder decoder = charset.newDecoder();
CharBuffer charBuffer = decoder.decode(byteBuffer);
String text = charBuffer.toString();
client.getInputBuffer().append(text);
if ( text.indexOf( "\n" ) >= 0 )
client.input();
} catch (IOException ioe) {
logger.warning("Unexpected quit...");
client.disconnect();
logger.exiting("jamb.server.Server", "readKey");
private void writeKey (SelectionKey writeKey) {
//logger.entering("jamb.server.Server", "writeKey");
Client client = (Client) writeKey.attachment();
if (!client.isConnected()) {
client.connect();
//logger.exiting("jamb.server.Server", "writeKey");From my own expierence with the NIO (Under Windows XP/ jdk1.4.1_01); you can't seem to set READ and WRITE at the same time.
The program flow I usually end up with for a echo server is:
When the selector.isAcceptable(): accept a connection; register for READs
In the read event; write the incoming characters to a buffer; register for a WRITE and add the buffer as an attachment.
In the write event; write the data to the socket If all the data was written; register for a READ; otherwise register for another WRITE so that you can write the rest.
Not sure if that the "proper" way; but it works well for me.
- Chris -
Writing Java Non-Blocking Socket Server for Winsock Client
Hi.
Im a newbie to Sockets. I need to write a Non-Blocking Socket server (using java.nio) that accepts connection and reads the data sent by WinSock clients, and write them to another winsock client. Its quite intriguing. How do I do that? Is it possible?
Thanks in advance
ArunWell, in traditional 4.2 BSD sockets, you'd fill up a set of filedescriptors of connections (an array of bits usually), and push that in the read parameter of a call to 'select'. Select then blocks until at least one of the file descriptors become available for reading, or writing if you also made an fd_set full of file descriptors (but you can usually write freely to a socket, so there is not much use for that). Then you start finding out which of these file descriptors have actually become available for reading, and you pass those to a bunch of worker-threads. The advantage is that your set of worker-threads can be quite small, while your number of connections can still be quite large (unless, of course, everyone of your clients start talking all at once, but that is no different from the one-socket-one-thread-model that java.net.* forces upon you).
In java, the 'select' call is replaced by a call to java.nio.channels.Selector.select(); and then the fishing out of the selected stuff comes from java.nio.channels.Selector.selectedKeys().
To add a thingy to a selector, use (for example) java.nio.channel.ServerSocketChannel.register(Selector, ops, att);
whereby the ops parameter is the kind of action you'd like to select this channel for; SelectionKey.OP_READ etc..
The workerthread bit is also easy to write, but I leave that to you as an exercise. -
Multiple non-blocking selectors in threads
About networking in java and sockets i read alot. I know about the 2 implementations witk thread per client and non-blocking IO (usage of select()). I ran a few tests of my own and i could't decide. So I wrote a server that uses them both. The Ideea is to have no more than 100 clients per each selector and each selector runs in a thread. so when you have 4000 clients you actualy have 40 threads running each 100 clients:D
However I'm stuck. I wrote this piece of code that has 2 objects one work thread with his own selector and a main thread with an accepting selector. The problem is that when I connect to it via telnet it only works when there is an even number of clients:(( when client nr 1 connects it blocks on channel.register(workSelector,SelectionKey.OP_READ);when client 2 gets connected it works:((
HELP!!!
here is the
import java.nio.*;
import java.nio.channels.*;
import java.nio.charset.*;
import java.io.*;
import java.net.*;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
//work thread with own selector
class workThread implements Runnable{
public int userCount=0;
public int ID=0;
private Charset charset;
private CharsetEncoder encoder;
private CharsetDecoder decoder;
private ByteBuffer buffer;
private ConcurrentHashMap userMap;
public Selector workSelector = null;
public void addClient(SelectableChannel channel, int ops) throws IOException{
if (channel == null) return;
//channel.configureBlocking(false);
System.out.println("Channel was added in workSelector : "+channel);
channel.register(workSelector,SelectionKey.OP_READ);
System.out.println("Client was added in work selector : " + ID);
userCount++;
System.out.println("Client nr : "+userCount);
public boolean find(String name){
return true;
public workThread() throws IOException{
charset = Charset.forName("ISO-8859-1");
encoder = charset.newEncoder();
decoder = charset.newDecoder();
buffer = ByteBuffer.allocate(256);
// System.out.println("S-a creat selectorul");
workSelector = Selector.open();
System.out.println("Selector open ");
private void doRead(SelectionKey key) throws IOException{
SocketChannel clientChannel = (SocketChannel)key.channel();
buffer.clear();
System.out.println("Speaking to client");
if(clientChannel.read(buffer) == -1){
key.cancel();
clientChannel.close();
}else{
buffer.flip();
String message = decoder.decode(buffer).toString();
System.out.println(message);
if(message.trim().equals(".")){
clientChannel.write(encoder.encode(CharBuffer.wrap("BYE")));
key.cancel();
clientChannel.close();
}else{
buffer.flip();
clientChannel.write(buffer);
private void doWrite(SelectionKey key){
System.out.println("WRITE!!!");
public void run(){
while(true){
try{
if(workSelector.select() > 0){
Set keys1 = workSelector.selectedKeys();
for(Iterator itr = keys1.iterator();itr.hasNext();){
SelectionKey key = (SelectionKey)itr.next();
itr.remove();
if(key.isReadable()) {doRead(key);}
// if(key.isWritable()) {doWrite(key);}
}catch(IOException e){
System.out.println("Error in workthread "+ID+" "+e);
//main thread
public class msselect implements Runnable{
private ServerSocketChannel server;
private ServerSocket sock;
private Selector acceptSelector;
private workThread wk;
public static void main(String args[]){
try{
msselect x = new msselect("80.97.89.73",6500,5);
new Thread(x).start();
}catch(IOException ex){
System.out.println("Probleme"+ex);
msselect(String host, int port, int numT) throws IOException{
server = ServerSocketChannel.open();
sock = server.socket();
sock.bind(new InetSocketAddress(port));
server.configureBlocking(false);
acceptSelector = Selector.open();
server.register(acceptSelector, SelectionKey.OP_ACCEPT);
/* for(int i=0;i<numT;i++){
wk[i] = new workThread();
wk.ID = i;
new Thread(wk[i]).start();
wk = new workThread();
wk.ID = 1;
server.register(wk.workSelector, SelectionKey.OP_ACCEPT);
new Thread(wk).start();
public void run(){
while(true){
try{
acceptSelector.select();
Set keys = acceptSelector.selectedKeys();
for(Iterator itr = keys.iterator();itr.hasNext();){
SelectionKey key = (SelectionKey)itr.next();
itr.remove();
if(key.isAcceptable()){
SocketChannel client = null;
client = server.accept();
client.configureBlocking(false);
System.out.println("Accepted "+client);
wk.addClient(client,SelectionKey.OP_READ);
}catch(IOException e){
System.out.println("Main Thread error"+e);I compiled and ran your code.
The call channel.register(...) should block while selector is blocked in workSelector.select(). The fact that the registration succeeds is that you registered the server channel with both the work selector and the acceptor selector. The work selector is returning from select() because of a new client connection is accepted and is a matter of thread scheduling that the client.register() succeeeds (but only sometimes).
Possible solution:
- don't register the server channel with the work selector;
- probably call workSelector.wakeup() to make the client channel registration possible. -
JDK 1.4 nio non-blocking connects don't finish
I am working with the Linux version of JDK1.4, doing some testing of the
non-blocking capability. The few tests that I have done have shown some
strange results:
- When opening a non-blocking connection to a server which has an
announcement banner, say such as POP which gives something like:
+OK pop3 server ready
then connection seems to be OK and proceed through to completion.
- When opening a non-blocking connection to a server which is silent, and
expects me to start the conversation, such as contacting an HTTP server,
then the Selector.select() never returns to allow me to finish the
connection.
Below is a test program which illustrates the problem for me.
It attempts to open a non-blocking connection to www.yahoo.com on port 80.
It then drops into a Selector.select() where I am waiting for the chance to
catch the SelectionKey.OP_CONNECT event and finish connecting, and then send
a simple HTTP GET request down the wire. That OP_CONNECT event
never seems to arrive though, and I remain stuck in the select().
'netstat -na' shows that I am in a connection established state.
Any insight appreciated,
-Steve M [email protected]
------------------>8 cut here 8<-----------------------
import java.io.*;
import java.net.*;
import java.util.*;
import java.nio.*;
import java.nio.channels.*;
public class NoWorkee
public static String GET_REQUEST = "GET / HTTP/1.0\r\n\r\n";
static class Context
String host;
int port;
ByteBuffer request;
public Context (String h, int p, ByteBuffer r)
{host=h; port=p; request=r;}
Selector sel = null;
boolean keepGoing = true;
public NoWorkee()
throws IOException
sel = Selector.open();
public void add(String host, int port)
throws IOException
int ops = SelectionKey.OP_CONNECT;
// create non-blocking socket, and initiate connect operation
SocketChannel sc = SocketChannel.open();
sc.configureBlocking(false);
sc.connect(new InetSocketAddress(InetAddress.getByName(host), port));
SelectionKey sk;
try
sk = sc.register(sel, ops);
System.out.println ( "sc.register looks good connected=" +
sc.isConnected() + ", isConnectionPending=" +
sc.isConnectionPending());
sk.attach(new Context(host, port, ByteBuffer.wrap(GET_REQUEST.getBytes())));
catch (IOException x)
x.printStackTrace();
sc.close();
public void run()
throws IOException
keepGoing = true;
System.out.println ( "Selecting " + sel.keys().size() + " SelectKeys");
while (keepGoing)
final long before = System.currentTimeMillis();
final int numReady = sel.select();
//final int numReady = sel.select(1000);
final long after = System.currentTimeMillis();
System.out.println ( "Blocked " + (after-before) + " ms, numReady=" + numReady);
if (numReady > 0)
Set readyKeys = sel.selectedKeys();
System.out.println ( "Selected keys size " + readyKeys.size());
for (Iterator it = readyKeys.iterator(); it.hasNext();)
SelectionKey sk = (SelectionKey)it.next();
SocketChannel sockChan = (SocketChannel)sk.channel();
Context ctx = (Context)sk.attachment();
System.out.println ( "Servicing host " + ctx.host + " port "
ctx.port);
System.out.println ( "1");
if (sk.isConnectable())
if (sockChan.finishConnect())
System.out.println ( "Finished connecting success "
+ sockChan);
int ops = SelectionKey.OP_READ | SelectionKey.OP_WRITE;
sk.interestOps (ops);
else
System.out.println ( "Could not finishConnect for "
sockChan);
sk.cancel();
sockChan.close();
System.out.println ( "2");
if (sk.isAcceptable())
System.out.println ( "in sk.isAcceptable() block");
System.out.println ( "3");
if (sk.isReadable())
System.out.println ( "in sk.isReadable() block");
byte rawBuff[] = new byte[32 * 1024];
ByteBuffer buff = ByteBuffer.wrap(rawBuff);
int numRead = -1;
while (0 < (numRead = sockChan.read(buff)))
System.out.println ( "numRead = " + numRead);
for (int i = 0; i < numRead; i++)
System.out.print((char)rawBuff);
System.out.println ( "numRead = " + numRead);
System.out.println ( "4");
if (sk.isWritable())
System.out.println ( "in sk.isReadable() block");
int numWritten = -1;
if (null != ctx.request)
numWritten = sockChan.write(ctx.request);
if (!ctx.request.hasRemaining())
sk.interestOps(sk.interestOps() &
~SelectionKey.OP_WRITE);
System.out.println ( "numWritten = " + numWritten);
//else
// service timeouts
public static void main (String arg[])
try
NoWorkee bla = new NoWorkee();
bla.add ("www.yahoo.com", 80);
bla.run();
catch (Exception e)
e.printStackTrace();
------------------>8 cut here 8<-----------------------Just for the benefit of anyone who might be seeing the same problem, it looks like bug 4457776 accurately describes the problem that I was having above.
I am downloading j2sdk-1_4_0-beta2-linux-i386-rpm.bin to see if that fixed it.
Reference:
http://developer.java.sun.com/developer/bugParade/bugs/4457776.html -
A few general java.nio.* questions
Hi,
I've been reading up on selectors and channels for networking programming. During my reading I came across this website, http://www.javaworld.com/javaworld/jw-09-2001/jw-0907-merlin.html
It's kind of dated (3 years ago) but the example they provide on non-blocking I/O still compiles.
Further in the article (http://www.javaworld.com/javaworld/jw-09-2001/jw-0907-merlin-p4.html) they mention a few "gotcha's" with using selectors. One in particular that caught my attention was "a selector can have only 63 channels registered, which is probably not a big deal."
So only 63 channels max can be registered to a selector ? I searched around the web and the java API looking to confirm this. Does anyone know this to be true or if the max 63 registered channels was dropped before java 1.4 beta became the stable release of java 1.4? The article was written when java 1.4 was in beta.
Lastly, do selectors weed out dropped socket connections? For example. Say I have 500 socket (TCP/IP) connections to a server of mine. Of those 500 , 100 are now closed by the client by the time the server comes around to send them data. As the server loops through the 500 connections using a selector, will there be a delay because 100 connections are dropped and the server must wait for a timeout before proceeding to send the data to the next connection or does the selector detect these 100 dropped connections and only use the 400 still connected?
I hope this last question makes sense.
Thanks in advance,
NickHi,
I've been reading up on selectors and channels for
networking programming. During my reading I came
across this website,
http://www.javaworld.com/javaworld/jw-09-2001/jw-0907-m
rlin.html
It's kind of dated (3 years ago) but the example they
provide on non-blocking I/O still compiles.
Further in the article
(http://www.javaworld.com/javaworld/jw-09-2001/jw-0907-
erlin-p4.html) they mention a few "gotcha's" with
using selectors. One in particular that caught my
attention was "a selector can have only 63 channels
registered, which is probably not a big deal."
So only 63 channels max can be registered to a
selector ? I searched around the web and the java API
looking to confirm this. Does anyone know this to be
true or if the max 63 registered channels was dropped
before java 1.4 beta became the stable release of java
1.4? The article was written when java 1.4 was in
beta.I don't know if that limit of 63 is still there, but it probably makes sense to use a pool of threads each supporting a Selector if the number of connections gets that high.
Lastly, do selectors weed out dropped socket
connections? For example. Say I have 500 socket
(TCP/IP) connections to a server of mine. Of those 500
, 100 are now closed by the client by the time the
server comes around to send them data. As the server
loops through the 500 connections using a selector,
will there be a delay because 100 connections are
dropped and the server must wait for a timeout before
proceeding to send the data to the next connection or
does the selector detect these 100 dropped connections
and only use the 400 still connected?closed channels are removed by the select statement. When using non-blocking IO, nothing waits anyway.
I hope this last question makes sense.
Thanks in advance,
Nick -
NIO: Strange problem when using ByteBuffer with non-blocking SocketChannel
Hi,
I have a server that uses multiplexed, non-blocking I/O with java.nio. When a client connects, the server waits for the message: <system cmd="knock"/>, returns a message and disconnects the client. The clients are shortly serviced in less than a second.
But the server newer receive anything from about 20% of the clients - even though it is sent. Or with other words: it is received and the data is contained in the ByteBuffer - SocketChannel.read(ByteBuffer) - but a call to ByteBuffer.remaing() returns 0 !!
ByteBuffer receiveBuf = ByteBuffer.allocate(65536);
receiveBuf.clear(); // the code is elsewhere used for longer living clients
int readBytes = channel.read(receiveBuf);
receiveBuf.flip();
StringBuffer sb = new StringBuffer();
System.out.println(" * Remaining: "+receiveBuf.remaining()); // writes: ' * Remaining: 0'
System.out.println(" * Received: "+new String(receiveBuf.array())); // writes: ' * Received: <system cmd="knock"/>'
while(receiveBuf.remaining() >= 2) {
byte b = receiveBuf.get();
sb.append((char)b);
System.out.println(" * sb content: "+sb.toString()); // writes: ' * sb content: 'The ByteBuffer clearly receives the correct data, but the ByteBuffer.remaining() returns 0 and therefore the StringBuffer will never have any content.
The problem seems to occur randomly and for about 20% of the clients (simulated from the same computer and therefore has the same bandwidth and so on).
Anyone knows what is going on, and how to solve the problem !?It's always possible in any read that the number of bytes read is less than the number of bytes requested. You need to keep reading until you have got everything you expected, and cope with every possible error condition on each iteration.
EJP -
SO timeout relevance in non-blocking NIO ServerSocketChannel
I converted a threaded blocking server from using the standard blocking IO with a thread for each connection to using non-blocking NIO channels. Works great but I'm trying to understand the SO timeout relevance in the context of non-blocking NIO channels. Is there any relevance and should I set an SO timeout on the ServerSocket associated with a ServerSocketChannel and if so how should I handle it?
No. Socket timeouts are for blocking mode. If you need timeouts in non-blocking mode you have to do them yourself, taking advantage of the Selector.select(long timeout) method and keeping track of activity times per channel yourself. You should use that select() method anyway, rather than just blocking indefinitely, as it gives you a chance to catch up on housekeeping, dead channels, etc. For example as a very naive approach if nothing happens in say a select timeout of 10 minutes you might want to close all accepted SocketChannels.
-
About NIO non-blocking socket write method behavior
Hi all -
I hope anyone could help in this issue.
Assume you create a NIO socket and configure its channel in non blocking mode:
channel.configureBlocking(false);
then you write a byte buffer into it:
int sentBytesNum = channel.write(byteBuffer);
What are the possibilities of sentBytesNum value? I am sure of two possibilities, which are:
- all data sent so sentBytesNum equals byteBuffer data size (limit minus position before sending) (after sending the limit and position of byteBuffer are the same)
- no data is sent so sentBytesNum is zero and byteBuffer limit and postion did not change, so we shall use a write selector to test when the channel will be ready for write to write the data.
What I am not sure about and need someone to confirm is the third possibility:
- only a part of data is sent (according to the available free space of socket output buffer) so sentBytesNum is more than zero but less than data size and byteBuffer position is advanced by the value of sentBytesNum, might this possibility happen??
If yes, so we should manage to hold the non sent part of byteBuffer till the channel becomes ready for write (we can know that using write selector) so we can write it to channel???
Thanks for help,
RockaYes, case three can occur. The usual NIO write loop looks like this:
int count = 0;
while (buf.position() > 0)
buf.flip();
count = ch.write(buf);
buf.compact();
if (count == 0)
break;
}This will run until buf.position() == 0 or count == 0. If the partial write case happens it will loop one more time and probably get a 0 count.
At the end of the loop, if buf.position() > 0 there is still unwritten data and at this point you should register for OP_WRITE and return to the selector. Otherwise if there is no data unwritten you should deregister OP_WRITE as you have nothing to write and aren't interested in the event any longer.
If you are reading something and writing at the same time the logic goes like this:
while (inch.read(buf) >= 0 || buf.position() > 0)
// etc
}This will read until EOF occurs and either buf.position() is zero or a zero length write occurred. -
Thread blocking on java.nio.charset.CoderResult
Hello all,
I have a multi-threaded app which does some fairly intestive string operations (basically extracts text from documents for indexing a search system).
I am seeing a massive bottleneck around the java.nio.charset.CoderResult class. When profiling, I see a whole stack of threads blocking on (waiting for) a monitor on this java.nio.charset.CoderResult class. Seems to be a result of string encoding/decoding (I am often encoding strings as UTF-8).
Anyone know why the JVM would want my threads to sync on this class? It's creating a huge performance issue for my app. Approximately 15% of ALL the processing time is spent waiting for this class.
Help!I would guess that you're using some of the static methods in the CoderResult class. The static methods CoderResult.unmappableCache(), CoderResult.malformedForLength() and CoderResult.malformedCache all use a static inner class called Cache. Its get() method is synchronized on Cache.class. Since the Cache inner-class is static, any part of your multi-threaded application that goes through the Cache.get() method is going to be waiting for the lock on Cache.class.
Could you create a CoderResult instance for each thread? That would mean that there would be a different static Cache class for each thread, reducing the number of threads competing for the Cache.class lock.
I'd have to see some of your code to give a better answer.
Brian -
Hello,
I just started to learn the java.nio package so I decided to make a simple Echo server. I made a client which reads a line from the keyboard, sends it to the server and the server returns it. It all works except one little detail. Here's little code from the server:
int n = client.read(buffer);
if ( n > 0)
buffer.flip();
client.write(buffer);
Charset charset = Charset.forName("ISO-8859-1");
CharsetDecoder decoder = charset.newDecoder();
charBuffer = decoder.decode(buffer);
System.out.println(charBuffer.toString());
buffer.clear();
}So that works, I send the data and then I receive it back. But only for the client. I also wanted the server to print the line which is the reason for the charset and the decoder. The above code however prints only a blank line. Thus I tried this:
int n = client.read(buffer);
if ( n > 0)
buffer.flip();
Charset charset = Charset.forName("ISO-8859-1");
CharsetDecoder decoder = charset.newDecoder();
charBuffer = decoder.decode(buffer);
System.out.println(charBuffer.toString());
client.write(buffer);
buffer.clear();
}Or in other words I just moved the write() part downwards. So far so good, now the server was actually printing the lines that the client was sending but nothing was sent back to the client.
The question is how to make both, the send back line and the print line on the server side to work as intended. Also a little explanation why the events described above are happening is going to be more than welcome :)
Thanks in advance!Strike notice
A number of the regular posters here are striking in protest at the poor
management of these forums. Although it is our unpaid efforts which make the
forums function, the Sun employees responsible for them seem to regard us as
contemptible. We hope that this strike will enable them to see the value
which we provide to Sun. Apologies to unsuspecting innocents caught up in
the cross-fire.
Maybe you are looking for
-
How do I make a background photo in a folder permanent on a burned CD?
I'm trying to burn a CD with a folder that has a custom picture in the background. I can get the burned CD to show up on my own computer but when I put the disc in another computer, the picture disappears. Does anyone have a trick for making the back
-
How to deal with table.Field/sum(table.Field) BIEE
I have a question with this table.Field/sum(table.Field), and another case is: one field of table /another field of a table, but these two tables not directly relation, such as: one day sales(one table)/the last day sales(another table)
-
Support for Co-ordinate system?
Does anyone know whether Oracle Spatial supports the mapping co-ordinate system used in Romania (Stereo70)?
-
Reminder badge count is incorrect
My Reminder badge count shows that I have 4 tasks outstanding - when I have none. This is true for the machine I just upgraded to Yosemite and the one that is also running Mavericks. My iOS devices are fine. When I go into Reminders and click on the
-
Simple cfc mapping issue = I am a muppet
Right, there's something blissfully simple that I'm just missing here, but I've had by C# hat on for weeks and now have had to come back to CF and I've gotten stuck on a simple issue. Yes, I've trawled Google and the search on here but I just can't s