Implementing non-blocking read
Hi all
I have some doubts about implementing non-blocking io in my application.
I am using Jsch library (an ssh implementation) to open an ssh connection with a host. The API only provides me with methods to open a connection and retreive the input & output streams. I want to make my read on inputstream non-blocking.In such a case is it possible to use nio for the purpose?
If it's not possible then I am planning to use threading to make read() non-blocking. Here also i need some clarifications. I am planning to use a ThreadPoolExecutor to create a thread pool for reading data. SO whenever i have a read i'll assign this task to the pool which will use one of the free threads to execute the inputStresm.read().
Now the question is if one of the threads in this pool blocks forever during a read since it didn't get any response from the other side, is there a way to stop that read and make that thread free again to execute more tasks? or will the thread block forever till the application is closed?
In my case i cannot afford to have too many such blocked threads, since this application will not be restarted very often. Once it is started it can go on for may be days or months.
Please suggest what would be best in my case taking into account performance as most important factor.
Thanks in advance.
Anu
endasil wrote:
First of all, let me state that I agree with the others in saying that I don't fully agree with your premises.
That said, I believe that this does a non-blocking read based on the contract of InputStream.available() and .read(byte[], int, int):
private int nonBlockingRead(InputStream in, byte[] buffer) throws IOException {
return in.read(buffer, 0, Math.min(in.available(), buffer.length));
If the InputStream is obtained from a JSSE socket then it is my understanding that available() always returns zero. This is allowed under the InputStream.available() contract as defined in the Javadoc - http://java.sun.com/javase/6/docs/api/java/io/InputStream.html#available() . If I am right then your code will never read anything from a JSSE socket InputStream and I would suspect that Jsch is using JSSE sockets.
Similar Messages
-
Non-blocking read from an InputStream??
Hello all, I have a bit of a problem. I have a GUI based app that, through the mindterm SSH classes, runs a "tail -f /var/log/somelog" command on a *nix server. The purpose of the app is to make looking through the log's easier for the non-computer lay-person. The problem is that if they click the stop button to stop the output of the tail it doesn't actually stop that thread until the next line is appended to the end of the log file. That could be a second or an hour. So what I need is a way to somehow stop that tail -f command when the user hits stop and not have to wait for the read to occur.
What I'm working with is a com.mindbright.util.InputStreamPipe that is a subclass of java.io.InputStream. I've tried several things. Such as the seda.nbio.NonblockingInputStream which gives me a runtime classCastException when trying to cast the com.mindbright.util.InputStreamPipe to a seda.nbio.NonblockingInputStream. I've tried creating a java.nio.channels.ReadableByteChannel and using it's read method but that also blocks.
So my question is, does anyone have any clever solutions to this particular problem? I thought the way to beat it was to find some mechanism to do a read that will just get passed by if there's nothing to read then have it wait an arbitrary amount of time before reading again. So the longest the user would have to wait would be that arbitrary amount of time. But I don't know how to implement this or if it's even a good way to do it.
Thanks in advance!Thanks for the help dubwai. I actually found a way to accomplish this without any non-blocking balony.
public void run () {
try {
java.io.InputStream is = tbs.getConsoleOut();
java.io.BufferedReader in = new java.io.BufferedReader(new java.io.InputStreamReader(is));
String line = null;
// continue is a volatile boolean that gets set to false when user clicks stop
while ( cont ) {
if ( is.available() > 0 ) {
line = in.readLine();
System.out.println("in while: "+line);
} // end
else {
try {
java.lang.Thread.sleep(500);
} // end try
catch ( java.lang.InterruptedException ie ) {
System.err.println("Error sleeping: "+ie);
} // end catch
} // end else
} // end while
is.close(); in.close();
System.out.println("After while");
System.exit(0);
} // end try
catch ( Exception e ) {
System.err.println("Error reading lines: "+e);
} // end catch
} // end runThis seems to work on a few trial runs.. Does anyone see any possible random timing issues from this?? -
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();
} -
Easy way to non-blocked sockets
Use JSSE and NIO for a quick way to implement non-blocking communications
October 22, 2003
Although SSL blocking operations -- in which the socket is blocked from access while data is being read from or written to -- provide better I/O-error notification than the non-blocking counterpart, non-blocking operations allow the calling thread to continue. In this article, the author will cover both the client and server side as he describes how to create non-blocking secure connections using the Java Secure Socket Extensions (JSSE) and the Java NIO (new I/O) library, and he will explain the traditional approach to creating a non-blocking socket, as well as an alternative (and necessary) method if you want to use JSSE with NIO.
http://www-106.ibm.com/developerworks/java/library/j-sslnb.html?ca=dgr-jw03j-sslnbMORE IBM SPAM Previous discussion
I find it interesting spam, but thats a matter of taste. If the OP was truly interested in "trying to get new information out there" he would answer the mulitple questions about NIO and especially NIO mixed with traditional Sockets and NIO vs Secure Sockets. These are all on ALT, NIO is of no interest to New to Java folk.
Given their budget I think IBM could do a better job of publishing their research. -
Hi All,
Anybody have some idea about how to implement non blocking sockets using two threads/Can u help me with some sites where i can get more information in this topic.
Regards
Priyahi,
you could have a look at the nonblocking io (nio) api's present in jdk1.4, that should do the trick. this link ought to get u started i suppose..
http://developer.java.sun.com/developer/technicalArticles/releases/nio/
hope this helps.
cheerz
ynkrish -
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,
FrankYou 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 -
Non-blocking Vectored read from SocketChannel fails
Hi.
Please look at the code example:
ByteBuffer tmp1 = ByteBuffer.allocate(500);
ByteBuffer tmp2 = ByteBuffer.allocate(500);
ByteBuffer[] tmp = {tmp1, tmp2};
while ((count = socketChannel.read(tmp)) > 0) {
+ "bytes.");
When I run this code, (using non-blocking socket channel), the first buffer is read ok, but on the second round of the while loop, I get an IOException, saying: "a non blocking socket operation could not be completed immediately".
If I put 'tmp1' instead of 'tmp' in the read(), (i.e - use a normal buffer), the read successfully finishes.
Why?
I found nothing in the documentation, and in the 'Java NIO' book (Ron Hitchens).
Help?
Thanks.Please ignore the + "bytes."); inside the while.
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Read Timeout on non-blocking sockets
Hi,
I was wondering if there is a way to specify a read timeout (like setSoTimeout for synchronous sockets) when using a non-blocking socket.
I'd like to have the select() method return is a sockets timeout expires, puting in the selected key set the timedout socket and have it's read operation return -1, something like what happens when a socket is closed by the other side.
The thing is I need this to be a timeout specific to each socket, thus the select(millis) isn't apropriate.
Anyone knows of something like this?
Thanks....Yeah, select() is the only thing built in for that, and you have to do the bookkeeping yourself. You would start something like forming the disjunction of the ready keys and the registered keys after each select, to get the unready keys, and then looking at their history to see how long they have been unready via a Map{key,Long(time)).
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Non-Blocking call to read the Keyboard
does anyone know how to make a JAVA program make a non-blocking call to read the keyboard? eg. write a program which generates prime number until a keyboard key is pressed.
if you use a gui you can use keyListener
Would work only if your gui elements have focus right now. -
Non-blocking file behaviour for Reader
Hi!
It will be really nice if Reader will not block files for writing. Assume someone is using TeX to create PDF and he need to
a) examine the results;
b) edit the source.
It becomes a nightmare with Reader. Compile TeX, open Reader, examine results, close Reader. And then from the beginning.
Auto reloading of a PDF file is also a nice feature!
So far I can see only one solution: one should use another viewer to get non-blocking behaviour.
Thanks!OK, let us see. I use Windows XP Professional SP3, Reader version is 9.3.0. I made a simple test: openned a file in Reader, then openned this file in my favourite text editor and tried to change "%PDF-1.4" to "%PDF-1.3" and save. I received a "File sharing violation error". I can also try to do it in Ubuntu.
As for the open source viewers. That is what I am currently doing: I use additional viewer to view the files I create and I use Adobe Reader to view all other files. One can leave with such solution, but it is not the best. -
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 -
Non-Blocking I/O Implementation Issue
Hi All,
I am trying out the latest JDK 1.4 java.nio.* package. I modified the NBTimeServer and wrote a client which connects to the NBTimeServer and tries to pass messages to and fro. I always succeed to pass on roundtrip of msgs and then Server blocks my client forever. I have modified NBTimeServer to accomodate one client only. Any help or comments on this would be really appreciated. Code is below. Feel free to try it out if want to see what I am trying to convey in this message.
/*******Server Code*******/
Modified this code to test mulitple to and fro msgs between client and server.
Only one client will ever be able to connect to this server during life of a server.
My point here is to demonstrate the to and fro comm between one client and one server
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.channels.spi.*;
import java.net.*;
import java.util.*;
import java.nio.charset.*;
import java.util.regex.*;
// Listen on a port for connections and write back the current time.
public class NBTimeServer {
private static final int DEFAULT_TIME_PORT = 8900;
// Constructor with no arguments creates a time server on default port.
public NBTimeServer() throws Exception {
acceptConnections(this.DEFAULT_TIME_PORT);
// Constructor with port argument creates a time server on specified port.
public NBTimeServer(int port) throws Exception {
acceptConnections(port);
// Accept connections for current time. Lazy Exception thrown.
private static void acceptConnections(int port) throws Exception {
// Selector for incoming time requests
Selector acceptSelector = SelectorProvider.provider().openSelector();
Selector rwSelector = SelectorProvider.provider().openSelector();
// Create a new server socket and set to non blocking mode
ServerSocketChannel ssc = ServerSocketChannel.open();
ssc.configureBlocking(false);
// Bind the server socket to the local host and port
InetAddress lh = InetAddress.getLocalHost();
InetSocketAddress isa = new InetSocketAddress(lh, port);
ssc.socket().bind(isa);
// Register accepts on the server socket with the selector. This
// step tells the selector that the socket wants to be put on the
// ready list when accept operations occur, so allowing multiplexed
// non-blocking I/O to take place.
SelectionKey acceptKey = ssc.register(acceptSelector,
SelectionKey.OP_ACCEPT);
int keysAdded = 0;
// Here's where everything happens. The select method will
// return when any operations registered above have occurred, the
// thread has been interrupted, etc.
while ((keysAdded = acceptSelector.select()) > 0) {
// Someone is ready for I/O, get the ready keys
Set readyKeys = acceptSelector.selectedKeys();
Iterator i = readyKeys.iterator();
// Walk through the ready keys collection and process date requests.
while (i.hasNext()) {
SelectionKey sk = (SelectionKey)i.next();
i.remove();
// The key indexes into the selector so you
// can retrieve the socket that's ready for I/O
ServerSocketChannel nextReady =
(ServerSocketChannel)sk.channel();
// Accept the date request and send back the date string
Socket s = nextReady.accept();
SocketChannel sc = s.getChannel();
System.out.println("Got client channel..");
sc.configureBlocking(false);
SelectionKey readKey = sc.register(rwSelector,
SelectionKey.OP_READ|SelectionKey.OP_WRITE);
int count = 0;
while(true) {
if((count = rwSelector.select(1000L)) > 0) {
Set readKeys = rwSelector.selectedKeys();
Iterator i1 = readKeys.iterator();
while(i1.hasNext()) {
System.out.println("Loop in Iterator");
SelectionKey sk1 = (SelectionKey)i1.next();
i1.remove();
if(sk1.isReadable()) {
DataInputStream sin = new DataInputStream(new BufferedInputStream(s.getInputStream(), 4096));
System.out.println(sin.readInt());
if(sk1.isWritable()) {
DataOutputStream sout = new DataOutputStream(new BufferedOutputStream(s.getOutputStream(), 4096));
PrintWriter out = new PrintWriter(sout, true);
Date now = new Date();
out.println(now);
// Entry point.
public static void main(String[] args) {
// Parse command line arguments and
// create a new time server (no arguments yet)
try {
NBTimeServer nbt = new NBTimeServer();
} catch(Exception e) {
e.printStackTrace();
/******End Server Code********/
/*****Begin Client Code********/
import java.io.*;
import java.net.*;
import java.util.*;
// Listen on a port for connections and write back the current time.
public class NBTimeClient {
private static final int DEFAULT_TIME_PORT = 8900;
public static void main(String args[]) throws Exception {
InetAddress lh = InetAddress.getLocalHost();
Socket s = new Socket(lh, DEFAULT_TIME_PORT);
DataInputStream din = new DataInputStream(new BufferedInputStream(s.getInputStream(), 4096));
DataOutputStream dout = new DataOutputStream(new BufferedOutputStream(s.getOutputStream(), 4096));
//Read the time
System.out.println(din.readLine());
//send some junk which is read by server
dout.writeInt(1299);
dout.flush();
//read time again -- I never get anything here and I am blocked here...
System.out.println(din.readLine());
//send some junk back to the server
dout.writeInt(1299);
dout.flush();
s.close();
/*******End Client Code**********/
thanks,
XtrimityThe reason it blocks forever is that you need to keep reusing your main select. That is where the non-blocking event will come from. Here is a bit of code that doesn't block forever.
Tim
http://tim.owlmountain.com
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.channels.spi.*;
import java.net.*;
import java.util.*;
import org.apache.log4j.*;
public class NBServer3 {
int _port = 4000;
Selector _selector = null;
ServerSocketChannel _selectableChannel = null;
int _keysAdded = 0;
static Category log =
Category.getInstance(NBServer3.class.getName());
static String QUIT_SERVER = "quit";
public NBServer3() {
public NBServer3( int port ) {
this._port = port;
public void initialize()
throws IOException {
this._selector = SelectorProvider.provider().openSelector();
this._selectableChannel = ServerSocketChannel.open();
this._selectableChannel.configureBlocking(false);
InetAddress lh = InetAddress.getLocalHost();
InetSocketAddress isa = new InetSocketAddress(lh, this._port );
this._selectableChannel.socket().bind(isa);
public void finalize()
throws IOException {
this._selectableChannel.close();
this._selector.close();
public void acceptConnections()
throws IOException {
Selects a set of keys whose corresponding channels are ready for I/O
operations. This method performs a non-blocking selection operation.
If no channels have become selectable since the previous selection
operation then this method immediately returns zero.
Returns:
The number of keys, possibly zero, whose ready-operation sets
were updated by the selection operation
do {
SelectionKey acceptKey =
this._selectableChannel.register( this._selector,
SelectionKey.OP_ACCEPT );
log.debug( "Acceptor loop..." );
while (( this._keysAdded = acceptKey.selector().select()) > 0 ) {
log.debug( "Selector returned "
+ this._keysAdded + " ready for IO operations" );
Set readyKeys = this._selector.selectedKeys();
Iterator i = readyKeys.iterator();
while (i.hasNext()) {
SelectionKey key = (SelectionKey)i.next();
i.remove();
if ( key.isAcceptable() ) {
ServerSocketChannel nextReady =
(ServerSocketChannel)key.channel();
log.debug( "Processing selection key read="
+ key.isReadable() + " write=" + key.isWritable() +
" accept=" + key.isAcceptable() );
Socket s = nextReady.accept();
s.getChannel().configureBlocking( false );
SelectionKey readKey =
s.getChannel().register( this._selector,
SelectionKey.OP_READ );
readKey.attach( s );
else if ( key.isReadable() ) {
SelectableChannel nextReady =
(SelectableChannel) key.channel();
log.debug( "Processing selection key read="
+ key.isReadable() + " write=" + key.isWritable() +
" accept=" + key.isAcceptable() );
Socket socket = (Socket) key.attachment();
BufferedReader in = new BufferedReader(
new InputStreamReader( socket.getInputStream() ));
String line = null;
if ( (line = in.readLine() ) != null )
log.debug( line );
log.debug( "End acceptor loop..." );
} while ( false ); //FIXIT tim this should be false. justa test
public static void main( String[] args ) {
BasicConfigurator.configure();
NBServer3 nbServer = new NBServer3();
try {
nbServer.initialize();
} catch ( IOException e ) {
e.printStackTrace();
System.exit( -1 );
try {
nbServer.acceptConnections();
catch ( IOException e ) {
e.printStackTrace();
log.error( e ); -
Non-Blocking SocketChannel read give Connection timed out
Hi,
My program is using Non-Block NIO SocketChannel for both Server and Client sides. However, if the connection between server and client has been idle for a while, when the client try to communicate with server, it gets blocked for a while (15 minutes) and then receives the following exception:
java.io.IOException: Connection timed out
at sun.nio.ch.FileDispatcher.read0(Native Method)
at sun.nio.ch.SocketDispatcher.read(Unknown Source)
at sun.nio.ch.IOUtil.readIntoNativeBuffer(Unknown Source)
at sun.nio.ch.IOUtil.read(Unknown Source)
at sun.nio.ch.SocketChannelImpl.read(Unknown Source)
How can this be since the read is in Non-Blocking mode? Also, is there anyway to determine the timeout without being blocked?This would mean that you are trying to read from the socket without having properly completed the connection, which timed out. I would say you are connecting in non-blocking mode but not calling finishConnect() when you get OP_CONNECT.
-
Hi friends,
I have an issue to implement. We implemented a web application and now I want to separate some parts of this application. I can express the action shorter and simple below.
if a user requests x-y ticket {
���try {
������check db if x-y is available
������if true send available
������else not
���}catch (NonExistentData) {
������addJob2Queue(x-y)
������send available
���}
class AvailabilityHelper extends Thread{
���run(){
������while(true){
���������checkQueue
���������if there is job, updateDBfor(org,dest)
������}
���}
* updateDBfor(org,dest) includes other ejb references to request data and insert it into db
This web aplication runs with JSP 1.2 EL support on WAS 5.1
the check for db and the thread implementation was in a seperate package and were used in actions of web application. Now they also want to use these checks for other (non web) applications. So i think there should be a session bean in ejb and should process these requests and I used JMS (Message-Driven Bean) with WebSphere MQ to undertake the thread and queue mechanism.
Now it works like this :
class exampleSessionBean{
���getAvalibability(org, dest){
������try {
���������check db if x-y is available
���������if true send available
���������else not
������}catch (NonExistentData) {
���������queueSender.sendMessage(update x-y)
���������send available;
������}
���}
class queueMDB{
���onMessage(){
������updateDBfor(org,dest)
���}
The purpose of this applications was, not to block the user for db update process, if there is data in db select and show to the user. the update process of db includes other ejb references and takes so much time. It is nonsense to make the user wait such time. And the info is not a big deal, because of this when i put a request for update, without blocking i send it as available...
So what is the problem ? :) Deployment team don't want to pay for websphere MQ and hire a admin for that. Also they don't want to use Embedded Messaging server client (JMS Server) of WAS. So I am asking you: is there a way to implement a non blocking servlet so that i will just make a request and don't wait for response, or a servlet will take the request and release the connection but will continue to process the request ?
thanks for all...I tried to implement a web application and add a ServletContextListener as stevejluke suggests. But on contextInitialized function when creating new JobConsumer it gets following errors on server.
6/27/06 12:51:36:391 CEST] 33238138 WebContainer A SRVE0169I: Loading Web Module: non-blocking.
[6/27/06 12:51:36:438 CEST] 33238138 SystemOut O OlaListener initializing...
[6/27/06 12:51:36:516 CEST] 33238138 SystemOut O OlaListener - Queue added
[6/27/06 12:51:36:516 CEST] 33238138 SystemOut O OlaListener - keepWorking
[6/27/06 12:51:36:672 CEST] 33238138 WebApp E SRVE0015E: Failure to initialize Web application non-blocking
[6/27/06 12:51:36:844 CEST] 33238138 WebGroup E SRVE0054E: An error occurred while loading Web application
public class OlaListener implements ServletContextListener {
* @see javax.servlet.ServletContextListener#void (javax.servlet.ServletContextEvent)
public void contextDestroyed(ServletContextEvent event) {
ServletContext context = event.getServletContext();
context.setAttribute("keepWorking", Boolean.FALSE);
* @see javax.servlet.ServletContextListener#void (javax.servlet.ServletContextEvent)
public void contextInitialized(ServletContextEvent event) {
System.out.println("OlaListener initializing...");
ServletContext context = event.getServletContext();
WorkQueue newJobs = new WorkQueue();
context.setAttribute("newJobs", newJobs);
System.out.println("OlaListener - Queue added");
context.setAttribute("keepWorking", Boolean.TRUE);
System.out.println("OlaListener - keepWorking");
JobConsumer jc = new JobConsumer();
System.out.println("OlaListener - Job Consumer created");
jc.setContext(context);
System.out.println("OlaListener - context set");
Thread t = new Thread(jc);
t.start();
public class JobConsumer implements Runnable{
/*log4j*/ private static final Logger logger = Logger.getLogger(JobConsumer.class);
private ServletContext context = null;
public void run(){
System.out.println("JobConsumer - run Start");
processQueue(context);
System.out.println("JobConsumer - run End");
} -
Does Java provide any way to implement a non-blocking accept()?
There is no 'more or less' about it, it can be done. See java.nio.channels.ServerSocketChannel.
You could also set a very short timeout on a ServerSocket if you don't want the entire NIO circus ...
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