[java] When is "java.io.IOException:Connection reset by peer" thrown?

ERROR GServerHandler  - java.io.IOException: Connection reset by peer
java.io.IOException: Connection reset by peer
        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)
        at org.jboss.netty.channel.socket.nio.NioWorker.read(NioWorker.java:323)
        at org.jboss.netty.channel.socket.nio.NioWorker.processSelectedKeys(NioWorker.java:282)
        at org.jboss.netty.channel.socket.nio.NioWorker.run(NioWorker.java:202)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)

This log is from a game server implemented using netty. What can cause this exception ?

To expand on BalusC's answer, any scenario where the sender continues to write after the peer has stopped reading and closed its socket will produce this exception, as will the peer closing while it still had unread data in its own socket receive buffer. In other words, an application protocol error. For example, if you write something to the peer that the peer doesn't understand, and then it closes its socket in protest, and you then continue to write, the peer's TCP stack will issue an RST, which results in this exception and message at the sender.

It can also mean that the server is completely inaccessible - I was getting this when trying to hit a server that was offline

My client was configured to connect to localhost:3000, but no server was running on that port.

I think this should be java.net.SocketException as its definition is stated for a TCP error.

/**
 * Thrown to indicate that there is an error in the underlying 
 * protocol, such as a TCP error. 
 *
 * @author  Jonathan Payne
 * @version %I%, %G%
 * @since   JDK1.0
 */
public 
class SocketException extends IOException {

There are lot of factors , first see whether server returns the result, then check between server and client.

rectify them from server side first,then check the writing condition between server and client !

server side rectify the time outs between the datalayer and server from client side rectify the time out and number of available connections !

java.io.IOException: Connection reset by peer

In my case, the problem was with PUT requests (GET and POST were passing successfully).

Communication went through VPN tunnel and ssh connection. And there was a firewall with default restrictions on PUT requests... PUT requests haven't been passing throughout, to the server...

Problem was solved after exception was added to the firewall for my IP address.

java.io.IOException in Netty means your game server tries to send data to a client, but that client has closed connection to your server.

And that exception is not the only one! There're several others. See BadClientSilencer in Xitrum. I had to add that to prevent those errors from messing my log file.

For me useful code witch help me was http://rox-xmlrpc.sourceforge.net/niotut/src/NioServer.java

// The remote forcibly closed the connection, cancel

// the selection key and close the channel.

    private void read(SelectionKey key) throws IOException {
            SocketChannel socketChannel = (SocketChannel) key.channel();

            // Clear out our read buffer so it's ready for new data
            this.readBuffer.clear();

            // Attempt to read off the channel
            int numRead;
            try {
                numRead = socketChannel.read(this.readBuffer);
            } catch (IOException e) {
                // The remote forcibly closed the connection, cancel
                // the selection key and close the channel.
                key.cancel();
                socketChannel.close();
                return;
            }

            if (numRead == -1) {
                // Remote entity shut the socket down cleanly. Do the
                // same from our end and cancel the channel.
                key.channel().close();
                key.cancel();
                return;
            }
...