Examples of io.netty.channel.ChannelEvent

io.netty.channel.ChannelEvent

An I/O event or I/O request associated with a {@link Channel}.

A {@link ChannelEvent} is handled by a series of {@link ChannelHandler}s in a {@link ChannelPipeline}.

Upstream events and downstream events, and their interpretation

Every event is either an upstream event or a downstream event. If an event flows forward from the first handler to the last handler in a {@link ChannelPipeline}, we call it an upstream event and say "an event goes upstream." If an event flows backward from the last handler to the first handler in a {@link ChannelPipeline}, we call it a downstream event and say "an event goes downstream." (Please refer to the diagram in {@link ChannelPipeline} for more explanation.)

When your server receives a message from a client, the event associated with the received message is an upstream event. When your server sends a message or reply to the client, the event associated with the write request is a downstream event. The same rule applies for the client side. If your client sent a request to the server, it means your client triggered a downstream event. If your client received a response from the server, it means your client will be notified with an upstream event. Upstream events are often the result of inbound operations such as {@link InputStream#read(byte[])}, and downstream events are the request for outbound operations such as {@link OutputStream#write(byte[])}, {@link Socket#connect(SocketAddress)}, and {@link Socket#close()}.

Upstream events

Event name	Event type and condition	Meaning
{@code "messageReceived"}	{@link MessageEvent}	a message object (e.g. {@link ChannelBuffer}) was received from a remote peer
{@code "exceptionCaught"}	{@link ExceptionEvent}	an exception was raised by an I/O thread or a {@link ChannelHandler}
{@code "channelOpen"}	{@link ChannelStateEvent} (state = {@link ChannelState#OPEN OPEN}, value = {@code true})	a {@link Channel} is open, but not bound nor connected	Be aware that this event is fired from within the Boss-Thread so you should not execute any heavy operation in there as it will block the dispatching to other workers!
{@code "channelClosed"}	{@link ChannelStateEvent} (state = {@link ChannelState#OPEN OPEN}, value = {@code false})	a {@link Channel} was closed and all its related resources were released
{@code "channelBound"}	{@link ChannelStateEvent} (state = {@link ChannelState#BOUND BOUND}, value = {@link SocketAddress})	a {@link Channel} is open and bound to a local address, but not connected	Be aware that this event is fired from within the Boss-Thread so you should not execute any heavy operation in there as it will block the dispatching to other workers!
{@code "channelUnbound"}	{@link ChannelStateEvent} (state = {@link ChannelState#BOUND BOUND}, value = {@code null})	a {@link Channel} was unbound from the current local address
{@code "channelConnected"}	{@link ChannelStateEvent} (state = {@link ChannelState#CONNECTED CONNECTED}, value = {@link SocketAddress})	a {@link Channel} is open, bound to a local address, and connected to a remote address
{@code "writeComplete"}	{@link WriteCompletionEvent}	something has been written to a remote peer
{@code "channelDisconnected"}	{@link ChannelStateEvent} (state = {@link ChannelState#CONNECTED CONNECTED}, value = {@code null})	a {@link Channel} was disconnected from its remote peer
{@code "channelInterestChanged"}	{@link ChannelStateEvent} (state = {@link ChannelState#INTEREST_OPS INTEREST_OPS}, no value)	a {@link Channel}'s {@link Channel#getInterestOps() interestOps} was changed

These two additional event types are used only for a parent channel which can have a child channel (e.g. {@link ServerSocketChannel}).

Event name	Event type and condition	Meaning
{@code "childChannelOpen"}	{@link ChildChannelStateEvent} ( {@code childChannel.isOpen() = true})	a child {@link Channel} was open (e.g. a server channel accepted a connection.)
{@code "childChannelClosed"}	{@link ChildChannelStateEvent} ( {@code childChannel.isOpen() = false})	a child {@link Channel} was closed (e.g. the accepted connection was closed.)

Downstream events

Event name	Event type and condition	Meaning
{@code "write"}	{@link MessageEvent}	Send a message to the {@link Channel}.
{@code "bind"}	{@link ChannelStateEvent} (state = {@link ChannelState#BOUND BOUND}, value = {@link SocketAddress})	Bind the {@link Channel} to the specified local address.
{@code "unbind"}	{@link ChannelStateEvent} (state = {@link ChannelState#BOUND BOUND}, value = {@code null})	Unbind the {@link Channel} from the current local address.
{@code "connect"}	{@link ChannelStateEvent} (state = {@link ChannelState#CONNECTED CONNECTED}, value = {@link SocketAddress})	Connect the {@link Channel} to the specified remote address.
{@code "disconnect"}	{@link ChannelStateEvent} (state = {@link ChannelState#CONNECTED CONNECTED}, value = {@code null})	Disconnect the {@link Channel} from the current remote address.
{@code "close"}	{@link ChannelStateEvent} (state = {@link ChannelState#OPEN OPEN}, value = {@code false})	Close the {@link Channel}.

Other event types and conditions which were not addressed here will be ignored and discarded. Please note that there's no {@code "open"} in thetable. It is because a {@link Channel} is always open when it is createdby a {@link ChannelFactory}.

Additional resources worth reading

Please refer to the {@link ChannelHandler} and {@link ChannelPipeline}documentation to find out how an event flows in a pipeline and how to handle the event in your application. @apiviz.landmark @apiviz.composedOf io.netty.channel.ChannelFuture

     *         if failed to receive a new message
     * @throws InterruptedException
     *         if the operation has been interrupted
     */
    public E read(long timeout, TimeUnit unit) throws IOException, InterruptedException {
        ChannelEvent e = readEvent(timeout, unit);
        if (e == null) {
            return null;
        }


        if (e instanceof MessageEvent) {

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            if (getQueue().isEmpty()) {
                return null;
            }
        }


        ChannelEvent e = getQueue().take();
        if (e instanceof ChannelStateEvent) {
            // channelClosed has been triggered.
            assert closed;
            return null;
        } else {

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            if (getQueue().isEmpty()) {
                return null;
            }
        }


        ChannelEvent e = getQueue().poll(timeout, unit);
        if (e == null) {
            throw new BlockingReadTimeoutException();
        } else if (e instanceof ChannelStateEvent) {
            // channelClosed has been triggered.
            assert closed;

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     *         if failed to receive a new message
     * @throws InterruptedException
     *         if the operation has been interrupted
     */
    public E read() throws IOException, InterruptedException {
        ChannelEvent e = readEvent();
        if (e == null) {
            return null;
        }


        if (e instanceof MessageEvent) {

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     * make sure important tasks are not counted.
     */
    protected boolean shouldCount(Runnable task) {
        if (task instanceof ChannelUpstreamEventRunnable) {
            ChannelUpstreamEventRunnable r = (ChannelUpstreamEventRunnable) task;
            ChannelEvent e = r.getEvent();
            if (e instanceof WriteCompletionEvent) {
                return false;
            } else if (e instanceof ChannelStateEvent) {
                if (((ChannelStateEvent) e).getState() == ChannelState.INTEREST_OPS) {
                    return false;

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Related Classes of io.netty.channel.ChannelEvent

io.netty.handler.execution.MemoryAwareThreadPoolExecutor

io.netty.handler.queue.BlockingReadHandler

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