/**
* A list of {@link ChannelHandler}s which handles or intercepts inbound events and outbound operations of a
* {@link Channel}. {@link ChannelPipeline} implements an advanced form of the
* <a href="http://www.oracle.com/technetwork/java/interceptingfilter-142169.html">Intercepting Filter</a> pattern
* to give a user full control over how an event is handled and how the {@link ChannelHandler}s in a pipeline
* interact with each other.
*
* <h3>Creation of a pipeline</h3>
*
* Each channel has its own pipeline and it is created automatically when a new channel is created.
*
* <h3>How an event flows in a pipeline</h3>
*
* The following diagram describes how I/O events are processed by {@link ChannelHandler}s in a {@link ChannelPipeline}
* typically. An I/O event is handled by either a {@link ChannelInboundHandler} or a {@link ChannelOutboundHandler}
* and be forwarded to its closest handler by calling the event propagation methods defined in
* {@link ChannelHandlerContext}, such as {@link ChannelHandlerContext#fireChannelRead(Object)} and
* {@link ChannelHandlerContext#write(Object)}.
*
* <pre>
* I/O Request
* via {@link Channel} or
* {@link ChannelHandlerContext}
* |
* +---------------------------------------------------+---------------+
* | ChannelPipeline | |
* | \|/ |
* | +---------------------+ +-----------+----------+ |
* | | Inbound Handler N | | Outbound Handler 1 | |
* | +----------+----------+ +-----------+----------+ |
* | /|\ | |
* | | \|/ |
* | +----------+----------+ +-----------+----------+ |
* | | Inbound Handler N-1 | | Outbound Handler 2 | |
* | +----------+----------+ +-----------+----------+ |
* | /|\ . |
* | . . |
* | ChannelHandlerContext.fireIN_EVT() ChannelHandlerContext.OUT_EVT()|
* | [ method call] [method call] |
* | . . |
* | . \|/ |
* | +----------+----------+ +-----------+----------+ |
* | | Inbound Handler 2 | | Outbound Handler M-1 | |
* | +----------+----------+ +-----------+----------+ |
* | /|\ | |
* | | \|/ |
* | +----------+----------+ +-----------+----------+ |
* | | Inbound Handler 1 | | Outbound Handler M | |
* | +----------+----------+ +-----------+----------+ |
* | /|\ | |
* +---------------+-----------------------------------+---------------+
* | \|/
* +---------------+-----------------------------------+---------------+
* | | | |
* | [ Socket.read() ] [ Socket.write() ] |
* | |
* | Netty Internal I/O Threads (Transport Implementation) |
* +-------------------------------------------------------------------+
* </pre>
* An inbound event is handled by the inbound handlers in the bottom-up direction as shown on the left side of the
* diagram. An inbound handler usually handles the inbound data generated by the I/O thread on the bottom of the
* diagram. The inbound data is often read from a remote peer via the actual input operation such as
* {@link SocketChannel#read(ByteBuffer)}. If an inbound event goes beyond the top inbound handler, it is discarded
* silently, or logged if it needs your attention.
* <p>
* An outbound event is handled by the outbound handler in the top-down direction as shown on the right side of the
* diagram. An outbound handler usually generates or transforms the outbound traffic such as write requests.
* If an outbound event goes beyond the bottom outbound handler, it is handled by an I/O thread associated with the
* {@link Channel}. The I/O thread often performs the actual output operation such as
* {@link SocketChannel#write(ByteBuffer)}.
* <p>
* For example, let us assume that we created the following pipeline:
* <pre>
* {@link ChannelPipeline} p = ...;
* p.addLast("1", new InboundHandlerA());
* p.addLast("2", new InboundHandlerB());
* p.addLast("3", new OutboundHandlerA());
* p.addLast("4", new OutboundHandlerB());
* p.addLast("5", new InboundOutboundHandlerX());
* </pre>
* In the example above, the class whose name starts with {@code Inbound} means it is an inbound handler.
* The class whose name starts with {@code Outbound} means it is a outbound handler.
* <p>
* In the given example configuration, the handler evaluation order is 1, 2, 3, 4, 5 when an event goes inbound.
* When an event goes outbound, the order is 5, 4, 3, 2, 1. On top of this principle, {@link ChannelPipeline} skips
* the evaluation of certain handlers to shorten the stack depth:
* <ul>
* <li>3 and 4 don't implement {@link ChannelInboundHandler}, and therefore the actual evaluation order of an inbound
* event will be: 1, 2, and 5.</li>
* <li>1 and 2 don't implement {@link ChannelOutboundHandler}, and therefore the actual evaluation order of a
* outbound event will be: 5, 4, and 3.</li>
* <li>If 5 implements both {@link ChannelInboundHandler} and {@link ChannelOutboundHandler}, the evaluation order of
* an inbound and a outbound event could be 125 and 543 respectively.</li>
* </ul>
*
* <h3>Forwarding an event to the next handler</h3>
*
* As you might noticed in the diagram shows, a handler has to invoke the event propagation methods in
* {@link ChannelHandlerContext} to forward an event to its next handler. Those methods include:
* <ul>
* <li>Inbound event propagation methods:
* <ul>
* <li>{@link ChannelHandlerContext#fireChannelRegistered()}</li>
* <li>{@link ChannelHandlerContext#fireChannelActive()}</li>
* <li>{@link ChannelHandlerContext#fireChannelRead(Object)}</li>
* <li>{@link ChannelHandlerContext#fireChannelReadComplete()}</li>
* <li>{@link ChannelHandlerContext#fireExceptionCaught(Throwable)}</li>
* <li>{@link ChannelHandlerContext#fireUserEventTriggered(Object)}</li>
* <li>{@link ChannelHandlerContext#fireChannelWritabilityChanged()}</li>
* <li>{@link ChannelHandlerContext#fireChannelInactive()}</li>
* <li>{@link ChannelHandlerContext#fireChannelUnregistered()}</li>
* </ul>
* </li>
* <li>Outbound event propagation methods:
* <ul>
* <li>{@link ChannelHandlerContext#bind(SocketAddress, ChannelPromise)}</li>
* <li>{@link ChannelHandlerContext#connect(SocketAddress, SocketAddress, ChannelPromise)}</li>
* <li>{@link ChannelHandlerContext#write(Object, ChannelPromise)}</li>
* <li>{@link ChannelHandlerContext#flush()}</li>
* <li>{@link ChannelHandlerContext#read()}</li>
* <li>{@link ChannelHandlerContext#disconnect(ChannelPromise)}</li>
* <li>{@link ChannelHandlerContext#close(ChannelPromise)}</li>
* <li>{@link ChannelHandlerContext#deregister(ChannelPromise)}</li>
* </ul>
* </li>
* </ul>
*
* and the following example shows how the event propagation is usually done:
*
* <pre>
* public class MyInboundHandler extends {@link ChannelInboundHandlerAdapter} {
* {@code @Override}
* public void channelActive({@link ChannelHandlerContext} ctx) {
* System.out.println("Connected!");
* ctx.fireChannelActive();
* }
* }
*
* public clas MyOutboundHandler extends {@link ChannelOutboundHandlerAdapter} {
* {@code @Override}
* public void close({@link ChannelHandlerContext} ctx, {@link ChannelPromise} promise) {
* System.out.println("Closing ..");
* ctx.close(promise);
* }
* }
* </pre>
*
* <h3>Building a pipeline</h3>
* <p>
* A user is supposed to have one or more {@link ChannelHandler}s in a pipeline to receive I/O events (e.g. read) and
* to request I/O operations (e.g. write and close). For example, a typical server will have the following handlers
* in each channel's pipeline, but your mileage may vary depending on the complexity and characteristics of the
* protocol and business logic:
*
* <ol>
* <li>Protocol Decoder - translates binary data (e.g. {@link ByteBuf}) into a Java object.</li>
* <li>Protocol Encoder - translates a Java object into binary data.</li>
* <li>Business Logic Handler - performs the actual business logic (e.g. database access).</li>
* </ol>
*
* and it could be represented as shown in the following example:
*
* <pre>
* static final {@link EventExecutorGroup} group = new {@link DefaultEventExecutorGroup}(16);
* ...
*
* {@link ChannelPipeline} pipeline = ch.pipeline();
*
* pipeline.addLast("decoder", new MyProtocolDecoder());
* pipeline.addLast("encoder", new MyProtocolEncoder());
*
* // Tell the pipeline to run MyBusinessLogicHandler's event handler methods
* // in a different thread than an I/O thread so that the I/O thread is not blocked by
* // a time-consuming task.
* // If your business logic is fully asynchronous or finished very quickly, you don't
* // need to specify a group.
* pipeline.addLast(group, "handler", new MyBusinessLogicHandler());
* </pre>
*
* <h3>Thread safety</h3>
* <p>
* A {@link ChannelHandler} can be added or removed at any time because a {@link ChannelPipeline} is thread safe.
* For example, you can insert an encryption handler when sensitive information is about to be exchanged, and remove it
* after the exchange.
*/
