一、ChannelHandler

1. Channel的继承关系

2.ChannelHandler

处理I/O事件或者拦截一个I/O操作，并且把操作forward到其ChannelPipeline的下一个Handler

用户通过自定义ChannelHandler来实现自己的业务逻辑

ChannelHandler的类注释比较多，把ChannelHandler的相关类都说清楚了，可以源码中细看

public interface ChannelHandler {

    /**
     * Gets called after the {@link ChannelHandler} was added to the actual context and it's ready to handle events.
     * 当handler加入到上下文中调用
     */
    void handlerAdded(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called after the {@link ChannelHandler} was removed from the actual context and it doesn't handle events
     * anymore.
     * 当handler加入到上下文中调用
     */
    void handlerRemoved(ChannelHandlerContext ctx) throws Exception;

    /**
     * Indicates that the same instance of the annotated {@link ChannelHandler}
     * can be added to one or more {@link ChannelPipeline}s multiple times
     * without a race condition.
     * <p>
     * If this annotation is not specified, you have to create a new handler
     * instance every time you add it to a pipeline because it has unshared
     * state such as member variables.
     * <p>
     * This annotation is provided for documentation purpose, just like
     * <a href="http://www.javaconcurrencyinpractice.com/annotations/doc/">the JCIP annotations</a>.
     * 可以共享实例的handler，但是可能会存在线程安全问题
     */
    @Inherited
    @Documented
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @interface Sharable {
        // no value
    }
}

3.ChannelInboundHandler

处理输入的IO操作

public interface ChannelInboundHandler extends ChannelHandler {

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered with its {@link EventLoop}
     */
    void channelRegistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was unregistered from its {@link EventLoop}
     */
    void channelUnregistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} is now active
     */
    void channelActive(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered is now inactive and reached its
     * end of lifetime.
     */
    void channelInactive(ChannelHandlerContext ctx) throws Exception;

    /**
     * Invoked when the current {@link Channel} has read a message from the peer.
     */
    void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception;

    /**
     * Invoked when the last message read by the current read operation has been consumed by
     * {@link #channelRead(ChannelHandlerContext, Object)}.  If {@link ChannelOption#AUTO_READ} is off, no further
     * attempt to read an inbound data from the current {@link Channel} will be made until
     * {@link ChannelHandlerContext#read()} is called.
     * 在channelRead方法后调用
     */
    void channelReadComplete(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if an user event was triggered.
     */
    void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception;

    /**
     * Gets called once the writable state of a {@link Channel} changed. You can check the state with
     * {@link Channel#isWritable()}.
     */
    void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if a {@link Throwable} was thrown.
     * 发生异常时调用
     */
    @Override
    @SuppressWarnings("deprecation")
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
}

4.ChannelOutboundHandler

处理输出的数据

public interface ChannelOutboundHandler extends ChannelHandler {
    /**
     * Called once a bind operation is made.
     *
     * @param ctx           the {@link ChannelHandlerContext} for which the bind operation is made
     * @param localAddress  the {@link SocketAddress} to which it should bound
     * @param promise       the {@link ChannelPromise} to notify once the operation completes
     * @throws Exception    thrown if an error occurs
     */
    void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception;

    /**
     * Called once a connect operation is made.
     *
     * @param ctx               the {@link ChannelHandlerContext} for which the connect operation is made
     * @param remoteAddress     the {@link SocketAddress} to which it should connect
     * @param localAddress      the {@link SocketAddress} which is used as source on connect
     * @param promise           the {@link ChannelPromise} to notify once the operation completes
     * @throws Exception        thrown if an error occurs
     */
    void connect(
            ChannelHandlerContext ctx, SocketAddress remoteAddress,
            SocketAddress localAddress, ChannelPromise promise) throws Exception;

    /**
     * Called once a disconnect operation is made.
     */
    void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;

    /**
     * Called once a close operation is made.
     */
    void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;

    /**
     * Called once a deregister operation is made from the current registered {@link EventLoop}.
     *
     * @param ctx               the {@link ChannelHandlerContext} for which the close operation is made
     * @param promise           the {@link ChannelPromise} to notify once the operation completes
     * @throws Exception        thrown if an error occurs
     */
    void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;

    /**
     * Intercepts {@link ChannelHandlerContext#read()}.
     */
    void read(ChannelHandlerContext ctx) throws Exception;

    /**
    * Called once a write operation is made. The write operation will write the messages through the
     * {@link ChannelPipeline}. Those are then ready to be flushed to the actual {@link Channel} once
     * {@link Channel#flush()} is called
     *
     */
    void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception;

    /**
     * Called once a flush operation is made. The flush operation will try to flush out all previous written messages
     * that are pending.
     */
    void flush(ChannelHandlerContext ctx) throws Exception;
}

5.ChannelHandlerAdapter

提供了一些简单的实现，为实现类提供公共方法

public abstract class ChannelHandlerAdapter implements ChannelHandler {

    // Not using volatile because it's used only for a sanity check.
    boolean added;

    /**
     * Throws {@link IllegalStateException} if {@link ChannelHandlerAdapter#isSharable()} returns {@code true}
     */
    protected void ensureNotSharable() {
        if (isSharable()) {
            throw new IllegalStateException("ChannelHandler " + getClass().getName() + " is not allowed to be shared");
        }
    }

    /**
     * Return {@code true} if the implementation is {@link Sharable} and so can be added
     * to different {@link ChannelPipeline}s.
     */
    public boolean isSharable() {
        /**
         * Cache the result of {@link Sharable} annotation detection to workaround a condition. We use a
         * {@link ThreadLocal} and {@link WeakHashMap} to eliminate the volatile write/reads. Using different
         * {@link WeakHashMap} instances per {@link Thread} is good enough for us and the number of
         * {@link Thread}s are quite limited anyway.
         *
         * See <a href="https://github.com/netty/netty/issues/2289">#2289</a>.
         */
        Class<?> clazz = getClass();
        Map<Class<?>, Boolean> cache = InternalThreadLocalMap.get().handlerSharableCache();
        Boolean sharable = cache.get(clazz);
        if (sharable == null) {
            sharable = clazz.isAnnotationPresent(Sharable.class);
            cache.put(clazz, sharable);
        }
        return sharable;
    }

    /**
     * Do nothing by default, sub-classes may override this method.
     */
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        // NOOP
    }

    /**
     * Do nothing by default, sub-classes may override this method.
     */
    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        // NOOP
    }

    /**
     * Calls {@link ChannelHandlerContext#fireExceptionCaught(Throwable)} to forward
     * to the next {@link ChannelHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     *
     * @deprecated is part of {@link ChannelInboundHandler}
     */
    @Skip
    @Override
    @Deprecated
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        ctx.fireExceptionCaught(cause);
    }
}

6.ChannelInboundHandlerAdapter

只是简单的将事件forward到下一个handler中

一般用户自定义Inbound类型的handler都是继承ChannelInboundHandlerAdapter，用户想要自定义哪些方法就可以重写对应的逻辑

/**
 * Abstract base class for {@link ChannelInboundHandler} implementations which provide
 * implementations of all of their methods.
 *
 * <p>
 * This implementation just forward the operation to the next {@link ChannelHandler} in the
 * {@link ChannelPipeline}. Sub-classes may override a method implementation to change this.
 * </p>
 * <p>
 * Be aware that messages are not released after the {@link #channelRead(ChannelHandlerContext, Object)}
 * method returns automatically. If you are looking for a {@link ChannelInboundHandler} implementation that
 * releases the received messages automatically, please see {@link SimpleChannelInboundHandler}.
 * </p>
 */
public class ChannelInboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelInboundHandler {

    /**
     * Calls {@link ChannelHandlerContext#fireChannelRegistered()} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelRegistered();
    }

    /**
     * Calls {@link ChannelHandlerContext#fireChannelUnregistered()} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelUnregistered(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelUnregistered();
    }

    /**
     * Calls {@link ChannelHandlerContext#fireChannelActive()} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelActive();
    }

    /**
     * Calls {@link ChannelHandlerContext#fireChannelInactive()} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelInactive();
    }

    /**
     * Calls {@link ChannelHandlerContext#fireChannelRead(Object)} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        ctx.fireChannelRead(msg);
    }

    /**
     * Calls {@link ChannelHandlerContext#fireChannelReadComplete()} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelReadComplete();
    }

    /**
     * Calls {@link ChannelHandlerContext#fireUserEventTriggered(Object)} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {
        ctx.fireUserEventTriggered(evt);
    }

    /**
     * Calls {@link ChannelHandlerContext#fireChannelWritabilityChanged()} to forward
     * to the next {@link ChannelInboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelWritabilityChanged();
    }

    /**
     * Calls {@link ChannelHandlerContext#fireExceptionCaught(Throwable)} to forward
     * to the next {@link ChannelHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    @SuppressWarnings("deprecation")
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
            throws Exception {
        ctx.fireExceptionCaught(cause);
    }
}

7.ChannelOutboundHandlerAdapter

方法的实现仅仅是通过ctx将事件传递到下个handler

一般用户自定义Outbound类型的handler都是继承ChannelOutboundHandlerAdapter，用户想要自定义哪些方法就可以重写对应的逻辑

/**
 * Skeleton implementation of a {@link ChannelOutboundHandler}. This implementation just forwards each method call via
 * the {@link ChannelHandlerContext}.
 */
public class ChannelOutboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelOutboundHandler {

    /**
     * Calls {@link ChannelHandlerContext#bind(SocketAddress, ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void bind(ChannelHandlerContext ctx, SocketAddress localAddress,
            ChannelPromise promise) throws Exception {
        ctx.bind(localAddress, promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#connect(SocketAddress, SocketAddress, ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress,
            SocketAddress localAddress, ChannelPromise promise) throws Exception {
        ctx.connect(remoteAddress, localAddress, promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#disconnect(ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise)
            throws Exception {
        ctx.disconnect(promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#close(ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void close(ChannelHandlerContext ctx, ChannelPromise promise)
            throws Exception {
        ctx.close(promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#deregister(ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
        ctx.deregister(promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#read()} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void read(ChannelHandlerContext ctx) throws Exception {
        ctx.read();
    }

    /**
     * Calls {@link ChannelHandlerContext#write(Object, ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        ctx.write(msg, promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#flush()} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void flush(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }
}

8.ChannelDuplexHandler

既能处理输入事件也能处理输出事件

public class ChannelDuplexHandler extends ChannelInboundHandlerAdapter implements ChannelOutboundHandler {

    /**
     * Calls {@link ChannelHandlerContext#bind(SocketAddress, ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void bind(ChannelHandlerContext ctx, SocketAddress localAddress,
                     ChannelPromise promise) throws Exception {
        ctx.bind(localAddress, promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#connect(SocketAddress, SocketAddress, ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress,
                        SocketAddress localAddress, ChannelPromise promise) throws Exception {
        ctx.connect(remoteAddress, localAddress, promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#disconnect(ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise)
            throws Exception {
        ctx.disconnect(promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#close(ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
        ctx.close(promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#deregister(ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {
        ctx.deregister(promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#read()} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void read(ChannelHandlerContext ctx) throws Exception {
        ctx.read();
    }

    /**
     * Calls {@link ChannelHandlerContext#write(Object, ChannelPromise)} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        ctx.write(msg, promise);
    }

    /**
     * Calls {@link ChannelHandlerContext#flush()} to forward
     * to the next {@link ChannelOutboundHandler} in the {@link ChannelPipeline}.
     *
     * Sub-classes may override this method to change behavior.
     */
    @Skip
    @Override
    public void flush(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }
}

二、ChannelHandlerContext

ChannelHandler的上下文，用来使ChannelHandler和ChannelPipeline以及其他ChannelHandler之间进行交互

你可以调用ChannelHandlerContext的方法来通知ChannelPipeline的下一个Handler来处理

AttributeKey用来存储和当前Handler和context相关的信息，比如传一些中间参数，它是和Context绑定在一起的

ChannelInboundInvoker和ChannelOutboundInvoker就是定义触发下一个hander的方法

public interface ChannelHandlerContext extends AttributeMap, ChannelInboundInvoker, ChannelOutboundInvoker {

    /**
     * Return the {@link Channel} which is bound to the {@link ChannelHandlerContext}.
     */
    Channel channel();

    /**
     * Returns the {@link EventExecutor} which is used to execute an arbitrary task.
     */
    EventExecutor executor();

    /**
     * The unique name of the {@link ChannelHandlerContext}.The name was used when then {@link ChannelHandler}
     * was added to the {@link ChannelPipeline}. This name can also be used to access the registered
     * {@link ChannelHandler} from the {@link ChannelPipeline}.
     */
    String name();

    /**
     * The {@link ChannelHandler} that is bound this {@link ChannelHandlerContext}.
     */
    ChannelHandler handler();

    /**
     * Return {@code true} if the {@link ChannelHandler} which belongs to this context was removed
     * from the {@link ChannelPipeline}. Note that this method is only meant to be called from with in the
     * {@link EventLoop}.
     */
    boolean isRemoved();

    @Override
    ChannelHandlerContext fireChannelRegistered();

    @Override
    ChannelHandlerContext fireChannelUnregistered();

    @Override
    ChannelHandlerContext fireChannelActive();

    @Override
    ChannelHandlerContext fireChannelInactive();

    @Override
    ChannelHandlerContext fireExceptionCaught(Throwable cause);

    @Override
    ChannelHandlerContext fireUserEventTriggered(Object evt);

    @Override
    ChannelHandlerContext fireChannelRead(Object msg);

    @Override
    ChannelHandlerContext fireChannelReadComplete();

    @Override
    ChannelHandlerContext fireChannelWritabilityChanged();

    @Override
    ChannelHandlerContext read();

    @Override
    ChannelHandlerContext flush();

    /**
     * Return the assigned {@link ChannelPipeline}
     */
    ChannelPipeline pipeline();

    /**
     * Return the assigned {@link ByteBufAllocator} which will be used to allocate {@link ByteBuf}s.
     */
    ByteBufAllocator alloc();

    /**
     * @deprecated Use {@link Channel#attr(AttributeKey)}
     */
    @Deprecated
    @Override
    <T> Attribute<T> attr(AttributeKey<T> key);

    /**
     * @deprecated Use {@link Channel#hasAttr(AttributeKey)}
     */
    @Deprecated
    @Override
    <T> boolean hasAttr(AttributeKey<T> key);
}

1. AbstractChannelHandlerContext

通过状态机来维护当前context对应handler的状态

使用类似bitmap的方式来标识bind、flush等操作

获取下一个最合适的context，触发相关事件，异常就会触发异常事件

abstract class AbstractChannelHandlerContext implements ChannelHandlerContext, ResourceLeakHint {

    volatile AbstractChannelHandlerContext next;
    volatile AbstractChannelHandlerContext prev;

    // 用来更新handlerState的值
    private static final AtomicIntegerFieldUpdater<AbstractChannelHandlerContext> HANDLER_STATE_UPDATER =
            AtomicIntegerFieldUpdater.newUpdater(AbstractChannelHandlerContext.class, "handlerState");

    // 这个值有4种，0标识初始化话状态，1表示context准备去添加handler，2标识context已经添加了handler，3标识context移除了handler
    private volatile int handlerState = INIT;

    private static final int ADD_PENDING = 1;
    private static final int ADD_COMPLETE = 2;
    private static final int REMOVE_COMPLETE = 3;
    private static final int INIT = 0;

    private final DefaultChannelPipeline pipeline;
    private final String name;
    private final boolean ordered;
    private final int executionMask;
    
    // 子EventGroupExecutor中的执行器，用来执行具体的task
    final EventExecutor executor;
    private ChannelFuture succeededFuture;

    // Lazily instantiated tasks used to trigger events to a handler with different executor.
    // There is no need to make this volatile as at worse it will just create a few more instances then needed.
    private Tasks invokeTasks;

    // 默认的初始化，context都会调用这个来初始化
    AbstractChannelHandlerContext(DefaultChannelPipeline pipeline, EventExecutor executor,
                                  String name, Class<? extends ChannelHandler> handlerClass) {
        this.name = ObjectUtil.checkNotNull(name, "name");
        this.pipeline = pipeline;
        this.executor = executor;
        this.executionMask = mask(handlerClass);
        // Its ordered if its driven by the EventLoop or the given Executor is an instanceof OrderedEventExecutor.
        ordered = executor == null || executor instanceof OrderedEventExecutor;
    }

    @Override
    public EventExecutor executor() {
        if (executor == null) {
            return channel().eventLoop();
        } else {
            return executor;
        }
    }

    // 这个方法会调用下一个context去处理ChannelRegister方法
    @Override
    public ChannelHandlerContext fireChannelRegistered() {
        invokeChannelRegistered(findContextInbound(MASK_CHANNEL_REGISTERED));
        return this;
    }

    static void invokeChannelRegistered(final AbstractChannelHandlerContext next) {
        EventExecutor executor = next.executor();
        // 判断当前线程是否和executor线程是一个线程
        if (executor.inEventLoop()) {
            // 真正触发调用下一个context的channelRegistered方法的地方
            next.invokeChannelRegistered();
        } else {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    next.invokeChannelRegistered();
                }
            });
        }
    }

    private void invokeChannelRegistered() {
        // 判断是否有handler，基于上面的handlerState来判断
        if (invokeHandler()) {
            try {
                ((ChannelInboundHandler) handler()).channelRegistered(this);
            } catch (Throwable t) {
                // 发生异常就会调用异常触发事件
                invokeExceptionCaught(t);
            }
        } else {
            // 如果handler还没有加入，那么就会一直循环调用
            fireChannelRegistered();
        }
    }
    
    /**
     * fireChannelUnregistered、fireChannelActive、fireChannelInactive、fireExceptionCaught、
     * fireUserEventTriggered、fireChannelRead、fireChannelReadComplete、fireChannelWritabilityChanged的实现和fireChannelRegistered类似
     */

    @Override
    public ChannelFuture bind(SocketAddress localAddress) {
        return bind(localAddress, newPromise());
    }
    
    // 调用上一个outbound的context执行bing操作
    @Override
    public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
        ObjectUtil.checkNotNull(localAddress, "localAddress");
        if (isNotValidPromise(promise, false)) {
            // cancelled
            return promise;
        }
    
        // 找到上一个outbound类型的context
        final AbstractChannelHandlerContext next = findContextOutbound(MASK_BIND);
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            // 调用context的bind方法
            next.invokeBind(localAddress, promise);
        } else {
            safeExecute(executor, new Runnable() {
                @Override
                public void run() {
                    next.invokeBind(localAddress, promise);
                }
            }, promise, null, false);
        }
        return promise;
    }
    
    private AbstractChannelHandlerContext findContextOutbound(int mask) {
        AbstractChannelHandlerContext ctx = this;
        EventExecutor currentExecutor = executor();
        do {
            ctx = ctx.prev;
        } while (skipContext(ctx, currentExecutor, mask, MASK_ONLY_OUTBOUND));
        return ctx;
    }

    private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
        if (invokeHandler()) {
            try {
                ((ChannelOutboundHandler) handler()).bind(this, localAddress, promise);
            } catch (Throwable t) {
                // 失败将异步future设置为失败，设置报错打印日志
                notifyOutboundHandlerException(t, promise);
            }
        } else {
            bind(localAddress, promise);
        }
    }
    
    private static void notifyOutboundHandlerException(Throwable cause, ChannelPromise promise) {
        // Only log if the given promise is not of type VoidChannelPromise as tryFailure(...) is expected to return
        // false.
        PromiseNotificationUtil.tryFailure(promise, cause, promise instanceof VoidChannelPromise ? null : logger);
    }
    
    /**
     * outbound的方法都和bind实现类似，如下
     * connect、disconnect、close、deregister
     * 它和上面inbound的处理区别在于失败会将promise设置为fail
     */

    /**
     * outbound的方法都和bind实现类似，如下
     * connect、disconnect、close、deregister、read、write、writeAndFlush
     * 它和上面inbound的处理区别在于失败会将promise设置为fail
     */

    // 查找下一个inbound的index
    private AbstractChannelHandlerContext findContextInbound(int mask) {
        AbstractChannelHandlerContext ctx = this;
        EventExecutor currentExecutor = executor();
        do {
            ctx = ctx.next;
            // 这里有对inbound过滤
        } while (skipContext(ctx, currentExecutor, mask, MASK_ONLY_INBOUND));
        return ctx;
    }

    // 查找上一个outbound的index
    private AbstractChannelHandlerContext findContextOutbound(int mask) {
        AbstractChannelHandlerContext ctx = this;
        EventExecutor currentExecutor = executor();
        do {
            ctx = ctx.prev;
        } while (skipContext(ctx, currentExecutor, mask, MASK_ONLY_OUTBOUND));
        return ctx;
    }

    // 判断是否跳过context，需要满足inbound、outbound方式
    private static boolean skipContext(
            AbstractChannelHandlerContext ctx, EventExecutor currentExecutor, int mask, int onlyMask) {
        // Ensure we correctly handle MASK_EXCEPTION_CAUGHT which is not included in the MASK_EXCEPTION_CAUGHT
        return (ctx.executionMask & (onlyMask | mask)) == 0 ||
                // We can only skip if the EventExecutor is the same as otherwise we need to ensure we offload
                // everything to preserve ordering.
                //
                // See https://github.com/netty/netty/issues/10067
                (ctx.executor() == currentExecutor && (ctx.executionMask & mask) == 0);
    }

    @Override
    public ChannelPromise voidPromise() {
        return channel().voidPromise();
    }


    /**
     * Makes best possible effort to detect if {@link ChannelHandler#handlerAdded(ChannelHandlerContext)} was called
     * yet. If not return {@code false} and if called or could not detect return {@code true}.
     *
     * If this method returns {@code false} we will not invoke the {@link ChannelHandler} but just forward the event.
     * This is needed as {@link DefaultChannelPipeline} may already put the {@link ChannelHandler} in the linked-list
     * but not called {@link ChannelHandler#handlerAdded(ChannelHandlerContext)}.
     */
    private boolean invokeHandler() {
        // Store in local variable to reduce volatile reads.
        int handlerState = this.handlerState;
        return handlerState == ADD_COMPLETE || (!ordered && handlerState == ADD_PENDING);
    }

    static final class WriteTask implements Runnable {
        private static final ObjectPool<WriteTask> RECYCLER = ObjectPool.newPool(new ObjectCreator<WriteTask>() {
            @Override
            public WriteTask newObject(Handle<WriteTask> handle) {
                return new WriteTask(handle);
            }
        });

        static WriteTask newInstance(AbstractChannelHandlerContext ctx,
                Object msg, ChannelPromise promise, boolean flush) {
            WriteTask task = RECYCLER.get();
            init(task, ctx, msg, promise, flush);
            return task;
        }

        private static final boolean ESTIMATE_TASK_SIZE_ON_SUBMIT =
                SystemPropertyUtil.getBoolean("io.netty.transport.estimateSizeOnSubmit", true);

        // Assuming compressed oops, 12 bytes obj header, 4 ref fields and one int field
        private static final int WRITE_TASK_OVERHEAD =
                SystemPropertyUtil.getInt("io.netty.transport.writeTaskSizeOverhead", 32);

        private final Handle<WriteTask> handle;
        private AbstractChannelHandlerContext ctx;
        private Object msg;
        private ChannelPromise promise;
        private int size; // sign bit controls flush

        @SuppressWarnings("unchecked")
        private WriteTask(Handle<? extends WriteTask> handle) {
            this.handle = (Handle<WriteTask>) handle;
        }

        protected static void init(WriteTask task, AbstractChannelHandlerContext ctx,
                                   Object msg, ChannelPromise promise, boolean flush) {
            task.ctx = ctx;
            task.msg = msg;
            task.promise = promise;

            if (ESTIMATE_TASK_SIZE_ON_SUBMIT) {
                task.size = ctx.pipeline.estimatorHandle().size(msg) + WRITE_TASK_OVERHEAD;
                ctx.pipeline.incrementPendingOutboundBytes(task.size);
            } else {
                task.size = 0;
            }
            if (flush) {
                task.size |= Integer.MIN_VALUE;
            }
        }

        @Override
        public void run() {
            try {
                decrementPendingOutboundBytes();
                if (size >= 0) {
                    ctx.invokeWrite(msg, promise);
                } else {
                    ctx.invokeWriteAndFlush(msg, promise);
                }
            } finally {
                recycle();
            }
        }

        void cancel() {
            try {
                decrementPendingOutboundBytes();
            } finally {
                recycle();
            }
        }

        private void decrementPendingOutboundBytes() {
            if (ESTIMATE_TASK_SIZE_ON_SUBMIT) {
                ctx.pipeline.decrementPendingOutboundBytes(size & Integer.MAX_VALUE);
            }
        }

        private void recycle() {
            // Set to null so the GC can collect them directly
            ctx = null;
            msg = null;
            promise = null;
            handle.recycle(this);
        }
    }

    private static final class Tasks {
        private final AbstractChannelHandlerContext next;
        private final Runnable invokeChannelReadCompleteTask = new Runnable() {
            @Override
            public void run() {
                next.invokeChannelReadComplete();
            }
        };
        private final Runnable invokeReadTask = new Runnable() {
            @Override
            public void run() {
                next.invokeRead();
            }
        };
        private final Runnable invokeChannelWritableStateChangedTask = new Runnable() {
            @Override
            public void run() {
                next.invokeChannelWritabilityChanged();
            }
        };
        private final Runnable invokeFlushTask = new Runnable() {
            @Override
            public void run() {
                next.invokeFlush();
            }
        };

        Tasks(AbstractChannelHandlerContext next) {
            this.next = next;
        }
    }
}

2.1 DefaultChannelHandlerContext

这个仅仅是默认的实现，提供handler()方法用于返回context中的handler

自定义的handler都会用这个默认的DefaultChannelHandlerContext来进行封装

final class DefaultChannelHandlerContext extends AbstractChannelHandlerContext {

    private final ChannelHandler handler;

    DefaultChannelHandlerContext(
            DefaultChannelPipeline pipeline, EventExecutor executor, String name, ChannelHandler handler) {
        super(pipeline, executor, name, handler.getClass());
        this.handler = handler;
    }

    @Override
    public ChannelHandler handler() {
        return handler;
    }
}

2.1 DefaultPipeline.HeadContext

通过Channel的Unsafe来进行连接、绑定以及读写数据到Buffer中

当有数据读时调用read方法，并封装成msg调用pipeline中的handler的ChannelRead方法，当数据可写时，会调用这个write方法写到buffer中，并且调用beginRead来进行Channel的感兴趣事件

这个context相当于是数据读取和写入的枢纽

final class HeadContext extends AbstractChannelHandlerContext
        implements ChannelOutboundHandler, ChannelInboundHandler {

    private final Unsafe unsafe;

    HeadContext(DefaultChannelPipeline pipeline) {
        super(pipeline, null, HEAD_NAME, HeadContext.class);
        unsafe = pipeline.channel().unsafe();
        setAddComplete(); // 标识handler设置成功
    }

    @Override
    public void bind(
            ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) {
        unsafe.bind(localAddress, promise);
    }

    @Override
    public void connect(
            ChannelHandlerContext ctx,
            SocketAddress remoteAddress, SocketAddress localAddress,
            ChannelPromise promise) {
        unsafe.connect(remoteAddress, localAddress, promise);
    }

    @Override
    public void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) {
        unsafe.disconnect(promise);
    }

    @Override
    public void close(ChannelHandlerContext ctx, ChannelPromise promise) {
        unsafe.close(promise);
    }

    @Override
    public void deregister(ChannelHandlerContext ctx, ChannelPromise promise) {
        unsafe.deregister(promise);
    }

    @Override
    public void read(ChannelHandlerContext ctx) {
        unsafe.beginRead();
    }

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) {
        unsafe.write(msg, promise);
    }

    @Override
    public void flush(ChannelHandlerContext ctx) {
        unsafe.flush();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        ctx.fireExceptionCaught(cause);
    }

    @Override
    public void channelRegistered(ChannelHandlerContext ctx) {
        invokeHandlerAddedIfNeeded();
        ctx.fireChannelRegistered();
    }

    @Override
    public void channelUnregistered(ChannelHandlerContext ctx) {
        ctx.fireChannelUnregistered();

        // Remove all handlers sequentially if channel is closed and unregistered.
        if (!channel.isOpen()) {
            destroy();
        }
    }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        ctx.fireChannelActive();

        readIfIsAutoRead();
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) {
        ctx.fireChannelInactive();
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        ctx.fireChannelRead(msg);
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) {
        ctx.fireChannelReadComplete();

        readIfIsAutoRead();
    }

    private void readIfIsAutoRead() {
        if (channel.config().isAutoRead()) {
            channel.read();
        }
    }

    @Override
    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) {
        ctx.fireUserEventTriggered(evt);
    }

    @Override
    public void channelWritabilityChanged(ChannelHandlerContext ctx) {
        ctx.fireChannelWritabilityChanged();
    }
}

2.1 DefaultPipeline.TailContext

这里有很多待扩展的方法，比如当数据读完之后的统一处理可以扩展onUnhandledInboundChannelReadComplete方法

主要的实现就是触发前一个handler进行处理

final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {

    TailContext(DefaultChannelPipeline pipeline) {
        super(pipeline, null, TAIL_NAME, TailContext.class);
        setAddComplete();
    }

    @Override
    public ChannelHandler handler() {
        return this;
    }

    @Override
    public void channelRegistered(ChannelHandlerContext ctx) { }

    @Override
    public void channelUnregistered(ChannelHandlerContext ctx) { }

    @Override
    public void channelActive(ChannelHandlerContext ctx) {
        onUnhandledInboundChannelActive();
    }

    @Override
    public void channelInactive(ChannelHandlerContext ctx) {
        onUnhandledInboundChannelInactive();
    }

    @Override
    public void channelWritabilityChanged(ChannelHandlerContext ctx) {
        onUnhandledChannelWritabilityChanged();
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) { }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) { }

    @Override
    public void userEventTriggered(ChannelHandlerContext ctx, Object evt) {
        onUnhandledInboundUserEventTriggered(evt);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        onUnhandledInboundException(cause);
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        onUnhandledInboundMessage(ctx, msg);
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) {
        onUnhandledInboundChannelReadComplete();
    }
}

【源码解析】Netty之ChannelHandler

一、ChannelHandler

1. Channel的继承关系

2.ChannelHandler

3.ChannelInboundHandler

4.ChannelOutboundHandler

5.ChannelHandlerAdapter

6.ChannelInboundHandlerAdapter

7.ChannelOutboundHandlerAdapter

8.ChannelDuplexHandler

二、ChannelHandlerContext

1. AbstractChannelHandlerContext

2.1 DefaultChannelHandlerContext

2.1 DefaultPipeline.HeadContext

2.1 DefaultPipeline.TailContext