ChannelHandler负责处理I/O事件或者拦截I/O操作，并且通过ChannelPipeline转发到下一个处理程序。

ChannelHandler中没有提供什么方法，要是实现只能选择ChannelInboundHandler和ChannelOutboundHandler去处理inbound和outbound操作。

ChannelHandler

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public interface ChannelHandler {
    // 添加和移除上下文
    void handlerAdded(ChannelHandlerContext ctx) throws Exception;
    void handlerRemoved(ChannelHandlerContext ctx) throws Exception;

    @Deprecated
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
    // 是否可分享的，可分享的handler能被多个ChannelPipeline持有。
    @Inherited
    @Documented
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @interface Sharable {
        // no value
    }
}

ChannelInboundHandler

在上一节ChannelPipeline中的解析其实对ChannelInboundHandler已经比较了解了，总共9个事件入口

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public interface ChannelInboundHandler extends ChannelHandler {
    // channel已注册
    void channelRegistered(ChannelHandlerContext ctx) throws Exception;
    // channel已注销
    void channelUnregistered(ChannelHandlerContext ctx) throws Exception;
    // channel存活
    void channelActive(ChannelHandlerContext ctx) throws Exception;
    // channel死亡
    void channelInactive(ChannelHandlerContext ctx) throws Exception;
    // channel读取
    void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception;
    // channel读取完成
    void channelReadComplete(ChannelHandlerContext ctx) throws Exception;
    // 用户事件触发
    void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception;
    // channel可写状态变更
    void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception;

    @Override
    @SuppressWarnings("deprecation")
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
}

ChannelOutboundHandler

共8个方法，Outbound需要主动操作

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public interface ChannelOutboundHandler extends ChannelHandler {
    // 绑定
    void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception;
    // 连接
    void connect(
            ChannelHandlerContext ctx, SocketAddress remoteAddress,
            SocketAddress localAddress, ChannelPromise promise) throws Exception;
    // 断开连接
    void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;
    // 关闭
    void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;
    // 撤销登记
    void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;
    // 读
    void read(ChannelHandlerContext ctx) throws Exception;
    // 写
    void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception;
    // 刷新
    void flush(ChannelHandlerContext ctx) throws Exception;
}

ChannelHandlerAdapter

ChannelHandler实现的基本骨架，主要解决handler是否可共享。

如果类上加上Sharable注释则代表可以共享。

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public abstract class ChannelHandlerAdapter implements ChannelHandler {

    boolean added;

    protected void ensureNotSharable() {
        if (isSharable()) {
            throw new IllegalStateException("ChannelHandler " + getClass().getName() + " is not allowed to be shared");
        }
    }
    // 是否可共享的，通过类是否有Sharable注解
    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;
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        // NOOP
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        // NOOP
    }

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

ChannelInboundHandlerAdapter

ChannelInboundHandlerAdapter中将所有的方法实现交由传入的ChannelHandlerContext去执行。

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public class ChannelInboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelInboundHandler {

    @Skip
    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelRegistered();
    }

    @Skip
    @Override
    public void channelUnregistered(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelUnregistered();
    }

    @Skip
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelActive();
    }

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

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

    @Skip
    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.fireChannelReadComplete();
    }

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

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

    @Skip
    @Override
    @SuppressWarnings("deprecation")
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
            throws Exception {
        ctx.fireExceptionCaught(cause);
    }
}

ChannelOutboundHandlerAdapter

ChannelOutboundHandlerAdapter与ChannelInboundHandlerAdapter逻辑一致将具体的执行逻辑交由ChannelHandlerContext处理。

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public class ChannelOutboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelOutboundHandler {

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

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

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

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

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

    @Skip
    @Override
    public void read(ChannelHandlerContext ctx) throws Exception {
        ctx.read();
    }

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

    @Skip
    @Override
    public void flush(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }
}