# 6.1 ChannelHandler的生命周期与事件处理机制

Netty的ChannelHandler是处理网络事件（如数据读取、数据写入、连接建立、连接关闭等）的核心组件. 日常开发，我们主要实现类：SimpleChannelInboundHandler，其核心方法主要是下边几个：

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);
    }

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在Netty中，ChannelHandler的生命周期与Channel的状态紧密相关，主要涉及到以下几个回调方法：

handlerAdded: 当一个新的ChannelHandler被添加到ChannelPipeline时调用。
channelRegistered: 当Channel成功注册到EventLoop上时调用。
channelActive: 当Channel激活，可以开始接收和发送数据时调用。
channelRead: 当从Channel中读取到数据时调用。
channelReadComplete: 当一次读取操作完成时调用。
channelInactive: 当Channel变为非激活状态时调用。
channelUnregistered: 当Channel从EventLoop上注销时调用。
handlerRemoved: 当ChannelHandler从ChannelPipeline中移除时调用。

详细请参看Netty Review - 深入理解Netty: ChannelHandler的生命周期与事件处理机制 (opens new window)

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