Netty之ChannelHandler

ChannelHandler

Handles an I/O event or intercepts an I/O operation, and forwards it to its next handler in its ChannelPipeline.
处理I/O事件或截取I/O操作，并将其转发到ChannelPipeline中的下一个Handler。
使用netty实现自我业务的扩展点。一般使用ChannelHandler的子类型ChannelInboundHandler和ChannelOutboundHandler。
但由于不想重写所有的接口方法，可以使用它们的Adapter。

ChannelHandler

public interface ChannelHandler {
   
    // Gets called after the ChannelHandler was added to the actual context and it's ready to handle events.
    // 当前Handler加入ChannelHandlerContext时的回调
    void handlerAdded(ChannelHandlerContext ctx) throws Exception;

    // Gets called after the ChannelHandler was removed from the actual context and it doesn't handle events anymore.
    // 当前Handler从ChannelHandlerContext移除时的回调
    void handlerRemoved(ChannelHandlerContext ctx) throws Exception;

    // 发生异常时的回调
    // Gets called if a Throwable was thrown.
    // if you want to handle this event you should implement ChannelInboundHandler and implement the method there.
    @Deprecated
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;

    @Inherited
    @Documented
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @interface Sharable {
        // no value
    }
}

ChannelInboundHandler

/**
 * 能够为状态变化添加回调的ChannelHandler. 在状态更改时，它使用户可以轻松的挂载钩子。
 * {@link ChannelHandler} which adds callbacks for state changes. This allows the user
 * to hook in to state changes easily.
 */
public interface ChannelInboundHandler extends ChannelHandler {

    /**
     * 触发时机：当前Channel注册到EventLoop
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered with its {@link EventLoop}
     */
    void channelRegistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * 触发时机：当前Chanel从EventLoop中取消注册
     * The {@link Channel} of the {@link ChannelHandlerContext} was unregistered from its {@link EventLoop}
     */
    void channelUnregistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * 触发时机：当前Channel活跃的时候
     * The {@link Channel} of the {@link ChannelHandlerContext} is now active
     */
    void channelActive(ChannelHandlerContext ctx) throws Exception;

    /**
     * 触发时机：当前Channel不活跃的时候，即Channel到了它的生命周期末
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered is now inactive and reached its
     * end of lifetime.
     */
    void channelInactive(ChannelHandlerContext ctx) throws Exception;

    /**
     * 触发时机：当前Channel从远程读到数据
     * Invoked when the current {@link Channel} has read a message from the peer.
     */
    void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception;

    /**
     * 触发时机：channelRead消费完读取的数据
     * 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.
     */
    void channelReadComplete(ChannelHandlerContext ctx) throws Exception;

    /**
     * 触发时机：用户事件触发时
     * Gets called if an user event was triggered.
     */
    void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception;

    /**
     * 触发时机：Chanel的写状态变化的时候触发
     * 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.
     */
    @Override
    @SuppressWarnings("deprecation")
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
}

ChannelOutboundHandler

/**
 * 收到IO出站操作通知的ChannelHandler
 * {@link ChannelHandler} which will get notified for IO-outbound-operations.
 *
 * 注意：有些方法有ChannelPromise参数，可以使用promise.addListener()注册监听，当回调方法执行完成后会触发监听
 */
public interface ChannelOutboundHandler extends ChannelHandler {
    /**
     * 触发时机：bind操作执行前
     * 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;

    /**
     * 触发时机：connect操作执行前
     * 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;

    /**
     * 触发时机：disconnect操作执行前
     * Called once a disconnect operation is made.
     *
     * @param ctx               the {@link ChannelHandlerContext} for which the disconnect operation is made
     * @param promise           the {@link ChannelPromise} to notify once the operation completes
     * @throws Exception        thrown if an error occurs
     */
    void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;

    /**
     * 触发时机：close操作执行前
     * Called once a close operation is made.
     *
     * @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 close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;

    /**
     * 触发时机：deregister操作执行前
     * 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;

    /**
     * 触发时机：read操作执行前
     * Intercepts {@link ChannelHandlerContext#read()}.
     */
    void read(ChannelHandlerContext ctx) throws Exception;

    /**
     * 触发时机：write操作执行前
     * 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
     *
     * @param ctx               the {@link ChannelHandlerContext} for which the write operation is made
     * @param msg               the message to write
     * @param promise           the {@link ChannelPromise} to notify once the operation completes
     * @throws Exception        thrown if an error occurs
     */
    void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception;

    /**
     * 触发时机：flush操作执行前
     * Called once a flush operation is made. The flush operation will try to flush out all previous written messages
     * that are pending.
     *
     * @param ctx               the {@link ChannelHandlerContext} for which the flush operation is made
     * @throws Exception        thrown if an error occurs
     */
    void flush(ChannelHandlerContext ctx) throws Exception;
}

DEMO

public void clientMode() throws Exception {
    NioEventLoopGroup thread = new NioEventLoopGroup(1);
    

}

参考文章：
https://juejin.cn/post/6844903597302349831
https://netty.io/4.1/api/io/netty/channel/ChannelHandler.html