改造timer，即客户端与服务端建立连接后，服务端主动向客户端发送当前时间。

server：

ch.pipeline().addLast(new TimeEncoder());
ch.pipeline().addLast(new TimeServerHandler());

TimeServerHandler

public class TimeServerHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {//(1)
        ChannelFuture f = ctx.writeAndFlush(new UnixTime());
        f.addListener(ChannelFutureListener.CLOSE);//因为是write，传递给的不是ByteToMessageDecoder
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
    
}

TimeEncoder

public class TimeEncoder extends ChannelOutboundHandlerAdapter {

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        UnixTime m = (UnixTime)msg;
        ByteBuf encoded = ctx.alloc().buffer(4);
        encoded.writeInt((int)m.value());
        ctx.write(encoded,promise);//(1)
    }
    
}

注意：

1、在TimeServerHandler中，ctx.writeAndFlush(new UnixTime())，已经writeAndFlush了，所以执行过程：TimeServerHandler(write) ---->TimeEncoder(write)--->ChannelOutboundHandlerAdapter(flush)。

 如果在TimeServerHandler中只写write不写flush，那么不会自动执行ChannelOutboundHandlerAdapter中的flush方法，从而无法发送到网络。

 如果在TimeServerHandler中只写write不写flush，但是在TimeEncoder中执行flush方法，不执行ChannelOutboundHandlerAdapter中的flush方法，但能发送到网络。

总结：在ChannelPipeline中传输的handler，只要在任意一个handler钟写flush，都会发送到网络。

2、TimeEncoder也可以写成：

public class TimeEncoder extends MessageToByteEncoder<UnixTime> {

    @Override
    protected void encode(ChannelHandlerContext ctx, UnixTime msg, ByteBuf out) throws Exception {
        out.writeInt((int)msg.value());
    }

}

TimeClient

ch.pipeline().addLast(new TimeDecoder());
ch.pipeline().addLast(new TimeClientHandler());

TimeDecoder

public class TimeDecoder extends ByteToMessageDecoder {
    @Override
    protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) throws Exception {
        if (in.readableBytes() < 4) {
            return ;
        }
        out.add(new UnixTime(in.readUnsignedInt()));
    }
}

TimeClientHandler

public class TimeClientHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        UnixTime m = (UnixTime) msg;
        log.info("{}", m);
        ctx.close();
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }

}