State management（handler的状态管理）

A ChannelHandler often needs to store some stateful information. The simplest and recommended approach is to use member variables:

public interface Message {
     // your methods here
 }
 public class DataServerHandler extends SimpleChannelInboundHandler<Message> {
     private boolean loggedIn;
     @Override
     public void channelRead0(ChannelHandlerContext ctx, Message message) {
         Channel ch = e.getChannel();
         if (message instanceof LoginMessage) {
             authenticate((LoginMessage) message);
             loggedIn = true;
         } else (message instanceof GetDataMessage) {
             if (loggedIn) {
                 ch.write(fetchSecret((GetDataMessage) message));
             } else {
                 fail();
             }
         }
     }
     ...
 }

Because the handler instance has a state variable（状态变量） which is dedicated（专用） to one connection, you have to create a new handler instance for each new channel to avoid a race condition（竞态条件） where a unauthenticated（未验证的） client can get the confidential（机密的） information:

 // Create a new handler instance per channel.
 // See ChannelInitializer.initChannel(Channel).
 public class DataServerInitializer extends ChannelInitializer<Channel> {
     @Override
     public void initChannel(Channel channel) {
         channel.pipeline().addLast("handler", new DataServerHandler());
     }
 }

is not a @Sharable handler 

当出现这样的错误提示时，有两种解决方法，要么就是加上注解@Sharable，要么就是每次都new一个新的handler的实例。

====END====