io.netty.channel

Interface ChannelHandler

All Known Subinterfaces:

ChannelInboundHandler, ChannelOutboundHandler

All Known Implementing Classes:

ChannelDuplexHandler, ChannelHandlerAdapter, ChannelInboundHandlerAdapter, ChannelInitializer, ChannelOutboundHandlerAdapter, CombinedChannelDuplexHandler, SimpleChannelInboundHandler

public interface ChannelHandler

Handles or intercepts a ChannelInboundInvoker or ChannelOutboundInvoker operation, and forwards it to the next handler in a ChannelPipeline.

Sub-types

ChannelHandler itself does not provide many methods. To handle a a ChannelInboundInvoker or ChannelOutboundInvoker operation you need to implement its sub-interfaces. There are many different sub-interfaces which handles inbound and outbound operations. But the most useful for developers may be:

• ChannelInboundHandlerAdapter handles and intercepts inbound operations
• ChannelOutboundHandlerAdapter handles and intercepts outbound operations

You will also find more detailed explanation from the documentation of each sub-interface on how an event is interpreted when it goes upstream and downstream respectively.

The context object

A ChannelHandler is provided with a ChannelHandlerContext object. A ChannelHandler is supposed to interact with the ChannelPipeline it belongs to via a context object. Using the context object, the ChannelHandler can pass events upstream or downstream, modify the pipeline dynamically, or store the information (using AttributeKeys) which is specific to the handler.

State management

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 which is dedicated to one connection, you have to create a new handler instance for each new channel to avoid a race condition where a unauthenticated client can get the confidential 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());
     }
 }

 

Using AttributeKey

Although it's recommended to use member variables to store the state of a handler, for some reason you might not want to create many handler instances. In such a case, you can use AttributeKeys which is provided by ChannelHandlerContext:

 public interface Message {
     // your methods here
 }

  @Sharable
 public class DataServerHandler extends SimpleChannelInboundHandler<Message> {
     private final AttributeKey<Boolean> auth =
           AttributeKey.valueOf("auth");

      @Override
     public void channelRead(ChannelHandlerContext ctx, Message message) {
         Attribute<Boolean> attr = ctx.attr(auth);
         Channel ch = ctx.channel();
         if (message instanceof LoginMessage) {
             authenticate((LoginMessage) o);
             attr.set(true);
         } else (message instanceof GetDataMessage) {
             if (Boolean.TRUE.equals(attr.get())) {
                 ch.write(fetchSecret((GetDataMessage) o));
             } else {
                 fail();
             }
         }
     }
     ...
 }
 

Now that the state of the handler isattached to the ChannelHandlerContext, you can add the same handler instance to different pipelines:

 public class DataServerInitializer extends ChannelInitializer<Channel> {

     private static final DataServerHandler SHARED = new DataServerHandler();

      @Override
     public void initChannel(Channel channel) {
         channel.pipeline().addLast("handler", SHARED);
     }
 }
 

The @Sharable annotation

In the example above which used an AttributeKey, you might have noticed the @Sharable annotation.

If a ChannelHandler is annotated with the @Sharable annotation, it means you can create an instance of the handler just once and add it to one or more ChannelPipelines multiple times without a race condition.

If this annotation is not specified, you have to create a new handler instance every time you add it to a pipeline because it has unshared state such as member variables.

This annotation is provided for documentation purpose, just like the JCIP annotations.

Additional resources worth reading

Please refer to the ChannelHandler, and ChannelPipeline to find out more about inbound and outbound operations, what fundamental differences they have, how they flow in a pipeline, and how to handle the operation in your application.

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static interface	ChannelHandler.Sharable Indicates that the same instance of the annotated ChannelHandler can be added to one or more ChannelPipelines multiple times without a race condition.

Method Summary

Modifier and Type	Method and Description
void	exceptionCaught(ChannelHandlerContext ctx, Throwable cause) Deprecated. is part of ChannelInboundHandler
void	handlerAdded(ChannelHandlerContext ctx) Gets called after the ChannelHandler was added to the actual context and it's ready to handle events.
void	handlerRemoved(ChannelHandlerContext ctx) Gets called after the ChannelHandler was removed from the actual context and it doesn't handle events anymore.

Method Detail

handlerAdded

void handlerAdded(ChannelHandlerContext ctx)
           throws Exception

Gets called after the ChannelHandler was added to the actual context and it's ready to handle events.

Throws:

Exception

handlerRemoved

void handlerRemoved(ChannelHandlerContext ctx)
             throws Exception

Gets called after the ChannelHandler was removed from the actual context and it doesn't handle events anymore.

Throws:

Exception

exceptionCaught

@Deprecated
void exceptionCaught(ChannelHandlerContext ctx,
                                 Throwable cause)
                          throws Exception

Deprecated. is part of ChannelInboundHandler

Gets called if a Throwable was thrown.

Throws:

Exception