Interface ChannelHandlerContext

All Known Implementing Classes:
DefaultChannelPipeline.DefaultChannelHandlerContext
public interface ChannelHandlerContext
Enables a ChannelHandler to interact with its ChannelPipeline and other handlers. A handler can send a ChannelEvent upstream or downstream, modify the ChannelPipeline it belongs to dynamically.

Sending an event

You can send or forward a ChannelEvent to the closest handler in the same ChannelPipeline by calling sendUpstream(ChannelEvent) or sendDownstream(ChannelEvent). Please refer to ChannelPipeline to understand how an event flows.

Modifying a pipeline

You can get the ChannelPipeline your handler belongs to by calling getPipeline(). A non-trivial application could insert, remove, or replace handlers in the pipeline dynamically in runtime.

Retrieving for later use

You can keep the ChannelHandlerContext for later use, such as triggering an event outside the handler methods, even from a different thread. 
public class MyHandler extends SimpleChannelHandler
                       implements LifeCycleAwareChannelHandler {

    private ChannelHandlerContext ctx;

    public void beforeAdd(ChannelHandlerContext ctx) {
        this.ctx = ctx;
    }

    public void login(String username, password) {
        Channels.write(
                this.ctx,
                Channels.succeededFuture(this.ctx.getChannel()),
                new LoginMessage(username, password));
    }
    ...
}

Storing stateful information

setAttachment(Object) and getAttachment() allow you to store and access stateful information that is related with a handler and its context. Please refer to ChannelHandler to learn various recommended ways to manage stateful information.

A handler can have more than one context

Please note that a ChannelHandler instance can be added to more than one ChannelPipeline. It means a single ChannelHandler instance can have more than one ChannelHandlerContext and therefore the single instance can be invoked with different ChannelHandlerContexts if it is added to one or more ChannelPipelines more than once.

For example, the following handler will have as many independent attachments as how many times it is added to pipelines, regardless if it is added to the same pipeline multiple times or added to different pipelines multiple times: 

public class FactorialHandler extends SimpleChannelHandler {

  // This handler will receive a sequence of increasing integers starting
  // from 1.
  @Override
  public void messageReceived(ChannelHandlerContext ctx, MessageEvent evt) {
    Integer a = (Integer) ctx.getAttachment();
    Integer b = (Integer) evt.getMessage();

    if (a == null) {
      a = 1;
    }

    ctx.setAttachment(Integer.valueOf(a * b));
  }
}

// Different context objects are given to "f1", "f2", "f3", and "f4" even if
// they refer to the same handler instance.  Because the FactorialHandler
// stores its state in a context object (as an attachment), the factorial is
// calculated correctly 4 times once the two pipelines (p1 and p2) are active.
FactorialHandler fh = new FactorialHandler();

ChannelPipeline p1 = Channels.pipeline();
p1.addLast("f1", fh);
p1.addLast("f2", fh);

ChannelPipeline p2 = Channels.pipeline();
p2.addLast("f3", fh);
p2.addLast("f4", fh);

Additional resources worth reading

Please refer to the ChannelHandler, ChannelEvent, and ChannelPipeline to find out what a upstream event and a downstream event are, what fundamental differences they have, how they flow in a pipeline, and how to handle the event in your application.