Netty ChannelPipeline

• Codecs

The Netty ChannelPipeline is a very central concept in Netty. Each Netty SocketChannel contains a ChannelPipeline . The ChannelPipeline contains a list of ChannelHandler instances. These ChannelHandler instances are called when data is moving in and out of the SocketChannel.

The ChannelHandler interface has two subinterfaces. These subinterfaces are:

• ChannelInboundHandler
• ChannelOutboundHandler

You can add both ChannelInboundHandler and ChannelOutboundHandler instances to the Netty ChannelPipeline. The following diagram illustrates a ChannelPipeline with ChannelInboundHandler and ChannelOutboundHandler instances added:

When data is received from the SocketChannel is is passed to the first ChannelInboundHandler in the ChannelPipeline. This ChannelInboundHandler processes the data, and then the data is passed on to the next ChannelInboundHandler in the ChannelPipeline.

Actually, a ChannelInboundHandler can transform the data it receives before it is passed on to the next handler in the pipeline. For instance, raw bytes can be transformed into an HTTP object, or some other object. The next handler in the pipeline then sees the HTTP object, not the raw data.

When writing data back to the SocketChannel it happens in the same fashion. The data is passed from ChannelOutboundHandler to ChannelOutboundHandler in the ChannelPipeline until it reaches the SocketChannel. The ChannelOutboundHandler instances can also transform the data in the process.

Even though the illustration shows the ChannelInboundHandler and ChannelOutboundHandler instances as separate lists, they are actually located in the same list (pipeline). Thus, if a ChannelInboundHandler decides to write something back to the SocketChannel, the data passes through all ChannelOutboundHandler instances located earlier in the ChannelPipeline than the ChannelInboundHandler writing the data. This is illustrated here:

Codecs

Netty has the concept of codecs (encoders + decoders). A Netty codec converts bytes to message objects (Java objects), or message objects to bytes. For instance, a codec might convert the raw bytes of an incoming HTTP request to an HTTP object, or convert an HTTP response object back to raw bytes.

A Netty codec object is really just one (or two) ChannelHandler implementations. A codec typically consists of a ChannelInboundHandler implementation which converts request bytes into objects, and a ChannelOutboundHandler which converts response objects into bytes.

Netty comes with codecs for several different protocols, like HTTP, WebSockets, SSL / TLS etc. In order to use those protocols with Netty, you have to add the corresponding protocol codec ChannelInboundHandler and ChannelOutboundHandler to the ChannelPipeline of the SocketChannel you want to use the protocol with. Netty codecs and protocols will be covered in more detail in their own tutorials.