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Netty Overview

Jakob Jenkov
Last update: 2017-10-15

In order to understand how Netty works it is useful to get an overview of Netty's internal design. There are several central concepts you need to know about. These are:

  • Bootstrap
  • EventLoopGroup
  • EventLoop
  • SocketChannel
  • ChannelInitializer
  • ChannelPipeline
  • ChannelHandler

How these concepts are related to each other is illustrated here:

An Netty overview giving an overview of Netty's internal design

Each of these parts of Netty's design will be described in more detail in the following sections.


The Bootstrap classes in Netty take care of bootstrapping Netty. The bootstrapping process includes starting threads, opening sockets etc. Bootstrapping will be explained in more detail in its own tutorial.


A Netty EventLoopGroup is a group of EventLoop's . Multiple EventLoop's can be grouped together. This way the EventLoop shares some resources like threads etc.


A Netty EventLoop is a loop that keeps looking for new events, e.g. incoming data from network sockets (from SocketChannel) instances). When an event occurs, the event is passed on to the appropriate event handler, for instance a ChannelHandler.


A Netty SocketChannel represents a TCP connection to another computer over a network. Whether you are using Netty as client or server, all data exchanged with other computers on the network are passed through a SocketChannel instance representing the TCP connection between the computers.

A SocektChannel is managed by an EventLoop, and always only by that same EventLoop. Since an EventLoop is always executed by the same thread, a SocketChannel instance is also only accessed by the same thread. Therefore you don't have to worry about synchronization when reading from a SocketChannel.


A Netty ChannelInitializer is a special ChannelHandler which is attached to the ChannelPipeline of a SocketChannel when the SocketChannel is created. The ChannelInitializer is then called so it can initialize the SocketChannel.

After initializing the SocketChannel the ChannelInitializer removes itself from the ChannelPipeline.


Each Netty SocketChannel has a ChannelPipeline. The ChannelPipeline contains a list of ChannelHandler instances. When the EventLoop reads data from a SocketChannel the data is passed to the first ChannelHandler in the ChannelPipeline. The first ChannelHandler processes the data and can choose to forward it to the next ChannelHandler in the ChannelPipeline, which then also processes the data and can choose to forward it to the next ChannelHandler in the ChannelPipeline etc.

When writing data out to a SocketChannel the written data is also passed through the ChannelPipeline before finally being written to the SocketChannel. Exactly how this works will be described in a separate tutorial.


A Netty ChannelHandler handles the data that is received from a Netty SocketChannel. A ChannelHandler can also handle data that is being written out to a SocketChannel. Exactly how this works will be described in a separate tutorial.

Jakob Jenkov

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