ChannelPipeline

ChannelPipeline is a core component of Netty’s architecture. It represents an ordered list of ChannelHandler instances associated with a Channel. Each handler implements logic for processing inbound events (such as connection establishment, data reads, and exceptions) or outbound operations (such as writes and flushes). By chaining handlers together, the pipeline allows separation of concerns: decoding and encoding of data, application protocol handling, and business logic can be developed and reused independently.

Inbound and outbound events traverse the pipeline in opposite directions. When an inbound event occurs, it is propagated from the head toward the tail by calling the corresponding ChannelInboundHandler methods on each handler. Outbound events, such as a write request, flow from the tail toward the head via ChannelOutboundHandler methods. Each handler is invoked through a ChannelHandlerContext, which also provides access to the pipeline, the associated Channel, and the next and previous handlers.

The pipeline includes special head and tail handlers. The head connects the pipeline to the transport and typically initiates outbound operations, while the tail acts as a sink for unhandled events and can handle or log errors.

Handlers can be added, removed, or replaced at runtime using methods like addLast, remove, or replace, often by name. All modifications are expected to occur within the Channel’s event loop thread to maintain thread safety.

Common components placed in a ChannelPipeline include decoders, encoders, business-logic handlers, and idle-state or exception handlers. Netty provides convenience base classes such as ChannelInboundHandlerAdapter and ChannelOutboundHandlerAdapter to simplify development.