ChannelHandlers

ChannelHandlers are components in Netty that intercept and process I/O events and data passing through a ChannelPipeline. They handle inbound data as it flows into the application and outbound data as it leaves, and may implement the inbound side, the outbound side, or both.

Handlers are added to a ChannelPipeline associated with a Channel. They implement the inbound interface (ChannelInboundHandler), the outbound interface (ChannelOutboundHandler), or the duplex interface (ChannelDuplexHandler). They can be stateless and sharable across pipelines if annotated with @Sharable, or stateful and bound to a single pipeline. They interact with the ChannelHandlerContext to communicate with other handlers and the framework, and they typically propagate events along the pipeline using methods such as ctx.fireChannelRead, ctx.write, and ctx.flush.

Lifecycle and events: Handlers receive lifecycle callbacks like handlerAdded and handlerRemoved, as well as channel lifecycle events such as channelRegistered, channelActive, and channelInactive. They also handle data and other events through methods like channelRead, channelReadComplete, userEventTriggered, exceptionCaught, and channelWritabilityChanged. Implementations should pass events along the pipeline unless they intend to consume or transform them.

Common patterns and considerations: ChannelHandlers are often used for decoding and encoding (protocol codecs), framing, business logic, and logging, or for ancillary tasks such as idle-state monitoring. Good practice emphasizes non-blocking, fast operations, and careful management of ByteBuf reference counts. For outbound processing, handlers may modify or buffer data before forwarding it; for inbound processing, they may transform or filter data before it reaches the business logic.

Ordering and interaction: Inbound events flow from the pipeline head toward the tail, while outbound events flow from the tail back toward the head. Proper ordering and minimal work per handler help maintain throughput and latency.