NIO

NIO, short for New I/O (or non-blocking I/O), is the Java API introduced in Java 1.4 for building high-throughput network and file operations that do not tie up a thread per connection. Where classic Java I/O gives you a blocking stream per socket, NIO gives you buffers, channels, and selectors so a single thread can watch thousands of connections and act only when one is ready. It is the plumbing beneath many of the servers that actually terminate API traffic.

• Channels and buffers - Data moves through Channel objects into and out of reusable ByteBuffer instances rather than one-byte-at-a-time streams.
• Selectors - A single Selector multiplexes readiness events across many channels, so one thread services many sockets.
• Non-blocking mode - Sockets can be configured to return immediately instead of blocking, which is what lets event-loop servers scale.
• Memory-mapped and direct buffers - File and network data can be handled with less copying between kernel and user space.
• Foundation for frameworks - Netty, Undertow, and other high-concurrency Java stacks are built directly on NIO.

In real API operations NIO rarely shows up in the contract or the documentation, but it shows up in the load numbers. The gateways, reverse proxies, and backend services that hold open many simultaneous connections — long-polling, streaming, webhooks, and increasingly agent traffic that fans out to many endpoints at once — lean on NIO-style event loops to stay efficient under concurrency. When you evaluate whether a provider’s infrastructure can absorb bursty, high-fan-out demand, the non-blocking I/O model underneath is a big part of the answer.