selectionkey

SelectionKey is a concept often encountered in Java's New I/O (NIO) framework, specifically within the `java.nio.channels` package. It represents the registration of a specific channel with a selector, along with the set of operations that the channel is interested in. When a selector detects that one or more of its registered channels are ready for an operation, it returns a set of SelectionKey objects.

Each SelectionKey object provides access to the channel it is associated with and the selector that registered it. Crucially, it also holds information about the type of operations for which the channel is ready. These operations are represented by integer constants, such as `OP_READ`, `OP_WRITE`, `OP_CONNECT`, and `OP_ACCEPT`. A SelectionKey can be interested in one or more of these operations.

Programmers use SelectionKey to determine which channels require attention. By examining the `readyOps()` method of a SelectionKey, one can ascertain the specific operations that are available on the associated channel. For instance, if `key.isReadable()` returns true, it signifies that the channel is ready to be read from. This allows for efficient, non-blocking I/O operations, as applications can multiplex the handling of multiple channels through a single thread. The SelectionKey is a fundamental element in managing and reacting to I/O events in Java NIO.