A file descriptor is a number that uniquely identifies an open file in a computer's operating system. It describes a data resource, and how that resource may be accessed. When a program asks to open a file — or another data resource, like a network socket — the kernel: Grants access.

What is the use of file descriptor in Java?

Instances of the file descriptor class serve as an opaque handle to the underlying machine-specific structure representing an open file, an open socket, or another source or sink of bytes. The main practical use for a file descriptor is to create a FileInputStream or FileOutputStream to contain it.

Why we are using FD?

For most operating systems, a file descriptor (FD) is a small non-negative integer that helps in identifying an open file within a process while using input/output resources like network sockets or pipes. In a way, it can be considered as an index table of open files.