FileDescriptor (Java Platform SE 8 )
Keywords: java.io.FileDescriptor class
Date: 2026-01-21

compact1, compact2, compact3
java.io

## Class FileDescriptor

- java.lang.Object
- - java.io.FileDescriptor

-     public final class FileDescriptor
      extends Object

  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.

  Applications should not create their own file descriptors.

  Since:
    JDK1.0
  See Also:
    `FileInputStream`, `FileOutputStream`

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static FileDescriptor` | `err` A handle to the standard error stream.
    `static FileDescriptor` | `in` A handle to the standard input stream.
    `static FileDescriptor` | `out` A handle to the standard output stream.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `FileDescriptor()` Constructs an (invalid) FileDescriptor object.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `sync()` Force all system buffers to synchronize with the underlying device.
    `boolean` | `valid()` Tests if this file descriptor object is valid.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Field Detail

    - #### in

          public static final FileDescriptor in

      A handle to the standard input stream. Usually, this file descriptor is not used directly, but rather via the input stream known as `System.in`.
      See Also:
        `System.in`

    - #### out

          public static final FileDescriptor out

      A handle to the standard output stream. Usually, this file descriptor is not used directly, but rather via the output stream known as `System.out`.
      See Also:
        `System.out`

    - #### err

          public static final FileDescriptor err

      A handle to the standard error stream. Usually, this file descriptor is not used directly, but rather via the output stream known as `System.err`.
      See Also:
        `System.err`

  - ### Constructor Detail

    - #### FileDescriptor

          public FileDescriptor()

      Constructs an (invalid) FileDescriptor object.

  - ### Method Detail

    - #### valid

          public boolean valid()

      Tests if this file descriptor object is valid.
      Returns:
        `true` if the file descriptor object represents a valid, open file, socket, or other active I/O connection; `false` otherwise.

    - #### sync

          public void sync()
                    throws SyncFailedException

      Force all system buffers to synchronize with the underlying device. This method returns after all modified data and attributes of this FileDescriptor have been written to the relevant device(s). In particular, if this FileDescriptor refers to a physical storage medium, such as a file in a file system, sync will not return until all in-memory modified copies of buffers associated with this FileDescriptor have been written to the physical medium. sync is meant to be used by code that requires physical storage (such as a file) to be in a known state For example, a class that provided a simple transaction facility might use sync to ensure that all changes to a file caused by a given transaction were recorded on a storage medium. sync only affects buffers downstream of this FileDescriptor. If any in-memory buffering is being done by the application (for example, by a BufferedOutputStream object), those buffers must be flushed into the FileDescriptor (for example, by invoking OutputStream.flush) before that data will be affected by sync.
      Throws:
        `SyncFailedException` - Thrown when the buffers cannot be flushed, or because the system cannot guarantee that all the buffers have been synchronized with physical media.
      Since:
        JDK1.1

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