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| Chapter 11 The java.io Package |

# FileDescriptor

## Name

FileDescriptor

## Synopsis

Class Name:

  `java.io.FileDescriptor`

Superclass:

  `java.lang.Object`

Immediate Subclasses:

  None

Interfaces Implemented:

  None

Availability:

  JDK 1.0 or later

## Description

The `FileDescriptor` class encapsulates system-specific handles for files and sockets. Instances of this class can be properly constructed only by native methods of other classes. In other words, you should not be constructing your own file descriptors. Currently, file descriptors are returned by the following methods:

- `DatagramSocketImpl.getFileDescriptor()`

- `FileInputStream.getFD()`

- `FileOutputStream.getFD()`

- `RandomAccessFile.getFD()`

- `SocketImpl.getFileDescriptor()`

A file descriptor can be used in the constructors for `FileInputStream`, `FileOutputStream`, `FileReader`, and `FileWriter`.

## Class Summary

    public final class java.io.FileDescriptor extends java.lang.Object {
      // Constants
      public final static FileDescriptor err;
      public final static FileDescriptor in;
      public final static FileDescriptor out;
      // Instance Methods
      public native void sync();                     // New in 1.1
      public native boolean valid();
    }

## Constants

### err

#### public final static FileDescriptor err

Description

  The file descriptor for standard error. See `System.err`, which is constructed from this constant.

### in

#### public final static FileDescriptor in

Description

  The file descriptor for standard input. See `System.in`, which is constructed from this constant.

### out

#### public final static FileDescriptor out

Description

  The file descriptor for standard output. See `System.out`, which is constructed from this constant.

## Instance Methods

### sync

#### public native void sync() throws SyncFailedException

Availability

  New as of JDK 1.1

Throws

  `SyncFailedException`

    If synchronization cannot be accomplished.

Description

  This method causes the underlying device to be updated to a current state, which typically involves asking the operating system to flush its buffer. For example, if this `FileDescriptor` refers to a file on a physical disk, the disk is physically updated to reflect the current state of the object this descriptor represents. This method allows an application to put a device in a known state, which could be useful for transaction processing.

### valid

#### public native boolean valid()

Returns

  `true` if the `FileDescriptor` represents a valid, open device; `false` otherwise.

Description

  This method returns a `boolean` value that indicates the validity of the file descriptor.

## Inherited Methods

Method | Inherited From | Method | Inherited From
--- | --- | --- | ---
`clone()` | `Object` | `equals(Object)` | `Object`
`finalize()` | `Object` | `getClass()` | `Object`
`hashCode()` | `Object` | `notify()` | `Object`
`notifyAll()` | `Object` | `toString()` | `Object`
`wait()` | `Object` | `wait(long)` | `Object`
`wait(long, int)` | `Object` |  |

## See Also

`FileInputStream`, `FileOutputStream`, `FileReader`, `FileWriter`, `SyncFailedException`, `System`

File |  | FileInputStream