Java FileDescriptor Class

The java.io.FileDescriptor class represents an open file or socket handle in the underlying operating system. It acts as an opaque reference to a particular file, device, or socket. This class is mainly used as a bridge between Java I/O streams and the native file system resources.

Class Declaration

public final class FileDescriptor extends Object

• 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.

Common FileDescriptor Objects

• FileDescriptor.in: Represents the standard input (keyboard)
• FileDescriptor.out: Represents the standard output (console)
• FileDescriptor.err: Represents the standard error (error output)

Constructor

• FileDescriptor(): Creates an invalid file descriptor that is not connected to any file or socket.

Methods

There are two main methods as mentioned below:

• void sync(): Forces all system buffers to synchronize with the underlying device, ensuring that data is physically written.
• java.io.File.valid(): Checks whether the file descriptor object is valid (i.e., connected to an open file or socket).

Method Example

1. sync() Method

Syntax:

public void sync()

Return : void

Exception: SyncFailedException - This is exception is thrown if there is no guarantee of synchronization of buffers with the device.

import java.io.*;

public class FileDescriptorValidity {
    public static void main(String[] args) throws Exception {
        FileOutputStream fos = new FileOutputStream("example.txt");
        FileDescriptor fd = fos.getFD();

        System.out.println("Is FileDescriptor valid? " + fd.valid());

        fos.close();
        System.out.println("Is FileDescriptor valid after close? " + fd.valid());
    }
}

Is FileDescriptor valid? true
Is FileDescriptor valid after close? false

2. valid() Method

Syntax:

public boolean valid()

Return: true if the FileDescriptor object is valid else, false

import java.io.*;

public class FileDescriptorSync {
    public static void main(String[] args) throws Exception {
        FileOutputStream fos = new FileOutputStream("syncDemo.txt");
        FileDescriptor fd = fos.getFD();

        fos.write("GeeksForGeeks FileDescriptor Example".getBytes());
        fd.sync();  // Ensures data is written to disk

        System.out.println("Data synchronized successfully.");
        fos.close();
    }
}

Data synchronized successfully.

Example with Standard FileDescriptors

import java.io.*;

public class StandardFileDescriptors {
    public static void main(String[] args) throws Exception {
        FileInputStream fis = new FileInputStream(FileDescriptor.in);
        FileOutputStream fos = new FileOutputStream(FileDescriptor.out);
        FileOutputStream fes = new FileOutputStream(FileDescriptor.err);

        fos.write("Enter something: ".getBytes());
        int data = fis.read();
        fes.write(("You entered: " + (char)data + "\n").getBytes());

        fis.close();
        fos.close();
        fes.close();
    }
}

Output:

Enter something: A

You entered: A