Java FileDescriptor Class

java.io.FileDescriptor class in Java works for opening a file having a specific name. If there is any content present in that file it will first erase all that content and put “Beginning of Process” as the first line. 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.

There are 3 major fields in this class:

• err: A handle to the standard error stream.
• in: A handle to the standard input stream.
• out: A handle to the standard output stream.

Declaration of FileDescriptor

public final class FileDescriptor
extends Object

Constructor – FileDescriptor(): constructs a FileDescriptor object

Methods Associated with FileDescriptor in Java 

There are two main methods as mentioned below:

• java.io.File.sync(): synchronizes all the buffers with the underlying device.
• java.io.File.valid(): checks whether the FileDescriptor object is valid or not.

1. sync() Method

java.io.File.sync() synchronizes all the buffers with the underlying device. When all the modified data of the FileDescriptor have been written to the underlying device, the method returns. 

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 SyncMethod
{
    public static void main(String[] args) 
                    throws IOException
    {
        // Initializing a FileDescriptor
        FileDescriptor geek_descriptor = null;
        FileOutputStream geek_out = null;

        // HERE I'm writing "GEEKS" in my file
        byte[] buffer = {71,69,69,75,83};

        try{
            geek_out = new FileOutputStream("FILE.txt");

            // This getFD() method is called before closing the output stream
            geek_descriptor = geek_out.getFD();

            // writes byte to file output stream
            geek_out.write(buffer);

            // USe of sync() : to sync data to the source file
            geek_descriptor.sync();
            System.out.print("\nUse of Sync Successful ");

        }
        catch(Exception except)
        {
            // if in case IO error occurs
            except.printStackTrace();
        }
        finally
        {
            // releases system resources
            if(geek_out!=null)
                geek_out.close();
        }
    }
}

Output : 

Note: You can not see the changes made by this code to “FILE.txt” file mentioned in the code as no such file exists on Online IDE. So, you need to copy the code to your SYSTEM compiler and observe the change to your file. Whatever may be the content present in the file, it will Sync your file to the device and overwrites the data. Now the content of file “FILE.txt” will be 

GEEKS

Even if no such file exists, it will create that file on its own, sync the file, and will write the content, you mention. 

2. valid() Method

java.io.File.valid() checks whether the FileDescriptor object is valid or not. 

Syntax:

public boolean valid()

Return true if the FileDescriptor object is valid else, false

import java.io.*;

public class ValidMethod
{
    public static void main(String[] args) throws IOException
    {
        // Initializing a FileDescriptor
        FileDescriptor geek_descriptor = null;
        FileInputStream geek_in = null;

        try
        {
            geek_in = new FileInputStream("FILE.txt");

            // get file descriptor
            geek_descriptor = geek_in.getFD();

            boolean check = false;

            // Use of valid() : checking the validity of FileDescriptor
            check = geek_descriptor.valid();

            System.out.print("FileDescriptor is valid : "+check);

        }
        catch(Exception except)
        {
            // if in case IO error occurs
            except.printStackTrace();
        }
        finally
        {
            // releases system resources
            if(geek_in!=null)
                geek_in.close();
        }
    }
}

Output : 

Note: You can not see the changes made by this code to the “FILE.txt” file mentioned in the code as no such file exists on Online IDE. So, you need to copy the code to your SYSTEM compiler and observe the change to your file. This method will check the validity of our FileDescriptor. Since, our FileDescriptor in the code was valid so, true is returned.