Java - FileDescriptor sync() method

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Description

The Java FileDescriptor sync() method forces all system buffers to be written to the disk. It ensures that data written to a file is physically stored on the disk, preventing data loss in case of a system crash.

Declaration

Following is the declaration for java.io.FileDescriptor.sync() method −

 public void sync() 

Parameters

NA

Return Value

The method does not return any value.

Exception

SyncFailedException− This exception is thrown when the buffer cannot be flushed or because the system cannot guarantee synchronization of all the buffers with the physical media.

Example - Usage of FileDescriptor sync() method

The following example shows the usage of Java FileDescriptor sync() method.

FileDescriptorDemo.java

 package com.tutorialspoint; import java.io.FileDescriptor; import java.io.FileInputStream; import java.io.FileOutputStream; public class FileDescriptorDemo { public static void main(String[] args) { FileInputStream fis = null; FileOutputStream fos = null; FileDescriptor fd = null; byte[] b = {65,66,67,68,69,70}; try { fos = new FileOutputStream("test.txt"); fd = fos.getFD(); // writes byte to file output stream fos.write(b); // flush data from the stream into the buffer fos.flush(); // confirms data to be written to the disk fd.sync(); // create input stream fis = new FileInputStream("test.txt"); int value = 0; // for every available bytes while((value = fis.read())!= -1) { // converts bytes to char char c = (char)value; // prints char System.out.print(c); } // print System.out.print("\nSync() successfully executed!!"); fos.close(); fis.close(); } catch(Exception e) { // if any error occurs e.printStackTrace(); } } } 

Output

Let us compile and run the above program, this will produce the following result−

 ABCDEF Sync() successfully executed!! 

Example - Using sync() to Ensure Data is Written to Disk

The following example shows the usage of Java FileDescriptor sync() method.

FileDescriptorDemo.java

 package com.tutorialspoint; import java.io.File; import java.io.FileDescriptor; import java.io.FileOutputStream; import java.io.IOException; public class FileDescriptorDemo { public static void main(String[] args) { File file = new File("sync_example.txt"); try (FileOutputStream fos = new FileOutputStream(file)) { FileDescriptor fd = fos.getFD(); // Get file descriptor // Write data to the file fos.write("Hello, this is a test for sync method.".getBytes()); // Force data to be written to disk fd.sync(); System.out.println("Data successfully written and synced to disk."); } catch (IOException e) { e.printStackTrace(); } } } 

Output

Let us compile and run the above program, this will produce the following result−

 Data successfully written and synced to disk. 

Explanation

• A FileOutputStream is created for "sync_example.txt".

• The getFD() method retrieves the file descriptor (FileDescriptor object).

• Data is written to the file using fos.write().

• The sync() method is called on the file descriptor, ensuring that the data is immediately written to disk.

• The file is automatically closed using try-with-resources.

Example - Using sync() in a Buffered Stream

The following example shows the usage of Java FileDescriptor sync() method.

FileDescriptorDemo.java

 package com.tutorialspoint; import java.io.BufferedOutputStream; import java.io.File; import java.io.FileDescriptor; import java.io.FileOutputStream; import java.io.IOException; public class FileDescriptorDemo { public static void main(String[] args) { File file = new File("buffered_sync_example.txt"); try (FileOutputStream fos = new FileOutputStream(file); BufferedOutputStream bos = new BufferedOutputStream(fos)) { FileDescriptor fd = fos.getFD(); // Get file descriptor // Write data in a buffered stream bos.write("Buffered stream sync example.".getBytes()); // Flush the buffer bos.flush(); // Force data to be written to disk fd.sync(); System.out.println("Buffered data successfully written and synced to disk."); } catch (IOException e) { e.printStackTrace(); } } } 

Output

Let us compile and run the above program, this will produce the following result−

 Buffered data successfully written and synced to disk. 

Explanation

• A BufferedOutputStream wraps the FileOutputStream for efficient writing.

• Data is written to the buffered stream.

• bos.flush() is called to ensure buffered data is passed to the underlying file stream.

• The sync() method is called on the file descriptor, ensuring the data is physically written to disk.

• The file is closed automatically using try-with-resources.