FileLock in Java NIO Channels

FileLock locks or tries to lock the given part of the file. It belongs to the package java.nio.channels and this feature is available in java from JDK 1.4.

FileLock is used for locking the files in either shared mode or non-shared mode. It has two important methods as given below:

FileLock.lock(long position, long size, boolean shared)
FileLock.tryLock(long position, long size, boolean shared)

The above method uses the argument as initial position, file size to lock and one more parameter to decide whether a lock is shared or not.

Creating a file lock

The object of file-lock is created when the lock is acquired on a file using one of the lock() or tryLock() methods of the FileChannel or AsynchronousFileChannel.

Basic FileLock Example

Let's see the program used to write (append) in a file using the channel with exclusive lock:

FileLockExample.java

package com.javatpoint;
import java.io.IOException;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.nio.ByteBuffer;
import java.nio.file.Paths;
import java.nio.file.Path;
import java.nio.file.StandardOpenOption;

public class FileLockExample {
    public static void main (String [] args)
            throws IOException {
        String input = "* end of the file.";
        System.out.println("Input string to the test file is: " + input);
        ByteBuffer buf = ByteBuffer.wrap(input.getBytes());
        String fp = "D:\\testout.txt";
        Path pt = Paths.get(fp);
        FileChannel fc = FileChannel.open(pt, StandardOpenOption.WRITE,
StandardOpenOption.APPEND);
        System.out.println("File channel is open for write and Acquiring lock...");
        fc.position(fc.size() - 1); // position of a cursor at the end of file     
        FileLock lock = fc.lock(); 
        System.out.println("The Lock is shared: " + lock.isShared());
        fc.write(buf);
        fc.close(); // Releases the Lock
        System.out.println("Content Writing is complete. Therefore close the channel and release the 

lock.");
        PrintFile.print(fp);
    }
}

PrintFile.java

package com.javatpoint;
import java.io.IOException;
import java.io.FileReader;
import java.io.BufferedReader;

public class PrintFile {
public static void print(String path)
throws IOException {
	FileReader filereader = new FileReader(path);
	BufferedReader bufferedreader = new BufferedReader(filereader);
	String tr = bufferedreader.readLine();	
	System.out.println("The Content of testout.txt file is: ");
	while (tr != null) {	
	 	System.out.println("    " + tr);
		tr = bufferedreader.readLine();
		}
	filereader.close();
	bufferedreader.close();
	}
}

Note: Before running the code a text file named as "testout.txt" is need to be created and the content of a text file is given below:

Welcome to javaTpoint.com

This is the example of FileLock in Java NIO channel.

Output:

Input string to the test file is: * end of the file.
File channel is open for write and Acquiring lock...
The Lock is shared: false
Content Writing is complete. Therefore close the channel and release the lock.
The Content of testout.txt file is: 
    Welcome to javaTpoint.com
    This is the example of FileLock in Java NIO channel.
    * end of the file.

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