OverlappingFileLockException in Java with Examples

An OverlappingFileLockException is thrown when attempting to acquire a lock that overlaps an existing or pending lock held by this process.

This exception is thrown by the lock( ) and tryLock( ) methods of FileChannel if the requested lock region overlaps a file lock that is already held by some thread in this JVM, or if there is already a thread in this JVM waiting to lock an overlapping region of the same file. The FileChannel file locking mechanism is designed to lock files against concurrent access by two separate processes. Two threads within the same JVM should not attempt to acquire a lock on overlapping regions of the same file, and any attempt to do so causes an exception of this type to be thrown.

Package View

java.lang.Object
    java.lang.Throwable
         java.lang.Exception
              java.lang.RuntimeException
                   java.lang.IllegalStateException
                    java.nio.channels.OverlappingFileLockException

Remember: It does implement Serializable interface.

Syntax:

public class OverlappingFileLockException
extends IllegalStateException

Note: An unchecked exception is thrown when an attempt is made to acquire a lock on a region of a file that overlaps a region already locked by the same Java virtual machine, or when another thread is already waiting to lock an overlapping region of the same file.

Constructor Summary

• OverlappingFileLockException(): Constructs an instance of this class.

Implementation:

In this example, we shall show you how to create a shared file lock in Java and handle OverlappingFileLockException. Creating shared file locks using Java NIO Channels implies that one should :

1. Create a File object to encapsulate an actual file in the file system that you want to lock to
2. Create a random access file stream (read-write). To do so you must first create a RandomAccessFile object to encapsulate the file object created above and open it for read-write operations. Then use the getChannel() API method of the RandomAccessFile object to get the file channel to read/write data from/to
3. Acquire an exclusive lock on this channel’s file by using the lock(long, long, boolean) API method of the FileChannel class. This method blocks until the region can be locked or this channel is closed or the invoking thread is interrupted. The boolean attribute marks the lock as shared or not. The method returns a handle to a FileLock class for utilizing the lock
4. Alternatively, we could use the tryLock(long, long, boolean) API method of the FileChannel class. This method attempts to acquire an exclusive lock on this channel’s file but does not block; an invocation always returns immediately, either having acquired a lock on the requested region or having failed to do so.

Example 1

Java

// Java Program to Illustrate Shared lock over File

// Importing required classes
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.nio.channels.OverlappingFileLockException;

// Main class
// CreateSharedFileLockOnFile
public class GFG {

    // Main driver method
    public static void main(String[] args)
    {

        // Try block to check for exceptions
        try {

            // Creating a file
            File file = new File("fileToLock.dat");

            // Creates a random access file stream to read
            // from, and optionally to write to
            FileChannel channel
                = new RandomAccessFile(file, "rw")
                      .getChannel();

            // Acquire an exclusive lock on this channel's
            // file ( block until the region can be locked,
            // this channel is closed, or the invoking
            // thread is interrupted)
            FileLock lock
                = channel.lock(0, Long.MAX_VALUE, true);

            // Attempts to acquire an exclusive lock on this
            // channel's file (does not block, an invocation
            // always returns immediately, either having
            // acquired a lock on the requested region or
            // having failed to do so.
            try {

                lock = channel.tryLock(0, Long.MAX_VALUE,
                                       true);
            }
            catch (OverlappingFileLockException e) {
                // thrown when an attempt is made to acquire
                // a lock on a a file that overlaps a region
                // already locked by the same JVM or when
                // another thread is already waiting to lock
                // an overlapping region of the same file
                System.out.println(
                    "Overlapping File Lock Error: "
                    + e.getMessage());
            }

            // Checking whether this lock is shared
            boolean isShared = lock.isShared();

            // Releasing the lock
            // using release() method
            lock.release();

            // Closing the channel
            // using standard close() method
            channel.close();
        }

        // Catch block to handle exceptions
        catch (IOException e) {

            // Display message(error) if I/O exception
            // occurs
            System.out.println("I/O Error: "
                               + e.getMessage());
        }
    }
}

Output:

Example 2:

Java

// Java Program to Lock a File before Writing into It

// Importing required classes
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;
import java.nio.channels.OverlappingFileLockException;
import java.util.concurrent.TimeUnit;

// Main class
public class Demo {

    // Main driver method
    public static void main(String[] args) throws Exception
    {
        // Creating object of RandomAccessFile
        RandomAccessFile file
            = new RandomAccessFile("accounts.txt", "rw");

        // Similarly creating object of FileChannel class
        FileChannel channel = file.getChannel();

        // Initially setting lock to file as null
        // as there is no lock by far
        FileLock lock = null;

        // Try block to check for exceptions
        try {
            lock = channel.tryLock();
        }

        // Catch block to handle exceptions
        catch (final OverlappingFileLockException e) {

            // Closing channel and file in order to
            // free up memory resources and avoid leakage
            // using close() method
            file.close();
            channel.close();
        }

        // Writing something while inlocked
        file.writeChars("writing after lock");

        // Making it to sleep for very small amount of time
        TimeUnit.HOURS.sleep(1);

        // Releasig lock over file using release() method
        lock.release();

        // Again closing channel and file in order to
        // free up memory resources and avoid leakage
        // using close() method
        file.close();
        channel.close();
    }
}

Output:

It will throw the OverlappingFileLockException, if a lock that overlaps the requested region is already held by this Java virtual machine, or if another thread is already blocked in this method and is attempting to lock an overlapping region.