Java FileLock blocking without exceptions; waiting on the lock
I've discovered the Java File locking mechanism is a real mistake since it cannot do the 1st rule of locking - blocking on a call to lock! Unless I'm wrong - a call to lock() will throw an Exception if an application already maintains a lock on the file, which I know will be the case in my application. I've looked through some answers here and unfortunately I am not using Java 7.
Does anyone have suggestions on how I might handle waiting on an exclusive file lock without putting myself in a while(true) loop :)
EDIT after Aubin's answer (copy from comment):
What I want to do is access a large file after its been copied to a directory and then process it. My code is simple...
public boolean ifFileReady(File file) {
boolean ready = false;
FileLock lock = null;
FileChannel channel = null;
try {
channel = new RandomAccessFile(file, "rw").getChannel();
lock = channel.lock();
lock.release();
ready = true;
}
catch( IOException e ) {
// Always Here
}
finally
{
if (channel != null)
channel.close();
}
return ready;
}
I never block at the lock. It always throws
Copy of the documentation of java.nio.channels.FileChannel :
public abstract FileLock lock( long position, long size, boolean shared ) throws IOException
Acquires a lock on the given region of this channel's file.
An invocation of this method will block until the region can be locked , this channel is closed, or the invoking thread is interrupted, whichever comes first.
Since: 1.4
The following code demonstrates that lock does indeed block without throwing an exception. I had a similar problem but discovered the exception was actually caused by opening the file rw when my user didn't have permission to do so.
It's a complete program; run two instances of it concurrently for the demonstration
import java.io.File;
import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;
public class Test {
public static void main(String args[]) throws Exception {
FileChannel channel = null;
File lockFile = new File("lock");
System.out.println ("Opening file...");
channel = new RandomAccessFile(lockFile, "rw").getChannel();
System.out.println ("File open. Locking channel...");
channel.lock();
System.out.println ("Channel locked. Sleep for 30 seconds");
Thread.sleep (30000);
System.out.println ("Process terminating");
}
}
To keep the code more consise I didn't include the unlocking, closing or exception handling (you will get a compiler warning). All the resources should be released automatically by the OS when the process exits.
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