multiple joins with timeouts

import java.io.File;

// PROBLEM...
// Worst case it will wait two minutes if both files do not exist,
// one minute if one or the other does not exist, but will typically
// only take several seconds if everything is working.
// Would line to wait a MAXIMUM of 1 minute for BOTH threads.

public class TryExistsThread
{
	public static void main( String[] args )
	{
		try
		{
			ExistsThread thread_A = new ExistsThread( "file_a.txt" );
			ExistsThread thread_B = new ExistsThread( "file_b.txt" );

thread_A.start();
			thread_B.start();

// Wait for thread_A and thread_B
			long start = System.currentTimeMillis();
			thread_A.join( 60000 );
			thread_B.join( 60000 );
			long stop = System.currentTimeMillis();

System.out.println( "Seconds = " + ((stop-start)/1000) );
			System.out.println( "(A) exists = " + thread_A.getExists() );
			System.out.println( "(B) exists = " + thread_B.getExists() );
		}
		catch( Exception ex )
		{
			ex.printStackTrace();
		}
	}

private static class ExistsThread
	extends Thread
	{
		private boolean exists;
		private File file;

public ExistsThread( String fileName )
		{
			file = new File( fileName );
			setDaemon( true );
		}

@Override
		public void run()
		{
			// Crude polling to emulate blocking IO
			while( ! file.exists() )
			{
				try
				{
					sleep( 1000 );
				}
				catch( InterruptedException e )
				{
				}
			}
			exists = true;
		}

public boolean getExists()
		{
			return exists;
		}
	}
}

Since neither thread ever finishes, the joins run consecutively. You wait 1 minute in the first join. Then you issue the second join which waits 1 minute.

Join is not your solution. Try wait/notify.

With a max of 60000 milliseconds. You can wait up to 30 seconds for the first thread to notify you and decrement the max wait time by how long it actually took. Then you can wait up to "what is left" milliseconds for the second thread to notify you.

Etc.

Ed

Edward Harned wrote:Since neither thread ever finishes...

This is not exactly the case. If, while the joins are pending, the files are created, then the ExistsThread will return.

Thanks for the reminder about wait/notify. I'll re-familiarize myself with it.