Under what conditions does the ScheduledExecutorService start the task on new threads (JDK 5)

[Robin Sharma]

Howdy!

I'm playing around with the ScheduledExecutorService scheduler. In the code i first init the scheduler with a fixed thread pool size and then pass it a simple task which keeps refreshing a shared list with new data, periodically. At random, the task might throw a runtime exception, in which case the scheduler should start a new thread, but i don't see it doing that. So, how do i simulate this scenario? I'm running this code on a single CPU m/c, using JDK 1.5.

import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.Random;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

import org.apache.log4j.Logger;

public final class Pinger {
    private static final Logger logger = Logger.getLogger(Pinger.class);
    
    private static final String NEW_LINE = System.getProperty("line.separator");
    private static int DEFAULT_WAIT_TIME = 1 * 30 * 1000;
    private static int DEFAULT_EPR_POLLER_POOL_SIZE = Runtime.getRuntime().availableProcessors();

    private List<Integer> list = new CopyOnWriteArrayList<Integer>();
    
    private final ScheduledExecutorService statusPollService;
    
	public Pinger() {
		// Initialize the thread pools.
        int poolSize = DEFAULT_EPR_POLLER_POOL_SIZE;
        logger.info("Using a pool size of: " + poolSize);
        statusPollService = Executors.newScheduledThreadPool(3);
        
        // Populate the shared list with some junk.
        list.addAll(createRandomList());
        logger.info("Original list... " + list);
        
        // Start the scheduler to ping.
        int waitTime = DEFAULT_WAIT_TIME;
        logger.info("Starting scheduler...");
		statusPollService.scheduleAtFixedRate(new Runnable() {
			public void run() {
				updateList();
			}
		}, 4000, waitTime, TimeUnit.MILLISECONDS);
		
		logger.info("Setting shutdown hook to cleanup thread pools...");
		// Add a shutdown hook to stop the thread pools gracefully when the application exits.
		Runtime.getRuntime().addShutdownHook(new Thread("[ThreadPool-Shutdown]") {
			
			@Override
			public void run() {
				if(statusPollService != null && !statusPollService.isShutdown()) {
					statusPollService.shutdownNow();
				}
				
			}
		});

	}
	
	public void updateList() {
		Thread t = Thread.currentThread();
		List<Integer> nl = createRandomList();
		logger.info(t.getName() + " > Refreshing list with new data: " + nl);
        Random r = new Random(System.nanoTime());
        
        if(r.nextInt(10) % 3 == 0) {
        	logger.info("Oops, gotta throw an exception :(");
        	throw (new RuntimeException("Aww Snap... Something went wrong :("));
        }
        
		list.clear();
		list.addAll(nl);
	}
	
	private List<Integer> createRandomList() {
		List<Integer> nl = new ArrayList<Integer>();
        Random r = new Random(System.nanoTime());
        for (int i = 0; i < 5; i++) {
        	nl.add(r.nextInt(100));
		}
        return nl;
	}
	
	
	public static void main(String[] args) {
		new Pinger();
	}
}

I just get the following log messages and then nothing happens:

[18:41:45,150] Using a pool size of: 1
[18:41:45,160] Original list... [33, 39, 4, 82, 22]
[18:41:45,160] Starting scheduler...
[18:41:45,160] Setting shutdown hook to cleanup thread pools...
[18:41:49,156] pool-1-thread-1 > Refreshing list with new data: [25, 42, 73, 17, 17]
[18:41:49,166] pool-1-thread-1 > zzzZZZ, gotta sleep before updating...
[18:41:51,169] Oops, gotta throw an exception

Thanks.

[Nitesh Kant]

Since your task ended up with an exception, it will not be executed anymore.

[Max Galushka]

Just don't throw an exception inside your task

[Christophe Verré]

public void run() {  
	updateList();  
}

Why not catch any exceptions here ? If you catch and log them, the executor will not be terminated.