synchronization

I have a Java program that executes from Spring Qquartz every 20 seconds. Sometimes it takes just few seconds to execute, but as data gets bigger I'm sure it run for 20 seconds or more. How can I prevent Quartz from firing/triggering the job while one instance is still being executed? Firing 2 jobs performing same operations on a database would not be so good. Is there a way I can do some kind of synchronization? If all you need to do is fire every 20 seconds, Quartz is serious overkill. The java.util.concurrent.ScheduledExecutorService should be perfectly sufficient for that job. The

List<String> list = Collections.synchronizedList(new ArrayList<String>()); synchronized (list) { list.add("message"); } Is the block "synchronized (list){} " really need here ? Sam Goldberg You don't need to synchronize as you put in your example. HOWEVER, very important, you need to synchronize around the list when you iterate it (as noted in the Javadoc): It is imperative that the user manually synchronize on the returned list when iterating over it: List list = Collections.synchronizedList(new ArrayList()); ... synchronized(list) { Iterator i = list.iterator(); // Must be in synchronized