Spring, background execution or on demand

Question

I have a spring application that uses org.springframework.scheduling.quartz.SimpleTriggerBean to schedule execution of a method on a regular basis.

Sometimes, I want to call the same method "on demand". It will be trigger by an action on the GUI. Since the method that I want to execute takes a couple of sec, I don't want to block the user GUI until the execution finish. Moreover, I want to coordinate the "on demand" execution with the background thread (mutually exclusive).

Here's one approach:

1. Create a Bean called Manager that use a TaskExecutor to schedule a Task. The Manager has a method Manager.scheduleTask()
2. Both the background and the "on demand" threads will call the same method on the Manager (Manager.scheduleTask)
3. The task runs in a synchronize method to assure that only one task is running.

I'm looking for more clever/cleaner solutions.

Answer 1

If you use Spring 3.0 or newer have a look at the Task Execution and Scheduling section of the reference doc.

It shows that there are two annotations:

• @Scheduled
• @Timer

The solution would be the same at least: having 3 methods:

private void  doIt() {...}

@Scheduled(cron="0 0 0 * * MON-FRI")
public void doItEveryDay() {doIt();}

@Async
public void doItOnDemand() {doIt();}

But with these annotations, it would be easy to read and easy to understand why there are three methods.

Answer 2

You can use a SingleThreadExecutor.

ExecutorService exec = Executors.newSingleThreadExecutor();
exec.submit(<task>);

When your quartz job fires it can submit a task to the executor. Similarly, when your job is run manually, it can also submit a task to the executor. Since the ExecutorService only has a single thread, the task can only run once at a time. The other instance of the task will be queued up until the one which is currently running completes. You don't need to worry about manual synchronisation in this case.