Not possible to configure TimerManagerFactoryBean for fixed-delay execution with period=0 [SPR-2856]

[spring-projects-issues]

Steven Devijver opened SPR-2856 and commented

It's not possible to configure TimerManagerFactoryBean (through ScheduledTimerListener) for fixed-delay execution with period=0. We use this scenario to immediately reschedule a job once it's finished.

We have to use fixed-delay and not fixed-rate since we only want one job to run at any given time (see http://e-docs.bea.com/wls/docs92/javadocs/commonj/timers/TimerManager.html for details). However, configuring a ScheduledTimerListener with fixedRate=false and period=0 puts us in neither fixed-rate nor fixed-delay but run-once.

The solution to get fixed-delay is to increase the value for period, for example to 1. I think the configuration would make more sense if there would be FixedRateScheduledTimerListener, FixedDelayScheduledTimerListener and RunOnceScheduledTimerListener configuration classes.

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Affects: 2.0 final

[spring-projects-issues]

Juergen Hoeller commented

I've simply redefined a period value of 0 to be valid now for a ScheduledTimerListener, with -1 being the default (leading to one-time execution, just like any negative number). This should still be reasonably intuitive, since no period specified results in one-time execution, a period specified results in fixed delay execution, a period specified with "fixedRate" flag results in fixed rate execution.

The odd thing - and the reason why 0 was not considered a valid actual period before - is that neither "java.util.Timer" nor "java.util.concurrent.ScheduledExecutorService" consider 0 as a valid period value! Which is why not only -1 (or any negative number) but also 0 does not trigger periodic execution there, but is rather considered as indicating a one-time execution - although noone is expected to specify that value explicitly in the first place. I guess we could consider throwing an IllegalArgumentException in case of 0 there, only accepting -1 (the default) as indication for one-time execution. On the other hand, that would break backwards compatibility for people who do specify 0 there... That said, declaratively configured one-time execution is rare anyway; there's almost always a concrete period value specified.

I'll consider not accepting a period value of 0 for ScheduledTimerTask and ScheduledExecutorTask in Spring 2.1, strictly only supporting valid periods when specified (and doing one-time execution in case of no period specified).

Juergen

[spring-projects-issues]

Steven Devijver commented

Juergen,

Where using the TimerManager for continuous execution, where not interested in pauzes although we can live with a very short pauze. How can we configure continuous execution?

Steven

[spring-projects-issues]

Juergen Hoeller commented

Did someone take over your account, Steven? That doesn't sound quite like yourself ;-)

As I indicated above, ScheduledTimerListener does now accept a period value 0 as valid, in particular for fixed delay execution. Only if you don't specify any period at all (or a negative value for the period property), you'll get one-time execution. So continuous immediate re-execution of the same job should work fine now (provided that the TimerManager implementation actually accepts a period value 0).

The rest of my comment above is only relevant for standard Timer and ScheduledExecutorService usage, where a period value 0 is definitely not accepted. We can't do much about that there; the only way around is to specify a low period value (such as 1) instead of 0. As I said, I'm wondering whether we should reject a 0 value with an IllegalArgumentException there... that doesn't affect TimerManager, though.

Juergen