Task Execution - Java Concurrency in Practice - page 142

construct a ScheduledThreadPoolExecutor through its constructor or through the
newScheduledThreadPool factory.
A Timer creates only a single thread for executing timer tasks. If a timer task takes too long
to run, the timing accuracy of other TimerTask s can suffer. If a recurring TimerTask is
scheduled to run every 10 ms and another Timer-Task takes 40 ms to run, the recurring
task either (depending on whether it was scheduled at fixed rate or fixed delay) gets called
four times in rapid succession after the long-running task completes, or “misses” four invoc-
ations completely. Scheduled thread pools address this limitation by letting you provide mul-
tiple threads for executing deferred and periodic tasks.
Another problem with Timer is that it behaves poorly if a TimerTask throws an un-
checked exception. The Timer thread doesn't catch the exception, so an unchecked excep-
tion thrown from a TimerTask terminates the timer thread. Timer also doesn't resurrect
the thread in this situation; instead, it erroneously assumes the entire Timer was cancelled.
In this case, TimerTask s that are already scheduled but not yet executed are never run, and
new tasks cannot be scheduled. (This problem, called “thread leakage” is described in Sec-
tion 7.3 , along with techniques for avoiding it.)
OutOfTime in Listing 6.9 illustrates how a Timer can become confused in this manner
and, as confusion loves company, how the Timer shares its confusion with the next hapless
caller that tries to submit a TimerTask . You might expect the program to run for six
seconds and exit, but what actually happens is that it terminates after one second with an
IllegalStateException whose message text is “Timer already cancelled”. Sched-
uledThreadPoolExecutor deals properly with ill-behaved tasks; there is little reason
to use Timer in Java 5.0 or later.
If you need to build your own scheduling service, you may still be able to take advantage of
the library by using a DelayQueue , a BlockingQueue implementation that provides the
scheduling functionality of ScheduledThreadPoolExecutor . A DelayQueue man-
ages a collection of Delayed objects. A Delayed has a delay time associated with it:
DelayQueue lets you take an element only if its delay has expired. Objects are returned
from a DelayQueue ordered by the time associated with their delay.
6.3. Finding Exploitable Parallelism
The Executor framework makes it easy to specify an execution policy, but in order to use
an Executor , you have to be able to describe your task as a Runnable . In most server