MAKE SURE A try-catch block is placed within the run() method of every single task that is submitted to a ScheduledThreadPoolExecutor in case an unexpected exception gets thrown and results in the hanging of the entire executor.
To elaborate:
Below is the run() overriden in the ScheduledFutureTask:
public void run() {
boolean periodic = isPeriodic();
if (! canRunInCurrentRunState(periodic ))
cancel(false );
else if ( !periodic )
ScheduledFutureTask .super.run() ;
else if ( ScheduledFutureTask. super. runAndReset()) {
setNextRunTime();
reExecutePeriodic(outerTask );
}
}
void reExecutePeriodic(RunnableScheduledFuture<?> task ) {
if (canRunInCurrentRunState(true)) {
super. getQueue() .add (task );
if (! canRunInCurrentRunState(true ) && remove( task))
task. cancel( false);
else
ensurePrestart();
}
}
An exception that gets thrown at super.runAndRest() makes it fail to re-add the task back to the execution-queue, therefore in ThreadPoolExecutor.runWorker(), the current thread wil never be able to retrieve another task in next run of the while loop. (See below) and it will hang at getTask().
In java.concurrent.ThreadPoolExecutor
final void runWorker( Worker w) {
Thread wt = Thread. currentThread();
Runnable task = w. firstTask;
w. firstTask = null;
w. unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
w. lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get() , STOP) ||
(Thread .interrupted() &&
runStateAtLeast(ctl.get (), STOP ))) &&
!wt .isInterrupted())
wt. interrupt();
try {
beforeExecute(wt, task) ;
Throwable thrown = null;
try {
task. run() ;
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task , thrown) ;
}
} finally {
task = null;
w. completedTasks++;
w. unlock();
}
}
completedAbruptly = false ;
} finally {
processWorkerExit(w , completedAbruptly);
}
}
