Scheduler must use AtomicBoolean instead of volatile boolean for job-is-running
check
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Key: SLING-1571
URL: https://issues.apache.org/jira/browse/SLING-1571
Project: Sling
Issue Type: Bug
Components: Commons
Affects Versions: Commons Scheduler 2.2.0
Reporter: Alexander Klimetschek
QuartzJobExecutor implements a check whether a job is already running if a job
is not allowed to run concurrently:
if (!canRunConcurrently) {
if ( handler.isRunning ) {
return;
}
handler.isRunning = true;
}
handler.isRunning is declared as "public volatile boolean".
This will fail if isRunning is false and two jobs start at exactly the same
time - both will see a value of false and start. The problem is that "volatile"
doesn't make the two commands "isRunning + set to true" atomic. It just makes
sure that updates can be read immediately by all threads, ie. it would only
help to see a _change_ of the variable, done by another thread, more quickly.
It doesn't help in this situation. See also [1] (the first reply).
Fortunately Java 5 comes with java.util.concurrent.atomic.AtomicBoolean, which
has an atomic operation compareAndSet [1]. Code would probably look like this:
public AtomicBoolean isRunning;
....
if (!canRunConcurrently) {
if ( !handler.isRunning.compareAndSet(false, true) ) {
return;
}
}
This is probably not so critical, since I guess it's very unlikely that the
scheduler triggers a job execution twice at the same time in the first place,
but you'll never know.
[1]
http://www.coderanch.com/t/233792/threads/java/Volatile-boolean-versus-AtomicBoolean
[2]
http://java.sun.com/j2se/1.5.0/docs/api/java/util/concurrent/atomic/AtomicBoolean.html#compareAndSet(boolean,%20boolean)
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