Cancellation and Shutdown - Java Concurrency in Practice - page 158

Listing 7.2. Generating a Second's Worth of Prime Numbers.
PrimeGenerator uses a simple cancellation policy: client code requests cancellation by
calling cancel , PrimeGenerator checks for cancellation once per prime found and
exits when it detects cancellation has been requested.
7.1.1. Interruption
The cancellation mechanism in PrimeGenerator will eventually cause the primeseeking
task to exit, but it might take a while. If, however, a task that uses this approach calls a block-
ing method such as BlockingQueue.put , we could have a more serious problem—the
task might never check the cancellation flag and therefore might never terminate.
BrokenPrimeProducer in Listing 7.3 illustrates this problem. The producer thread gen-
erates primes and places them on a blocking queue. If the producer gets ahead of the con-
sumer, the queue will fill up and put will block. What happens if the consumer tries to cancel
the producer task while it is blocked in put ? It can call cancel which will set the can-
celled flag—but the producer will never check the flag because it will never emerge from
the blocking put (because the consumer has stopped retrieving primes from the queue).
As we hinted in Chapter 5 , certain blocking library methods support interruption . Thread in-
terruption is a cooperative mechanism for a thread to signal another thread that it should, at
its convenience and if it feels like it, stop what it is doing and do something else.