...
This noncompliant code example violates the liveness property. A lock is held obtained using a raw Object lock and three threads are started. Two condition predicates are used. One checks whether the buffer has zero elements and the other checks if the buffer is full with ten elements (buffer is not shown for brevity, only the count of the number of elements in the buffer at any time is shown). Initially the buffer is neither full nor empty. Conditions are created so that the buffer becomes empty and thread 1 goes into wait state, followed by thread 2, when the buffer becomes full.
...
Note that when thread 2 goes into the wait state, the condition predicate of thread 1 becomes false. When notify() is invoked by thread 3, it can be delivered to either thread 1 or thread 2 depending on the particular Java Virtual Machine (JVM). If thread 1 is chosen to be notified, its condition turns out to be false, which terminates it. This is the required functionality, that is, any thread whose condition predicate is false must be terminated. However, if the notification is delivered to thread 2, it has no effect because its condition predicate is still true, and consequently, it goes into the wait state once again. Consequently, thread Thread 1 continues to wait despite its condition predicate being false and is not terminated in this case.
...
This compliant solution uses the notifyAll() method which sends notifications to all threads that wait on the same lock object. As a result, liveness is not affected unlike the noncompliant code example. The condition predicate controls which threads can resume their operations. Ensure that the lock is released promptly after the call to notifyAll(). This is not a requirement in this particular example because it is a producer-consumer process.
| Code Block | ||
|---|---|---|
| ||
else if(number == 3) {
list.notifyAll();
}
|
...