一, 使用conditionQueue需要注意的一些点:
一个conditionQueue被多种Predicate condition 使用是很正常的,所以当一个wait的线程被唤醒的时候,很有可能它监听的条件并没有成功,所以在wait醒后,最好再次进行条件的判断.
一定要在wait前就对条件进行判断,因为如果一个条件已经是true了,然后再进行wait,很可能会导致Missed Signals,
调用notify()比较危险,因为存在一个conditionQueue被多种Predicate condition 使用的情况,所以使用notify()可能会导致Missed Signals同样的问题,所以最好使用notifyAll() 这种的问题是,每次都notifyAll()可能会有过多的线程切换,会耗费性能
For all these reasons, when you wake up from wait you must test the condition predicate again, and go back to waiting
(or fail) if it is not yet true. Since you can wake up repeatedly without your condition predicate being true, you must
therefore always call wait from within a loop, testing the condition predicate in each iteration. The canonical form for a
condition wait is shown in Listing 14.7.
Listing 14.7. Canonical Form for Statedependent Methods.
void stateDependentMethod() throws InterruptedException {
// condition predicate must be guarded by lock
synchronized(lock) {
while (!conditionPredicate())
lock.wait();
// object is now in desired state
}
}
When using condition waits ( Object.wait or Condition.await ):
• Always have a condition predicatesome test of object state that must hold before proceeding;
• Always test the condition predicate before calling wait , and again after returning from wait ;
• Always call wait in a loop;
• Ensure that the state variables making up the condition predicate are guarded by the lock associated with the
condition queue;
• Hold the lock associated with the the condition queue when calling wait , notify , or notifyAll ; and
• Do not release the lock after checking the condition predicate but before acting on it.
Single notify can be used instead of notifyAll only when both of the following conditions hold:
• Uniform waiters. Only one condition predicate is associated with the condition queue, and each thread executes the
same logic upon returning from wait ; and
• One‐in, one‐out. A notification on the condition variable enables at most one thread to proceed.