Question about synchronize() statement and thread scheduling?

Question

I have threads accessing a shared LinkedList in the main thread. I made use of the Collections.synchronizedList() to avoid concurrent modifications. one of the threads is a timer thread and it has a priority of 5 and the other threads are message reception threads and i made them of a lower priority just to allow the timer thread to be executed on time. According to the javaAPI, all iterations on the synchronized list should be done using synchronized(synchronizedListRef){} and this is what i do,
The problem is even after setting the priority of the reception threads lower than the timer thread, the reception threads are sometimes executed more than once and the timer thread is delayed. Lets assume the situation where the timer reaches the point where it has to acquire the lock (the synchronized statement) and it discovers that the lock is already taken and it cant access the list because the other thread (of a lower priority) is iterating over it. shouldnt the higherpriority thread be executed after that the reception thread (that is blocking the timer thread) finishes its execution and release the lock (even if there are other reception thread waiting)??
does the priority play a role when a thread is blocked?

Answer 1

Asady wrote:
I have threads accessing a shared LinkedList in the main thread. I made use of the Collections.synchronizedList() to avoid concurrent modifications. Make sure you explicitly sync around all iterations. Just using Collections.sync is not enough.
The problem is even after setting the priority of the reception threads lower than the timer thread, the reception threads are sometimes executed more than once and the timer thread is delayed.There's no guarantee as to how priority will be interpreted. The scheduler implementation is free to do pretty much anything.
As for how to solve your problem, you might want to look at java.util.concurrent.*, e.g. PriorityQueue. If the classes there don't give you just what you need, you may be able to build on them, or at least get ideas from them. Bottom line is that if you want a specific scheduling algorithm, you'll have to implement it yourself, since Threads' priority semantics are only very loosely specified by the JLS.