Question

is it safe for the thread from start_thread, to access packetList while the start_activity thread(main thread) is sleeping on a semaphore. If not then how can I share the resource between them, please note the access will only be read and the packetList wont be changed.

            class A{
                Vector<packet> packetList;
                A()
                {
                    //packetList initialized here
                }

                void start_thread()
                {
                //start a thread here
                }
                void start_activity()
                {
                    while(true)
                    {
                        //possibly sleep on semephore
                        //do work
                    }
                }
            }

Answer 1

You can access the Vector, provided you don’t iterate over it. If you Iterate over it, you need to hold a lock with synchronized on that object.

If you want to queue “packets” of work, I suggest you look at using a BlockingQueue where you can wait for new tasks to be added, or use an ExecutorService which combines a Queue with a ThreadPool.