Inter-thread Communication

What is Polling and what are problems with it? The process of testing a condition repeatedly till it becomes true is known as polling.

Polling is usually implemented with the help of loops to check whether a particular condition is true or not. If it is true, certain action is taken. This waste many CPU cycles and makes the implementation inefficient. For example, in a classic queuing problem where one thread is producing data and other is consuming it.

How Java multi threading tackles this problem? To avoid polling, Java uses three methods, namely,wait(), notify() and notifyAll(). All these methods belong to object class as final so that all classes have them. They must be used within a synchronized block only.

• wait() - It tells the calling thread to give up the lock and go to sleep until some other thread enters the same monitor and calls notify().

• notify() - It wakes up one single thread that called wait() on the same object. It should be noted that calling notify() does not actually give up a lock on a resource.

• notifyAll() - It wakes up all the threads that called wait() on the same object.