What is Semaphore and its usage ?

Semaphores 

• used to control number of activities
• Number of permits are passed to Semaphore constructor.
• Activities can use acquire() or release() method to acquire / release the permit.
• If no permit is available, acquire blocks until any one permit is available.

Use case 1. Resource pool - like, Connection pool

• Initialize a Semaphore to the pool size
• acquire a permit before trying to fetch a resource from the pool
• release the permit after putting a resource back in the pool
• acquire blocks until the pool becomes nonempty

Use case 2. Bounded + blocking collection

• Initialize a Semaphore to max size of collection
• add acquires the permit before adding item
• remove releases permit so that more items can be added