Semaphores have a few differences from locks. First, the constructor requires the specification of the number of permits to be granted. There are also methods that return the number of total and free permits. This class implements only a grant and release algorithm; unlike the Lock interface, no attached condition variables are available with semaphores. There is no concept of nesting; multiple acquisitions by the same thread acquire multiple permits from the semaphore.
If a semaphore is constructed with its fair flag set to true, the semaphore tries to allocate the permits in the order that the requests are made--as close to first-come-first-serve as possible. The downside to this option is speed: it takes more time for the virtual machine to order the acquisition of the permits than to allow an arbitrary thread to acquire a permit.
Of all the different types of thread synchronization tools, the barrier is probably the easiest to understand and the least used. When we think of synchronization, our first thought is of a group of threads executing part of an overall task followed by a point at which they must synchronize their results. The barrier is simply a waiting point where all the threads can sync up either to merge results or to safely move on to the next part of the task. This is generally used when an application operates in phases. For example, many compilers make multiple passes between loading the source and generating the executable, with many interim files. A barrier, when used in this regard, can make sure that all of the threads are in the same phase.
