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Lesson 6 - Multithreading in Java - Semaphore

In the previous lesson, Multithreading in Java - Useful components for parallelism, we summarized other available techniques for parallel programming. Today, we're going to show the first of the advanced classes for parallel programming - the semaphore.

Critical Section

Before describing individual synchronization primitives, we'll explain what exactly a critical section is. In the previous lessons, we mentioned it briefly, but didn't explain it in detail. Let's fix that.

A critical section is where concurrency occurs, i.e. where common data is being accessed.

Critical section

Our goal is to ensure that only one process/thread works with the critical section at a time.

Each critical section refers to specific data being accessed in it (x, y, z, s1, ...). We try to not lock everything but only specific critical places where the data is shared.

We've already introduced one synchronization primitive: monitor. Monitor support must be implemented directly in the programming language. In Java, the monitor is implemented using the synchronized keyword. Now let's move on to other primitives.

The Semaphore in C

To understand the principles of the semaphore, we'll show a short abstract implementation in the C language. Dijkstra (1962/1963) designed a primitive that simplifies


 

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In this tutorial, we'll introduce advanced techniques for parallel programming in Java. We'll describe the semaphore.

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