Synchronization and concurrency in Java 5

The words synchronization and concurrency are overlapping and sometimes synonymous terms. The word synchronization generally means sharing data between multiple processors or threads, while concurrency refers to a measure of– or the art of improving– how effectively an application allows multiple jobs required by that application (e.g. serving web page requests from a web server) to run simultaneously.

In the past, synchronization and concurrency have been key concerns especially for "server" applications such as web servers that must deal with a continual stream of simultaneous requests, and which typically have more than one physical processor to deal with these conditions. But increasingly, they are key issues for much more mundane applications and more and more programmers will have to come to terms with them over the coming years. This is a result of the multicore era that we are now in: there is a trend for all computers– including humble desktops and games consoles–to be multiprocessor machines. Whereas in the past increased computing power came each year in the form of increased clock speeds (albeit with other architectural improvements), from now on such increases are likely to come in the form of increased multiprocessor capability. Programmers who don't get to grips with multiprocessor programming won't be able to take advantage of the increased capability of new machines.

Now the good news: Java is generally a good choice of language for multiprocessor applications, both because of in-built concepts fundamental to the language and because of a relatively rich concurrency library provided from Java version 5 onwards. On the following pages, we examine various issues of concurrent programming in Java:

Synchronization, concurrency and multitasking: why is it difficult?: In this section, we examine some basic issues of concurrent programming such as sharing resources between CPUs and instruction re-ordering to make best use of available parallel resources.
Synchronization and concurrency before Java 5: In this section, we review the "traditional" means of synchronizing data and improving concurrency in Java 5.
Problems with the Java 1.4 synchronization model: A discussion of some of the limitations which Java 5 attempts to resolve.
Synchronization under the hood: How synchronization actually works at the processor level, as a precursor to understanding how Java 5 improves synchronization and concurrency.
The Atomic classes in Java 5: An overview of classes such as AtomicInteger and AtomicLong, which effectively expose CAS instructions to the Java programmer.
ConcurrentHashMap: Arguably the most important of Java 5's concurrent collection classes. We examine some of the performance characteristics of ConcurrentHashMap, and example usage. Other concurrent collection classes are also examined.
Explicit locks: For cases where synchronized is not adequate, we examine the explicit lock classes provided in Java 5.
Semaphores: Using the Semaphore class, a construct designed to help in the creation of bounded resource pools such as database connection pools.