Java Threads, 3rd Edition

Chapter 5. Minimal Synchronization Techniques

In the previous two chapters, we discussed ways of making objects threadsafe, allowing them to be used by two or more threads at the same time. Thread safety is the most important aspect of good thread programming; race conditions are extremely difficult to reproduce and fix.

In this chapter, we complete our discussion of data synchronization and thread safety by examining two related topics. We begin with a discussion of the Java memory model, which defines how variables are actually accessed by threads. This model has some surprising ramifications; one of the issues that we’ll clear up from our previous chapters is just what it means for a thread to be modeled as a list of instructions. After explaining the memory model, we discuss how volatile variables fit into it and why they can be used safely among multiple threads. This topic is all about avoiding synchronization.

We then examine another approach to data synchronization: the use of atomic classes. This set of classes, introduced in J2SE 5.0, allows certain operations on certain types of data to be defined atomically. These classes provide a nice data abstraction for the operations while preventing the race conditions that would otherwise be associated with the operation. These classes are also interesting because they take a different approach to synchronization: rather than explicitly synchronizing access to the data, they use an approach that allows race conditions to occur but ...