Sunday, August 2, 2009

lockfree atomic variables -- JDK 1.5

(see article at http://www.ibm.com/developerworks/java/library/j-jtp11234/)

Until JDK 5.0, it was not possible to write wait-free, lock-free algorithms in the Java language without using native code. With the addition of the atomic variables classes in the java.util.concurrent.atomic package, that has changed. The atomic variable classes all expose a compare-and-set primitive (similar to compare-and-swap), which is implemented using the fastest native construct available on the platform.

The atomic variable classes can be thought of as a generalization of volatile variables, extending the concept of volatile variables to support atomic conditional compare-and-set update

Nearly all the classes in the java.util.concurrent package use atomic variables instead of synchronization, either directly or indirectly.

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http://java.sun.com/j2se/1.5.0/docs/guide/concurrency/overview.html has unusual comments
Atomic Variables - Classes for atomically manipulating single[1] variables (primitive types or references), providing high-performance atomic arithmetic and compare-and-set[2] methods. The atomic variable implementations offer higher[3] performance than would be available by using synchronization (on most platforms), making them useful for implementing high-performance concurrent algorithms as well as conveniently implementing counters and sequence number generators[4].
[1] atomic variables are quite low-level and only cover monolithic variables. No Object here. Object is different from reference (monolithic).
[2] CAS is the biggest feature
[3] synch performance hit is actually a real concern for java designers. I think synch severely limits multi-processor parallelism
[4] these simple integers are a key use case for atomic? You can see Atomic variables aren't for everyday use.

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