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Joe Darcy on JDK Compatibility Regions

Posted by editor on August 18, 2009 at 5:38 AM PDT

Joe Darcy has posted an interesting analysis titled JDK Release Types and Compatibility Regions, in which he uses a three-dimensional graph to represent different "axes" of compatibility between different JDK versions. In fact, this compatibility analysis could realistically be applied to many different software platforms that undergo sequential modification over time, but Joe talks specifically about how the JDK evolves in his post. Here's Joe's introduction to the concept:

There are three primary kinds of compatibility of concern when evolving the JDK, source, binary, and behavioral. These can be visualized as defining a three dimensional space.

The axes of this three-dimensional space are Behavioral, Source, and Binary, in Joe's representation. The farther away a point is from the origin, the more incompatible a change is; the origin itself represents perfect compatibility (no changes).

Joe specifically looks at the three main types of JDK release (platform, maintenance, update) from this point of view:

JDK 7 is a platform release since it is a new version of platform... The JDK 6 update releases are representative of update releases... While maintenance releases have not been formally issued since JDK 1.4.2, the changes in 6u10 were more on par with a maintenance release rather than a regular update release.

For update and maintenance releases, greater compatibility with the current platform release (for example, JDK 6) is required. Here, binary compatibility is required, as defined in the Java Language Specification.

Since binary incompatible changes are not allowed, the acceptable compatibility region for update and maintenance releases is confined to the (Behavioral x Source) plane, with more latitude on the behavioral axis.

Meanwhile:

The compatibility reference point for a platform release is an implementation of the previous platform specification. Compared to the previous platform specification, a platform release can add APIs and language feature that impact source compatibility (new keywords, etc.) and the implementation can have many changes in behavior (such as changing the iteration order of HashMap). In exceptional circumstances, there is the possibility of a sliver of binary incompatibility, such as to address a security issues in a rarely-used corner of the platform, but the central policy of preserving binary compatibility holds for platform releases as well.

All in all, Joe's post provides an interesting take on analyzing software platform life cycles. It's certainly not something I've ever thought about before, in my decades of working with evolving platforms.


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Joe Darcy has posted an interesting analysis titled "JDK Release Types and Compatibility Regions"...