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Notes on the Java SE Platform, the OpenJDK Project, and the JCP

Posted by editor on March 23, 2011 at 11:04 AM PDT

A few weeks ago I attended a Java-centric OTN Developer Day in Boston, MA (US). As I said in my initial blog about this, I found the sessions very interesting and took lots of notes.

I hadn't really thought much about the relationships between Java SE, the OpenJDK project, and the JCP in the past. Or, perhaps it's more correct to say that I had some vague notions, but had never taken the time to think about what the actual relationships might be.

Danny Coward, at the start of his presentation ("Java SE: The Road Forward, 2011 Edition"), clarified the situation, and showed that there really is a distinct separation between Java SE, OpenJDK, and the JCP. Here's what he said.

Java SE is a software platform. It consists of a set of documentation, prescribed implementations, Technology Compatibility Kits (TCK), etc. In other words, it's primarily a bunch of paper, all produced by the JCP. You can't use Java SE in itself, because what it really is is a description of a language platform; it exists prior to, or above, an actual language implementation.

Given this starting point, it becomes clear what the OpenJDK project is: it's an implementation of the Java SE platform -- specifically, Oracle's implementation (with recently growing participation from big companies like IBM, Apple, RedHat). So, if you look at OpenJDK, will you see a one-to-one correspondance between line items in the Java SE documentation and implementations within the OpenJDK? You'd certainly better! But, that's just the starting point for the OpenJDK project. Beyond implementing the specific requirements laid out in Java SE, the OpenJDK project includes many additional features, including tools, deployment capabilities, etc.

It's a bit like Linux distributions, where a single kernal is packaged in many different ways with many different added features, by different development teams. Except that Java SE would really be the description or definition of a kernel, not an actual downloadable software package.

An interesting point Danny made: the Java SE platform doesn't specify or require garbage collection. That's an addition that has been engineered in the various implementations. In other words, you could create a Java implementation that has no garbage collector and it could still be a valid Java SE implementation.

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-- Kevin Farnham

Twitter: @kevin_farnham