For over twenty years the Java SE Platform and the JDK have evolved in large, irregular, and somewhat unpredictable steps. Each feature release has been driven by one or a few significant features, and so the schedule of each release has been adjusted as needed — sometimes more than once! — in order to accommodate the development of those features.

This approach made it possible to deliver big new features at a high level of quality, after thorough review and testing by early adopters. The downside, however, was that smaller API, language, and JVM features could only be delivered when the big features were ready.

This was an acceptable tradeoff in the decades before and after the turn of the century, when Java competed with just a few platforms which evolved at a similar stately pace. Nowadays, however, Java competes with many platforms which evolve at a more rapid pace.

For Java to remain competitive it must not just continue to move forward — it must move forward faster.

Back on the train Five years ago I mused in this space on the tension between developers, who prefer rapid innovation, and enterprises, which prefer stability, and the fact that everyone prefers regular and predictable releases.

To address these differing desires I suggested, back then, that we switch from the historical feature-driven release model to a time-driven “train” model, with a feature release every two years. In this type of model the development process is a continuous pipeline of innovation that’s only loosely coupled to the actual release process, which itself has a constant cadence. Any particular feature, large or small, is merged only when it’s nearly finished. If a feature misses the current train then that’s unfortunate but it’s not the end of the world, since the next train will already be waiting and will also leave on schedule.

The two-year train model was appealing in theory, but proved unworkable in practice. We took an additional eight months for Java 8 in order to address critical security issues and finish Project Lambda, which was preferable to delaying Lambda by two years. We initially planned Java 9 as a two-and-a-half year release in order to include Project Jigsaw, which was preferable to delaying Jigsaw by an additional eighteen months, yet in the end we wound up taking an additional year and so Java 9 will ship this month, three and a half years after Java 8.

A two-year release cadence is, in retrospect, simply too slow. To achieve a constant cadence we must ship feature releases at a more rapid rate. Deferring a feature from one release to the next should be a tactical decision with minor inconveniences rather than a strategic decision with major consequences.

So, let’s ship a feature release every six months.

That’s fast enough to minimize the pain of waiting for the next train, yet slow enough that we can still deliver each release at a high level of quality.

Proposal Taking inspiration from the release models used by other platforms and by various operating-system distributions, I propose that after Java 9 we adopt a strict, time-based model with a new feature release every six months, update releases every quarter, and a long-term support release every three years.

• Feature releases can contain any type of feature, including not just new and improved APIs but also language and JVM features. New features will be merged only when they’re nearly finished, so that the release currently in development is feature-complete at all times. Feature releases will ship in March and September of each year, starting in March of 2018.

• Update releases will be strictly limited to fixes of security issues, regressions, and bugs in newer features. Each feature release will receive two updates before the next feature release. Update releases will ship quarterly in January, April, July, and October, as they do today.

• Every three years, starting in September of 2018, the feature release will be a long-term support release. Updates for these releases will be available for at least three years and quite possibly longer, depending upon your vendor.

In this model the overall rate of change should be about the same as it is today; what’s different is that there will be many more opportunities to deliver innovation. The six-month feature releases will be smaller than the multi-year feature releases of the past, and therefore easier to adopt. Six-month feature releases will also reduce the pressure to backport new features to older releases, since the next feature release will never be more than six months away.

Developers who prefer rapid innovation, so that they can leverage new features in production as soon as possible, can use the most recent feature release or an update release thereof and move on to the next one when it ships. They can deliver an application in a Docker image, or other type of container package, along with the exact Java release on which the application will run. Since the application and the Java release can always be tested together, in a modern continuous-integration and continuous-deployment pipeline, it will be straightforward to move from one Java release to the next.

Enterprises that prefer stability, so that they can run multiple large applications on a single shared Java release, can instead use the current long-term support release. They can plan ahead to migrate from one long-term support release to the next, like clockwork, every three years.

To make it clear that these are time-based releases, and to make it easy to figure out the release date of any particular release, the version strings of feature releases will be of the form $YEAR.$MONTH. Thus next year’s March release will be 18.3, and the September long-term support release will be 18.9.

Implications This proposal will, if adopted, require major changes in how contributors in the OpenJDK Community produce the JDK itself; I’ve posted some initial thoughts as to how we might proceed there. It will be made easier if we can reduce the overhead of the Java Community Process, which governs the evolution of the Java SE Platform; my colleagues Brian Goetz and Georges Saab have already raised this topic with the JCP Executive Committee.

This proposal will, ultimately, affect every developer, user, and enterprise that relies upon Java. It will, if successful, help Java remain competitive — while maintaining its core values of compatibility, reliability, and thoughtful evolution — for many years to come.

Comments and questions are welcome, either on the OpenJDK general discussion list (please subscribe to that list in order to post to it) or on Twitter, with the hashtag #javatrain.