On the one hand Java is very strict about backwards compatibility (still reserving keywords like const and goto even though they're not used (anymore)), but on the other hand Java 9's module system broke just about every Java <=8 project I've ever seen.

Project Loom also breaks tons of code because of locking and other threaded side effects that weren't even a consideration before, a trade-off between coloured functions and risking old code breaking.

It seems to me that Java is very conservative when it comes to the language itself, but actually compiling and running that language seems less of a concern. In this case, the "specify a flag to make all non-annotated variables non-null" approach would only add a single glyph to the language (the ? for denoting nullability), which I would've expected based on the way Java has evolved over time.

> Java 9's module system broke just about every Java <=8 project I've ever seen.

As far as I understand, some IDE's and built automation tools did not rollout support for module system immediately, yes?

Mixing a Java >= 9 project with Maven libraries made for <= 8 is one hell of a headache.

It took a while for IDEs and tools to catch up, but I still encounter issues with old codebases to this day, using the latest and greatest tools.

The problem isn't necessarily that the compiler isn't called right, but rather that a lot of existing library code suddenly didn't work anymore. By compartmentalising code (a welcome improvement!), projects that weren't designed for this compartmentalisation need a lot of "basically turn off Java module restrictions" command line flags to the JVM.

> but on the other hand Java 9's module system broke just about every Java <=8 project I've ever seen

That’s not true at all. Besides the package name change with jakarta, all it did is just expose a couple, popular libraries that were depending on runtime internals. Most of this could be fixed by bumping said libraries version. And the whole point of the module system is that it will never happen again, due to stricter encapsulation.

> Project Loom

How would it break any code? You have to explicitly use virtual threads, normal threads are not touched at all and will work as they have always did.

And yeah, java has always been “conservative on the language front, state-of-the-art on the runtime”

JDK 9 was the impetus, but the real nail was, I think, 17 when a lot of the constraints imposed by the module system became required. Before that folks could pretty much continue as usual with little regard to the module system.

After 17, legacy systems needed to start adding runtime options to work correctly. And, as I recall, this was mostly constrained to the dynamic, reflective, and introspective properties. Generic jars and code can still run without the module system.

Unfortunately, a lot of the magic in modern Java is through those reflection aspects. So it made moving to 17+ an effort for many legacy code bases.

And it should be clear, in general, all of the code was fine. It was all runtime options setting up package permissions that was the problem. Doesn’t make it less frustrating.

(And I’m pretty sure 17 was the rubicon for this, but it may have been another version.)

> That’s not true at all. Besides the package name change with jakarta, all it did is just expose a couple, popular libraries that were depending on runtime internals. Most of this could be fixed by bumping said libraries version.

I don't remember the exact details, but I worked on a project that was stuck on java 8 for a long time because some critical dependencies broke in java 9, and it took a long time before they released versions that were compatible with java 9.

> How would it break any code? You have to explicitly use virtual threads, normal threads are not touched at all and will work as they have always did.

You often have no control over what kind of thread the caller of your code is using. Code written for the old threading model can and will end up being called by these newfangled virtual threads. If it does something the virtual threads don't like (which, as far as I have heard, includes things like some uses of synchronized blocks), you can have unexpected breakage.

You're confusing the thread pinning problem with "breakage". The problem with some synchronized blocks is that they lead to thread pinning for the duration of the lock. Why is thread pinning bad? Because it means that if one core is idle, but all the threads are pinned to a specific core, you won't get full CPU utilisation. This undermines the performance benefit of virtual threads, because they are supposed to be schedulable via work stealing.

That’s not backwards compatibility, but forwards compatibility, though. And you would really have to go out of your way to introduce such an issue. Certain extreme (e.g. native calls) cases are handled by simply pinning a virtual thread to a real thread. So it would only cause an issue if it was already causing an issue at much smaller thread numbers.

And many of these extreme cases are getting “solved” to not pin the thread.

Another problem with (especially older versions of) Java is that the language was so restrictive that many of the popular frameworks rely on reflection and/or bytecode manipulation and these things can and do break frequently across JDK or JRE updates.