Basically you enforce static heap allocation. Meaning you cannot type new in a method only in the class definition. I don't even know if this can be enforced from the bytecode VM... just playing with it in my head.
That would violate the Java specification, i.e. it's not Java. On the other hand, what you could do is only allow allocations to succeed in class initialisation (Java's "static") and throw an error if done outside it. Note, however, that even adding an element to a Map outside of initialisers would fail when using OpenJDK's standard library even if both key and value have been preallocated in initialisers, as it may allocate an internal node and/or a new hash array. So you may want to change some of the standard library to make it more useful with your restrictions.
But you may want to ask yourself why you want to do that. OpenJDK's GCs have become really, really good in both throughput and latency, and the main thing you pay in exchange is memory footprint.
So the only place you really define what objects get created is in the class containing the main method? Seems fairly limited that you’d have to know upfront what your memory needs are? An app would have to allocate a large block of memory in the main class, and then handle memory management itself from this piece of memory?
That would remove a lot of the benefits of using Java, might as well use something like c. Garbage collection has been highly optimized in the jvm, and most would consider it a benefit of a jvm compared to managing memory yourself.
I’ve never understood it either. I’ve heard people doing similar things with Java in the low-latency trading area.
I mean Java isn’t the worst tool out there, but even C# outclasses it as a language. The biggest plus for Java is the massive, mature ecosystem which I imagine mostly evaporates when you’re only using a self restrictive subset of the language itself.
It doesn’t evaporate, more often than not only a small subset of these trading programs need that strict “no heap allocation” policy. The rest of the program is free to take advantage of the huge ecosystem.
