Java does memory recovery strictly by garbage collection. To delete an object remove all references to that object and wait.
A "delete now" operation imposes requirements on implementations that are not strictly required.
Any advantages gained from destructors can be realized via explicit cleanup methods and (as a hedge against mis-use) state checking for initialized and destroyed objects. A delete operation also adds risk that an object that has been deleted may be referenced again (although with esoteric stuff like PhantomReferences you can almost do it).
It's far from impossible. I would even say it's extremely trivial to add a C++ style delete keyword but then you'd lose memory safety because of the potential use after free bugs and it wouldn't even increase performance because the problem with garbage collectors is the stop the world pause, lack of off heap allocations for very large data and the needed value types to fully take advantage of that feature.
If you want manual memory allocation then you should instead look at sun.misc.Unsafe.
public native long allocateMemory(long bytes);
public native void freeMemory(long address);
Of course you still have to calculate the field offsets and all that stuff manually because it's not a part of the java language but you only asked about the JVM.
There are better ways to eliminate GC pauses than manual memory management (even ignoring the relatively new "pauseless" GCs). See the new RTSJ (realtime Java) specification here: https://www.aicas.com/cms/en/rtsj. Not that it's designed for far stricter requirements than games (cases where even microseconds of unexpected latency may result in actual life safety concerns).
Manual memory management is a good choice if you have both worst-case latency concerns as well as very restricted RAM and/or severe energy constraints (basically, the kind of applications that Rust was designed to handle).
I'm not a game developer, so why is that horrible?
consider I have an array/map of my game objects and the rest only works with methods, so most stuff does not lie on the heap, well that means basically no strings, since they would add to heap memory aswell, but only using basic datatypes and arrays.
why would some kind of that be problematic?
I mean graphics programmic is kind of new to me, but considering that you could offload that to a c/c++ engine via jni (that's another horrible thing of some kind), but just the game code itself doesn't seems to be to bad, does it?
> why would some kind of that be problematic? I mean graphics programmic is kind of new to me, but considering that you could offload that to a c/c++ engine via jni (that's another horrible thing of some kind), but just the game code itself doesn't seems to be to bad, does it?
The whole point is not to offload anything. If you need to offload anything, even the render loop, the language has failed.
You could also check out Eclipse OpenJ9's -Xgcpolicy:metronome option. It only works on Linux X86-64 at the moment, but it is designed to better regulate GC pauses. If you want to learn more, you can open an issue to ask for more details at https://github.com/eclipse/openj9/issues .
Even in game development in C/C++, you wouldn't want to be allocating/deleting heap objects in every frame. You'd probably end up pooling there too, unless you're referring to the problem that the JVM has no value types and forces heap allocation for all objects. This will be fixed in JDK 10 (see Project Valhalla).
That doesn't solve the fundamental problem of stop the world pauses. If you had 10 threads sharing the same gc heap that allocate as much garbage as they want and 1 thread that has it's own heap that doesn't allocate anything only the 10 threads would be stopped by the garbage collector and you would have reached your goal.
If those 11 threads would share the same heap and 1 thread still doesn't use the gc because it's using manual memory management then you'd still suffer from stop the world pauses.
What you want is isolated heaps like erlang does, not manual memory management. I'm still wondering why we have no other programming languages with multiple heaps. You could probably achieve something similar with D and use of memory mapped files to efficiently share memory without copying bewteen processes. Alternatively you could call into C to spawn OS threads that don't suffer from stop the world pauses. But those two options are a hack. You're not supposed to use D like that.
I chose D as an example because it's a programming language with both a garbage collector and manual memory management that still suffers from stop the world pauses.
A "delete now" operation imposes requirements on implementations that are not strictly required.
Any advantages gained from destructors can be realized via explicit cleanup methods and (as a hedge against mis-use) state checking for initialized and destroyed objects. A delete operation also adds risk that an object that has been deleted may be referenced again (although with esoteric stuff like PhantomReferences you can almost do it).