Use primitive objects once JDK supports

[JamesChenX]

Motivation

The primitive object supported by the Valhalla project will be introduced in JDK17 as a preview feature, and will significantly improve the memory layout of value-based classes, reduce 50% heap memory at least (we will try to reduce 80% heap memory after optimization), and make fewer Objects (the new name IdentityObject) with faster GC.

We will add "--enable-preview" to support primitive objects by default if we have enough unit/integration tests to ensure our projects can work as expected.

btw: The boxing and unboxing in Java should be considered as a design flaw, which will be refactored to the implementation primitive reference and value conversion in JEP 401.

Notes:

• JEP draft: Primitive Objects: https://openjdk.java.net/jeps/8251554
• EAP: https://builds.shipilev.net/openjdk-jdk-valhalla/
• Docs: https://github.com/openjdk/valhalla-docs/blob/main/site/design-notes/state-of-valhalla/03-vm-model.md
• We can track its process in https://bugs.openjdk.java.net/issues/?jql=project+%3D+JDK+AND+fixVersion+%3D+repo-valhalla
• Changes to the Java® Virtual Machine Specification: http://cr.openjdk.java.net/~dlsmith/jep401/jep401-20210702/specs/primitive-objects-jvms.html
• Benchmark: https://gist.github.com/demkom58/b743a70f49b0727b7334826e06e314c1
• Learning the exact behaviors from their tests: https://github.com/openjdk/valhalla/tree/227611116c706c13d3f367dd51e9fe231b37aad7/test/langtools/tools/javac/valhalla/lworld-values

Checklist:

• Use IdentityHashMap for Map<? extends PrimitiveObject, ?>
• Remove some object pools
• Adapt serialization logic because .getClass() will return different classes for the type like int and Integer
• new Object() -> Objects.newIdentity()
• Remove Eclipse Collections
• Convert most of records that we use in Turms to primitive record (In fact, I think we can convert 95% turms-defined class for models to primitive class)
• Replace most wrapper classes to primitive value types if they are non-nullable. Currently, we use wrapper class even for non-nullable primitives currently because even if we use primitives, they will be converted to wrappers somewhere, e.g. collections. If we only use primitives, it means that 1. We cannot use Java collections but collections in hppc, jctools, etc; 2. We need to refactor our deps because they use Java collections. Because of these reasons, it's impractical to use only primitives in all places. So what we need to do is just waiting for Universal Generics

[JamesChenX]

If we are lucky, we can take 1\~2 days to adapt our codes (we are very familiar with the usage of primitive class) to primitive classes, specifically Primitive Objects + Unify the Basic Primitives with Objects (btw, I have waited for the primitive object for years, if there is no this project, I may not write Turms in Java). And takes 2~3 days to test it to ensure it won't break the code of our dependencies. And we will just test the system by mainly sending 1 million client requests to check if any bug caused by primitive class occurs.

If it really breaks our deps, we will just try to not use the deps with primitive objects. But the good news is: A lot of important components (like requests/responses of RPC, entities serialization for MongoDB, etc) are written by us, meaning we can control and adapt them to primitive objects.

Note that we don't need to expect Universal Generics(https://openjdk.java.net/jeps/8261529) because I guess it will be published as a preview in JDK 19\~21. When it really comes, we will just adapt our code for it again, which is just a piece of cake.

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We will adapt our code for Valhalla once it publishes EAP builds so that we can develop and test based on the EAP version for months before it is merged into the baseline of JDK (just like what we had done to JDK 17 EAP). We will only commit our changes when the JDK supports primitive objects publishes.

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After playing with Valhalla EAP, I find that they even haven't delivered anything about JEP 402 currently (9/25) though they have taken 8 years for Valhalla project since 2014, so we hold on. btw, we test whether JEP 402 is available by:

Set<Integer> set = new HashSet<>();
set.add(new Integer(999999999));
set.add(new Integer(999999999));
System.out.println(set.size()); // should be 1 if JEP 402 is available