Java enumerations should *cache* the field values

[jonpryor]

Consider the Thread.State enum:

/* partial */ class Thread {
 public static final /* partial */ enum State {
 NEW, RUNNABLE, BLOCKED, WAITING, TIMED_WAITING, TERMINATED
 }
}

This "actually" is a class with fields for each enum value:

% javap java.lang.Thread.State
Compiled from "Thread.java"
public final class java.lang.Thread$State extends java.lang.Enum<java.lang.Thread$State> {
 public static final java.lang.Thread$State NEW;
 public static final java.lang.Thread$State RUNNABLE;
 public static final java.lang.Thread$State BLOCKED;
 public static final java.lang.Thread$State WAITING;
 public static final java.lang.Thread$State TIMED_WAITING;
 public static final java.lang.Thread$State TERMINATED;
 public static java.lang.Thread$State[] values();
 public static java.lang.Thread$State valueOf(java.lang.String);
 static {};
}

When we bind it, we bind the fields as properties:

https://github.com/dotnet/java-interop/blob/fcad3368815dffd0f38f64384aa21b0b65367d68/src/Java.Base-ref.cs#L5490-L5506

However, each of those properties involves a ValueManager lookup:

namespace Java.Lang {
	public partial class Thread {
		public sealed partial class State : global::Java.Lang.Enum {
			public static global::Java.Lang.Thread.State? Runnable {
				get {
					const string __id = "RUNNABLE.Ljava/lang/Thread$State;";

					var __v = _members.StaticFields.GetObjectValue (__id);
					return global::Java.Interop.JniEnvironment.Runtime.ValueManager.GetValue<global::Java.Lang.Thread.State? >(ref __v, JniObjectReferenceOptions.Copy);
				}
			}
		}
	}
}

This is JavaInterop1, not XAJavaInterop1, but .NET for Android isn't much different:

			public static Java.Lang.Thread.State? Runnable {
				get {
					const string __id = "RUNNABLE.Ljava/lang/Thread$State;";

					var __v = _members.StaticFields.GetObjectValue (__id);
					return global::Java.Lang.Object.GetObject<Java.Lang.Thread.State> (__v.Handle, JniHandleOwnership.TransferLocalRef);
				}
			}

The problem is that value lookup is not fast -- identity hash code needs to be obtained, locks obtained, etc. -- to the point that repeated enum lookups can actually show up in profiles.

TODO: @jonathanpeppers, please provide your profile data. :-D

Suggestion: could we update property generation for all final fields to cache the return value? This means we'd only need to call StaticFields.GetObjectValue() and "GetValue" once, instead of once per-access:

			global::Java.Lang.Thread.State? _Runnable_cache;
			public static global::Java.Lang.Thread.State? Runnable {
				get {
					if (_Runnable_cache != null) return _Runnable_cache;
					const string __id = "RUNNABLE.Ljava/lang/Thread$State;";

					var __v = _members.StaticFields.GetObjectValue (__id);
					return _Runnable_cache = global::Java.Interop.JniEnvironment.Runtime.ValueManager.GetValue<global::Java.Lang.Thread.State? >(ref __v, JniObjectReferenceOptions.Copy);
				}
			}

[jonpryor]

Updated description to mention that we should do caching for all final fields, not just fields of enum types. That's what final is for, after all, right?

[jonathanpeppers]

Here is an example trace:

• enum-traces.zip
• https://github.com/chabiss/periodictable (app)

Search for either Android.Graphics.Paint.Cap.get_Butt() or Android.Graphics.Paint.Join.get_Miter() as an example.

I'm doing this for now, which would also help .NET 8:

• [android] cache Join and Cap enum values maui#24248