Make it (mostly) work!

Commit 69c90ba expanded into a "Full(er)" sample, with just one issue:
it didn't fully work:

	Exception in thread "main" com.xamarin.java_interop.internal.JavaProxyThrowable: System.IO.FileNotFoundException: Could not resolve assembly 'Hello-NativeAOTFromJNI'.
	   at System.Reflection.TypeNameParser.ResolveAssembly(String) + 0x97
	   at System.Reflection.TypeNameParser.GetType(String, ReadOnlySpan`1, String) + 0x32
	   at System.Reflection.TypeNameParser.NamespaceTypeName.ResolveType(TypeNameParser&, String) + 0x17
	   at System.Reflection.TypeNameParser.GetType(String, Func`2, Func`4, Boolean, Boolean, Boolean, String) + 0x99
	   at Java.Interop.ManagedPeer.RegisterNativeMembers(IntPtr jnienv, IntPtr klass, IntPtr n_nativeClass, IntPtr n_assemblyQualifiedName, IntPtr n_methods) + 0x103
	        at com.xamarin.java_interop.ManagedPeer.registerNativeMembers(Native Method)
	        at example.ManagedType.<clinit>(ManagedType.java:15)
	        at com.microsoft.hello_from_jni.App.main(App.java:13)

The problem is that `ManagedPeer.RegisterNativeMembers()` calls
`Type.GetType("Example.ManagedType, Hello-NativeAOTFromJNI")`, which
throws `FileNotFoundException`.

Let's attempt to fix that:

Update `MangedPeer.RegisterNativeMembers()` to call the (new!) method:

	partial class JniRuntime {
	  partial class JniTypeManager {
	    public virtual void RegisterNativeMembers (
	        JniType nativeClass,
	        ReadOnlySpan<char> assmblyQualifiedTypeName,
	        ReadOnlySpan<char> methods);
	  }
	}

which allows a subclass to *avoid* the `Type.GetType()` call.

Add a `NativeAotTypeManager` class which subclasses
`JniRuntime.JniTypeManager`, overriding `RegisterNativeMembers()`
so as to avoid the `Type.GetType()` call.  (It also "fakes" its own
"typemap" implementation…)

Add `Hello-NativeAOTFromJNI.xml`, a Linker Descriptor, to preserve
the `JavaException` constructors, which are needed when things
break horrifically.

TODO: figure out the appropriate `DynamicDependencyAttribute`
incantations to replace `Hello-NativeAOTFromJNI.xml`.

Update the `_AddMarshalMethods` build task to *also* update
`$(IntermediateOutputPath)$(TargetFileName)`, as the copy in
`$(IntermediateOutputPath)` is used by the `IlcCompile` target.
*Not* updating the copy in `$(IntermediateOutputPath)` means that
we don't get *any* marshal methods, and things break.

Rename `ExpressionAssemblyBuilder.CreateRegistrationMethod()` to
Rename `ExpressionAssemblyBuilder.AddRegistrationMethod()`, so that
the `EmitConsoleWriteLine()` invocation can provide the *full*
type name of the `__RegisterNativeMembers()` method, which helps
when there is more than one such method running around…

Various changes to `JniRuntime.JniTypeManager.cs`, to increase
logging verbosity, and to make the optimization effort in
`TryLoadJniMarshalMethods()` actually work; `Type.GetRuntimeMethod()`
*will not find* non-public methods, and `__RegisterNativeMembers()`
is rarely/never public.  Thus, it was basically dead code, and we'd
always fallback to the "look at all methods and see which ones have
`[JniAddNativeMethodRegistration]`" codepath, which is by definition
slower.  Use `Type.GetMethod()` instead.

Update `jnimarshalmethod-gen` & co so that they're consistent with
the output of `jcw-gen`.  Without these changes, the JCW would declare
`n_GetString()`, while `jnimarshalmethod-gen` would try to register
`getString`, and Java would thrown an exception because there is no
`getString` member to register.  Doh!

Finally, and the one thing that keeps this from being "perfect",
add an "activation constructor"
`Example.ManagedType(ref JniObjectReference, JniObjectReferenceOptions)`.

This constructor is currently needed because "JavaInterop1"-style
Java Callable Wrappers don't contain constructors (?!), so no
`Example.ManagedType` instance is created *until* the
`ManagedType.n_GetString()` marshal method is executed and attempts
to invoke the `ManagedType.GetString()` method.

We'll need to update `jcw-gen` & related to address this.

Current output:

	% (cd bin/Release/osx-x64/publish ; java -cp hello-from-java.jar:java-interop.jar com/microsoft/hello_from_jni/App)
	Hello from Java!
	C# init()
	Hello from .NET NativeAOT!
	String returned to Java: Hello from .NET NativeAOT!
	C# RegisterNativeMembers(JniType(Name='example/ManagedType' PeerReference=0x7fe545812ed8/G), "Example.ManagedType, Hello-NativeAOTFromJNI", "n_GetString:()Ljava/lang/String;:__export__
	")
	# jonp: called `Example.ManagedType/__<$>_jni_marshal_methods.__RegisterNativeMembers()` w/ 1 methods to register.
	mt.getString()=Hello from C#, via Java.Interop!

samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.csproj

@@ -21,6 +21,11 @@
     <ProjectReference Include="..\..\src\Java.Interop.Export\Java.Interop.Export.csproj" />
   </ItemGroup>
 
+  <ItemGroup>
+    <TrimmerRootAssembly Include="Hello-NativeAOTFromJNI" />
+    <TrimmerRootDescriptor Include="Hello-NativeAOTFromJNI.xml" />
+  </ItemGroup>
+
   <ItemGroup>
     <Content Include="$(OutputPath)hello-from-java.jar" CopyToPublishDirectory="PreserveNewest" />
   </ItemGroup>

samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.targets

@@ -44,6 +44,9 @@
 
     <Exec Command="$(DotnetToolPath) $(_JnimarshalmethodGen) &quot;$(TargetPath)&quot; $(_Verbosity) $(_Libpath)" />
 
+    <!-- the IlcCompile target uses files from `$(IntermediateOutputPath)`, not `$(TargetPath)`, so… update both? -->
+    <Copy SourceFiles="$(TargetPath)" DestinationFolder="$(IntermediateOutputPath)" />
+
     <Touch Files="$(IntermediateOutputPath).added-marshal-methods" AlwaysCreate="True" />
   </Target>
 

samples/Hello-NativeAOTFromJNI/Hello-NativeAOTFromJNI.xml

@@ -0,0 +1,7 @@
+<linker>
+  <assembly fullname="Java.Interop">
+    <type fullname="Java.Interop.JavaException">
+      <method name=".ctor" />
+    </type>
+  </assembly>
+</linker>

samples/Hello-NativeAOTFromJNI/JavaInteropRuntime.cs

@@ -27,6 +27,7 @@ static void init (IntPtr jnienv, IntPtr klass)
 		try {
 			var options = new JreRuntimeOptions {
 				EnvironmentPointer  = jnienv,
+				TypeManager         = new NativeAotTypeManager (),
 			};
 			runtime = options.CreateJreVM ();
 		}

samples/Hello-NativeAOTFromJNI/ManagedType.cs

@@ -9,6 +9,12 @@ public ManagedType ()
 	{
 	}
 
+	// TODO: remove this
+	public ManagedType (ref JniObjectReference reference, JniObjectReferenceOptions options)
+		: base (ref reference, options)
+	{
+	}
+
 	[JavaCallable ("getString")]
 	public Java.Lang.String GetString ()
 	{

samples/Hello-NativeAOTFromJNI/NativeAotTypeManager.cs

@@ -0,0 +1,48 @@
+using Java.Interop;
+
+namespace Hello_NativeAOTFromJNI;
+
+class NativeAotTypeManager : JniRuntime.JniTypeManager {
+
+#pragma warning disable IL2026
+	Dictionary<string, Type> typeMappings = new () {
+		["com/xamarin/java_interop/internal/JavaProxyThrowable"] = typeof (JniEnvironment).Assembly.GetType ("Java.Interop.JavaProxyThrowable", throwOnError: true)!,
+		["example/ManagedType"] = typeof (Example.ManagedType),
+	};
+#pragma warning restore IL2026
+
+	public override void RegisterNativeMembers (JniType nativeClass, ReadOnlySpan<char> assemblyQualifiedTypeName, ReadOnlySpan<char> methods)
+	{
+		Console.WriteLine ($"C# RegisterNativeMembers({nativeClass}, \"{assemblyQualifiedTypeName}\", \"{methods}\")");
+		if (typeMappings.TryGetValue (nativeClass.Name, out var type)) {
+			base.TryRegisterNativeMembers (nativeClass, type, methods);
+		}
+		else {
+			throw new NotSupportedException ($"Unsupported registration type: \"{nativeClass.Name}\" <=> \"{assemblyQualifiedTypeName}\"!");
+		}
+	}
+
+	protected override IEnumerable<Type> GetTypesForSimpleReference (string jniSimpleReference)
+	{
+		if (typeMappings.TryGetValue (jniSimpleReference, out var target))
+			yield return target;
+		foreach (var t in base.GetTypesForSimpleReference (jniSimpleReference))
+			yield return t;
+	}
+
+	protected override IEnumerable<string> GetSimpleReferences (Type type)
+	{
+		return base.GetSimpleReferences (type)
+			.Concat (CreateSimpleReferencesEnumerator (type));
+	}
+
+	IEnumerable<string> CreateSimpleReferencesEnumerator (Type type)
+	{
+		if (typeMappings == null)
+			yield break;
+		foreach (var e in typeMappings) {
+			if (e.Value == type)
+				yield return e.Key;
+		}
+	}
+}

src/Java.Interop.Tools.Expressions/Java.Interop.Tools.Expressions/ExpressionAssemblyBuilder.cs

@@ -59,7 +59,7 @@ static MethodDefinition CreateMethodDefinition (AssemblyDefinition declaringAsse
 		return mmDef;
 	}
 
-	public MethodDefinition CreateRegistrationMethod (IList<ExpressionMethodRegistration> methods)
+	public void AddRegistrationMethod (TypeDefinition declaringType, IList<ExpressionMethodRegistration> methods)
 	{
 		var registrations = new MethodDefinition (
 			name:       "__RegisterNativeMembers",
@@ -71,6 +71,8 @@ public MethodDefinition CreateRegistrationMethod (IList<ExpressionMethodRegistra
 			},
 		};
 
+		declaringType.Methods.Add (registrations);
+
 		var ctor    = typeof (JniAddNativeMethodRegistrationAttribute).GetConstructor (Type.EmptyTypes);
 		var attr    = new CustomAttribute (DeclaringAssemblyDefinition.MainModule.ImportReference (ctor));
 		registrations.CustomAttributes.Add (attr);
@@ -84,7 +86,7 @@ public MethodDefinition CreateRegistrationMethod (IList<ExpressionMethodRegistra
 		registrations.Body.Variables.Add (array);
 
 		var il = registrations.Body.GetILProcessor ();
-		EmitConsoleWriteLine (il, $"# jonp: called __RegisterNativeMembers w/ {methods.Count} methods to register.");
+		EmitConsoleWriteLine (il, $"# jonp: called `{declaringType.FullName}.__RegisterNativeMembers()` w/ {methods.Count} methods to register.");
 		il.Emit (OpCodes.Ldc_I4, methods.Count);
 		il.Emit (OpCodes.Newarr, DeclaringAssemblyDefinition.MainModule.ImportReference (arrayType.GetElementType ()));
 		// il.Emit (OpCodes.Stloc_0);
@@ -117,9 +119,6 @@ public MethodDefinition CreateRegistrationMethod (IList<ExpressionMethodRegistra
 		il.Emit (OpCodes.Ldloc_0);
 		il.Emit (OpCodes.Call, DeclaringAssemblyDefinition.MainModule.ImportReference (addRegistrations.Method));
 		il.Emit (OpCodes.Ret);
-
-
-		return registrations;
 	}
 
 	void EmitConsoleWriteLine (ILProcessor il, string message)

src/Java.Interop/Java.Interop/JniEnvironment.Types.cs

@@ -265,6 +265,8 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 				int r   = _RegisterNatives (type, methods, numMethods);
 
 				if (r != 0) {
+					Console.WriteLine ($"# jonp: _RegisterNatives returned {r}");
+					JniNativeMethods.ExceptionDescribe (JniEnvironment.CurrentInfo.EnvironmentPointer);
 					throw new InvalidOperationException (
 							string.Format ("Could not register native methods for class '{0}'; JNIEnv::RegisterNatives() returned {1}.", GetJniTypeNameFromClass (type), r));
 				}

src/Java.Interop/Java.Interop/JniRuntime.JniTypeManager.cs

@@ -372,6 +372,9 @@ IEnumerable<Type> CreateGetTypesForSimpleReferenceEnumerator (string jniSimpleRe
 
 			protected virtual ReplacementMethodInfo? GetReplacementMethodInfoCore (string jniSimpleReference, string jniMethodName, string jniMethodSignature) => null;
 
+			public virtual void RegisterNativeMembers (JniType nativeClass, ReadOnlySpan<char> assmblyQualifiedTypeName, ReadOnlySpan<char> methods) =>
+				RegisterNativeMembers (nativeClass, Type.GetType (new string (assmblyQualifiedTypeName), throwOnError: true)!, methods);
+
 			public virtual void RegisterNativeMembers (JniType nativeClass, Type type, ReadOnlySpan<char> methods)
 			{
 				TryRegisterNativeMembers (nativeClass, type, methods);
@@ -412,10 +415,21 @@ protected bool TryRegisterNativeMembers (JniType nativeClass, Type type, string?
 			bool TryLoadJniMarshalMethods (JniType nativeClass, Type type, string? methods)
 			{
 				var marshalType = type?.GetNestedType ("__<$>_jni_marshal_methods", BindingFlags.NonPublic);
-				if (marshalType == null)
+				if (marshalType == null) {
+					Console.WriteLine ($"# jonp: could not find type `{(type?.FullName ?? "<no-type>")}.__<$>_jni_marshal_methods`");
 					return false;
+				}
 
-				var registerMethod = marshalType.GetRuntimeMethod ("__RegisterNativeMembers", registerMethodParameters);
+				var registerMethod = marshalType.GetMethod (
+						"__RegisterNativeMembers",
+						BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public,
+						binder: null,
+						callConvention: default,
+						types: registerMethodParameters,
+						modifiers: null);
+				if (registerMethod == null) {
+					Console.WriteLine ($"# jonp: could not find method `{marshalType.FullName}.__<$>_jni_marshal_methods.__RegisterNativeMembers`; will look for methods with `[JniAddNativeMethodRegistration]`…]");
+				}
 
 				return TryRegisterNativeMembers (nativeClass, marshalType, methods, registerMethod);
 			}
@@ -466,11 +480,14 @@ bool FindAndCallRegisterMethod (Type marshalType, JniNativeMethodRegistrationArg
 						continue;
 					}
 
+					var declaringTypeName = methodInfo.DeclaringType?.FullName ?? "<no-decl-type>";
+
 					if ((methodInfo.Attributes & MethodAttributes.Static) != MethodAttributes.Static) {
-						throw new InvalidOperationException ($"The method {methodInfo} marked with {nameof (JniAddNativeMethodRegistrationAttribute)} must be static");
+						throw new InvalidOperationException ($"The method `{declaringTypeName}.{methodInfo}` marked with [{nameof (JniAddNativeMethodRegistrationAttribute)}] must be static!");
 					}
 
 					var register = (Action<JniNativeMethodRegistrationArguments>)methodInfo.CreateDelegate (typeof (Action<JniNativeMethodRegistrationArguments>));
+					Console.WriteLine ($"# jonp: FindAndCallRegisterMethod: calling `{declaringTypeName}.{methodInfo}`…");
 					register (arguments);
 
 					found = true;

src/Java.Interop/Java.Interop/ManagedPeer.cs

@@ -197,22 +197,27 @@ static unsafe void RegisterNativeMembers (
 				var r_nativeClass   = new JniObjectReference (n_nativeClass);
 				var nativeClass     = new JniType (ref r_nativeClass, JniObjectReferenceOptions.Copy);
 
-				var assemblyQualifiedName   = JniEnvironment.Strings.ToString (new JniObjectReference (n_assemblyQualifiedName));
-				var type                    = Type.GetType (assemblyQualifiedName!, throwOnError: true)!;
 				var methodsRef              = new JniObjectReference (n_methods);
-
 #if NET
 
+				var aqnRef                  = new JniObjectReference (n_assemblyQualifiedName);
+				int aqnLength               = JniEnvironment.Strings.GetStringLength (aqnRef);
+				var aqnChars                = JniEnvironment.Strings.GetStringChars (aqnRef, null);
+				var aqn                     = new ReadOnlySpan<char>(aqnChars, aqnLength);
+
 				int methodsLength           = JniEnvironment.Strings.GetStringLength (methodsRef);
 				var methodsChars            = JniEnvironment.Strings.GetStringChars (methodsRef, null);
 				var methods                 = new ReadOnlySpan<char>(methodsChars, methodsLength);
 				try {
-					JniEnvironment.Runtime.TypeManager.RegisterNativeMembers (nativeClass, type, methods);
+					JniEnvironment.Runtime.TypeManager.RegisterNativeMembers (nativeClass, aqn, methods);
 				}
 				finally {
+					JniEnvironment.Strings.ReleaseStringChars (aqnRef, aqnChars);
 					JniEnvironment.Strings.ReleaseStringChars (methodsRef, methodsChars);
 				}
 #else   // NET
+				var assemblyQualifiedName   = JniEnvironment.Strings.ToString (new JniObjectReference (n_assemblyQualifiedName));
+				var type                    = Type.GetType (assemblyQualifiedName!, throwOnError: true)!;
 				var methods                 = JniEnvironment.Strings.ToString (methodsRef);
 				JniEnvironment.Runtime.TypeManager.RegisterNativeMembers (nativeClass, type, methods);
 #endif  // NET

tools/jnimarshalmethod-gen/App.cs

@@ -552,27 +552,22 @@ void CreateMarshalMethodAssembly (string path)
 					Log (TraceLevel.Verbose, $"## Dumping contents of marshal method for `{td.FullName}::{method.Name}({string.Join (", ", method.GetParameters ().Select (p => p.ParameterType))})`:");
 					Console.WriteLine (lambda.ToCSharpCode ());
 #endif  // _DUMP_REGISTER_NATIVE_MEMBERS
+					name = export?.Name ?? method.Name;
+
 					var mmDef = assemblyBuilder.Compile (lambda);
-					mmDef.Name = export?.Name ?? ("n_TODO" + lambda.GetHashCode ());
+					mmDef.Name = name;
 					mmTypeDef.Methods.Add (mmDef);
 
-					if (export != null) {
-						name = export.Name;
-						signature = export.Signature;
-					}
-
-					if (signature == null) {
-						signature = builder.GetJniMethodSignature (method);
-					}
+					signature = export?.Signature ?? builder.GetJniMethodSignature (method);
 
-					registrations.Add (new ExpressionMethodRegistration (name, signature, mmDef));
+					// Assume that `JavaCallableAttribute.Name` is "public" JCW name, and JCW's declare `n_`-prefixed `native` methods…
+					registrations.Add (new ExpressionMethodRegistration ("n_" + method.Name, signature, mmDef));
 
 					addedMethods.Add (methodName);
 				}
 				if (registrations.Count > 0) {
-					var m = assemblyBuilder.CreateRegistrationMethod (registrations);
-					mmTypeDef.Methods.Add (m);
 					td.NestedTypes.Add (mmTypeDef);
+					assemblyBuilder.AddRegistrationMethod (mmTypeDef, registrations);
 				}
 			}