[Java.Interop] Optional "Standalone" Build Config (#1049)

Context: 16e1ecd
Context: 312fbf4

It occurs to me that it would be easier for "external" developers to
use `Java.Interop.dll` if it didn't require building and distributing
any native libraries.  Furthermore, as of commit 312fbf4
(C#9 function pointer backend), it's *plausible* to make that work.

Let's do so.

Add a new `$(Standalone)` "configuration" property to
`src\Java.Interop\Java.Interop.csproj`; when True, the
`FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS` compiler define is set
instead of the `FEATURE_JNIENVIRONMENT_JI_PINVOKES` define.
This enables `JniEnvironment` to use C#9 function pointers instead of
P/Invokes to invoke the `JNIEnv` function pointers.

	% dotnet build src/Java.Interop/Java.Interop.csproj -p:Standalone=True
	% ikdasm src/Java.Interop/obj/Debug/net7.0/Java.Interop.dll | grep pinvoke
	# no matches
	# If `-p:Standalone=True` isn't set, then there are 188 matches.

Update `Java.Interop.dll` to compile when
`FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS` is set.

!!ABI BREAK!! `[Obsolete]` the method
`JniRuntime.GetAvailableInvocationPointers()`.  In retrospect this
never should have been exposed at this level of the stack, and its
existence was responsible for "really really bizarre" .NET Android
[app crashes][0] (due to static constructor orderings) when
*sometimes* `JniRuntime.Current` wasn't set "early enough":

	D Mono    : AOT: FOUND method Java.Interop.JniRuntime:GetAvailableInvocationPointers () [0x78e4da7960 - 0x78e4da7a7c 0x78e4de6840]
	D Mono    : AOT: FOUND method Java.Interop.JniRuntime:GetCreatedJavaVMs (intptr[],int,int&) [0x78e4ddd2b0 - 0x78e4ddd300 0x78e4de6bcd]
	D Mono    : AOT: NOT FOUND: Java.Interop.NativeMethods:java_interop_jvm_list (intptr[],int,int&).
	F monodroid-assembly: Internal p/invoke symbol 'java-interop @ java_interop_jvm_list' (hash: 0x58c48fc8b89cb484) not found in compile-time map.

*Nobody* should be using this method, largely given that only
Xamarin.Android and .NET Android apps currently use
`Java.Interop.dll`, and neither use
`JniRuntime.GetAvailableInvocationPointers()`.  Furthermore,
it *can't* work on Android, as Android doesn't provide a public
[`JNI_GetCreatedJavaVMs()`][1] symbol.

Update `build-tools/jnienv-gen` so that a `JniNativeMethods` class
is created which contains "human usable" ways to invoke `JNIEnv`
function pointers.  (Nobody wants to copy the expression
`(*((JNIEnv**)env))->ExceptionClear(env)` more than once, ever.
`JniNativeMethods.ExceptionClear(env)` is much nicer to write.)

Update `samples/Hello-Core` so that it sets `Standalone=True`.

Verification: After building `samples/Hello-Core`, the contained
`Java.Interop.dll` doesn't contain any `pinvokeimpl` methods:

	% dotnet build samples/Hello-Core
	% ikdasm samples/Hello-Core/bin/Debug/Java.Interop.dll | grep pinvoke
	# no matches

TODO: I also attempted to reduce the number of P/Invokes in
`Java.Runtime.Environment.dll`, with the hope that when *not* using
MonoVM it could be used without a native `java-interop` library.
This used [`System.Runtime.InteropServices.NativeLibrary`][2] to load
`JniRuntime.CreationOptions.JvmLibraryPath` and invoke the
`JNI_CreateJavaVM()` and `JNI_GetCreatedJavaVMs()` exports.

Unfortunately, this new backend crashes inexplicably when using
`dotnet test`.  The backend can now be selected by setting the
`JI_LOADER_TYPE` environment variable to one of:

  * `native-library`: the `NativeLibrary` backened, or
  * `java-interop`: the previous `java-interop` native lib backend.

This allows testing to work and CI to succeed:

	% dotnet test bin/TestDebug-net7.0/Java.Interop-Tests.dll
	# all good

while allowing us to separately explore why it crashes:

	% JI_LOADER_TYPE=native-library dotnet test bin/TestDebug-net7.0/Java.Interop-Tests.dll
	…
	# jonp: LoadJvmLibrary(…/libjli.dylib)=9056174496
	# jonp: JNI_CreateJavaVM=4561133901; JNI_GetCreatedJavaVMs=4561133970
	# jonp: executing JNI_CreateJavaVM=10fdd614d
	Error occurred during initialization of VM
	Could not reserve enough space in CodeHeap 'non-nmethods' (2496K)
	The active test run was aborted. Reason: Test host process crashed

	Test Run Aborted with error System.Exception: One or more errors occurred.
	 ---> System.Exception: Unable to read beyond the end of the stream.
	   at System.IO.BinaryReader.Read7BitEncodedInt()
	   at System.IO.BinaryReader.ReadString()
	   at Microsoft.VisualStudio.TestPlatform.CommunicationUtilities.LengthPrefixCommunicationChannel.NotifyDataAvailable()
	   at Microsoft.VisualStudio.TestPlatform.CommunicationUtilities.TcpClientExtensions.MessageLoopAsync(TcpClient client, ICommunicationChannel channel, Action`1 errorHandler, CancellationToken cancellationToken)
	   --- End of inner exception stack trace ---.

[0]: https://discord.com/channels/732297728826277939/732297837953679412/979054761603125319
[1]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/invocation.html#JNI_GetCreatedJavaVMs
[2]: https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.nativelibrary?view=net-7.0

Author: jonpryor

build-tools/jnienv-gen/Generator.cs

@@ -87,8 +87,8 @@ static void GenerateFile (TextWriter o)
 			o.WriteLine ("//");
 			o.WriteLine ("// To make changes, edit monodroid/tools/jnienv-gen-interop and rerun");
 			o.WriteLine ();
-			o.WriteLine ("#if !FEATURE_JNIENVIRONMENT_SAFEHANDLES && !FEATURE_JNIENVIRONMENT_JI_INTPTRS && !FEATURE_JNIENVIRONMENT_JI_PINVOKES && !FEATURE_JNIENVIRONMENT_XA_INTPTRS");
-			o.WriteLine ("#define FEATURE_JNIENVIRONMENT_SAFEHANDLES");
+			o.WriteLine ("#if !FEATURE_JNIENVIRONMENT_SAFEHANDLES && !FEATURE_JNIENVIRONMENT_JI_INTPTRS && !FEATURE_JNIENVIRONMENT_JI_PINVOKES && !FEATURE_JNIENVIRONMENT_XA_INTPTRS && !FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS");
+			o.WriteLine ("#define FEATURE_JNIENVIRONMENT_JI_PINVOKES");
 			o.WriteLine ("#endif  // !FEATURE_JNIENVIRONMENT_SAFEHANDLES && !FEATURE_JNIENVIRONMENT_JI_INTPTRS && !FEATURE_JNIENVIRONMENT_JI_PINVOKES && !FEATURE_JNIENVIRONMENT_XA_INTPTRS");
 			o.WriteLine ();
 			o.WriteLine ("#if FEATURE_JNIENVIRONMENT_SAFEHANDLES && FEATURE_JNIENVIRONMENT_JI_INTPTRS");
@@ -315,6 +315,9 @@ static void GenerateTypes (TextWriter o, HandleStyle style)
 			if (style == HandleStyle.JIIntPtrPinvokeWithErrors) {
 				GenerateNativeMethods (o, style);
 			}
+			if (style == HandleStyle.JIFunctionPtrWithErrors) {
+				GenerateJniNativeMethods (o, style);
+			}
 
 			var visibilities = new Dictionary<string, string> {
 				{ ArrayOperationsCategory,      "public" },
@@ -367,6 +370,46 @@ static void GenerateNativeMethods (TextWriter o, HandleStyle style)
 			o.WriteLine ();
 		}
 
+		static void GenerateJniNativeMethods (TextWriter o, HandleStyle style)
+		{
+			o.WriteLine ("\tstatic partial class JniNativeMethods {");
+			o.WriteLine ();
+			foreach (var entry in JNIEnvEntries) {
+				if (entry.Parameters == null)
+					continue;
+				if (entry.IsPrivate || entry.CustomWrapper)
+					continue;
+
+				var returnType = entry.ReturnType.GetMarshalType (HandleStyle.JIFunctionPtrWithErrors, isReturn: true, isPinvoke: true);
+
+				o.WriteLine ();
+				o.WriteLine ("\t\t[System.Runtime.CompilerServices.MethodImpl (System.Runtime.CompilerServices.MethodImplOptions.AggressiveInlining)]");
+				o.Write ("\t\tinternal static unsafe ");
+				o.Write (entry.ReturnType.GetMarshalType (HandleStyle.JIFunctionPtrWithErrors, isReturn: true, isPinvoke: true));
+				o.Write ($" {entry.Name} (IntPtr env");
+				foreach (var p in entry.Parameters) {
+					o.Write (", ");
+					o.Write (p.Type.GetMarshalType (HandleStyle.JIFunctionPtrWithErrors, isReturn: false, isPinvoke: true));
+					o.Write ($" {Escape (p.Name)}");
+				}
+				o.WriteLine (")");
+				o.WriteLine ("\t\t{");
+				o.Write ("\t\t\t");
+				if (returnType != "void") {
+					o.Write ("return ");
+				}
+				o.Write ($"(*((JNIEnv**)env))->{entry.Name} (env");
+				foreach (var p in entry.Parameters) {
+					o.Write (", ");
+					o.Write (Escape (p.Name));
+				}
+				o.WriteLine (");");
+				o.WriteLine ("\t\t}");
+			}
+			o.WriteLine ("\t}");
+			o.WriteLine ();
+		}
+
 		static string GetPinvokeName (string name)
 		{
 			var sb = new StringBuilder ("java_interop_jnienv_".Length + name.Length * 2);
@@ -427,7 +470,7 @@ static void GenerateJniEnv (TextWriter o, string type, string visibility, Handle
 								GetPinvokeName (entry.Name),
 								entry.Throws ? ", out thrown" : "");
 					} else if (style == HandleStyle.JIFunctionPtrWithErrors) {
-						o.Write ($"(*((JNIEnv**)__env))->{entry.Name} (__env");
+						o.Write ($"JniNativeMethods.{entry.Name} (__env");
 					} else {
 						o.Write ("__info.Invoker.{0} (__info.EnvironmentPointer", entry.Name);
 					}
@@ -444,7 +487,7 @@ static void GenerateJniEnv (TextWriter o, string type, string visibility, Handle
 					}
 					o.WriteLine (");");
 					if (style == HandleStyle.JIFunctionPtrWithErrors && entry.Throws) {
-						o.WriteLine ("\t\t\tIntPtr thrown = (*((JNIEnv**)__env))->ExceptionOccurred (__env);");
+						o.WriteLine ("\t\t\tIntPtr thrown = JniNativeMethods.ExceptionOccurred (__env);");
 					}
 					CleanupParameters (o, entry.Parameters, style);
 					RaiseException (o, entry, style);

samples/Hello-Core/Hello-Core.csproj

@@ -14,7 +14,9 @@
   </ItemGroup>
 
   <ItemGroup>
-    <ProjectReference Include="..\..\src\Java.Interop\Java.Interop.csproj" />
+    <ProjectReference Include="..\..\src\Java.Interop\Java.Interop.csproj"
+        AdditionalProperties="Standalone=True"
+    />
     <ProjectReference Include="..\..\src\Java.Runtime.Environment\Java.Runtime.Environment.csproj" />
   </ItemGroup>
 

samples/Hello-Java.Base/Program.cs

@@ -105,12 +105,12 @@ static unsafe void CreateAnotherJVM ()
 				t.Start ();
 				waitForCreation.Wait ();
 				*/
-				foreach (var h in JniRuntime.GetAvailableInvocationPointers ()) {
+				foreach (var h in vm.GetAvailableInvocationPointers ()) {
 					Console.WriteLine ("WITHIN: GetCreatedJavaVMs: {0}", h);
 				}
 				// exitThread.Signal ();
 			}
-			foreach (var h in JniRuntime.GetAvailableInvocationPointers ()) {
+			foreach (var h in JniRuntime.GetRegisteredRuntimes ()) {
 				Console.WriteLine ("POST: GetCreatedJavaVMs: {0}", h);
 			}
 		}

src/Java.Interop/Java.Interop.csproj

@@ -20,7 +20,6 @@
     <NoWarn>$(NoWarn);1591</NoWarn>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
-    <DefineConstants>INTEROP;FEATURE_JNIENVIRONMENT_JI_PINVOKES;FEATURE_JNIOBJECTREFERENCE_INTPTRS;INTERNAL_NULLABLE_ATTRIBUTES;$(JavaInteropDefineConstants)</DefineConstants>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <Nullable>enable</Nullable>
     <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
@@ -31,6 +30,7 @@
   </PropertyGroup>
   <Import Project="..\..\TargetFrameworkDependentValues.props" />
   <PropertyGroup>
+    <DefineConstants>INTEROP;FEATURE_JNIOBJECTREFERENCE_INTPTRS;INTERNAL_NULLABLE_ATTRIBUTES;$(JavaInteropDefineConstants)</DefineConstants>
     <IntermediateOutputPath>$(BaseIntermediateOutputPath)$(Configuration)\$(TargetFramework.ToLowerInvariant())\</IntermediateOutputPath>
     <OutputPath>$(ToolOutputFullPath)</OutputPath>
     <DocumentationFile>$(ToolOutputFullPath)Java.Interop.xml</DocumentationFile>
@@ -39,6 +39,13 @@
     <LangVersion Condition=" '$(LangVersion)' == '' ">8.0</LangVersion>
     <Version>$(JICoreLibVersion)</Version>
   </PropertyGroup>
+  <PropertyGroup Condition=" '$(Standalone)' == 'True' ">
+    <DefineConstants Condition=" '$(JIBuildingForNetCoreApp)' == 'True' ">FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS;$(DefineConstants)</DefineConstants>
+    <DefineConstants Condition=" '$(JIBuildingForNetCoreApp)' != 'True' ">FEATURE_JNIENVIRONMENT_JI_PINVOKES;$(DefineConstants)</DefineConstants>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Standalone)' != 'True' ">
+    <DefineConstants>FEATURE_JNIENVIRONMENT_JI_PINVOKES;$(DefineConstants)</DefineConstants>
+  </PropertyGroup>
   <ItemGroup>
     <Compile Condition=" '$(TargetFramework)' == 'netstandard2.0' " Include="..\utils\NullableAttributes.cs" />
     <Compile Remove="Java.Interop\JniLocationException.cs" />

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

@@ -4,6 +4,7 @@
 using System.Diagnostics;
 using System.Collections.Generic;
 using System.Text;
+using System.Runtime.InteropServices;
 
 namespace Java.Interop
 {
@@ -44,22 +45,13 @@ static unsafe JniObjectReference TryFindClass (string classname, bool throwOnErr
 					throw new ArgumentException ("'classname' cannot be a zero-length string.", nameof (classname));
 
 				var info    = JniEnvironment.CurrentInfo;
-#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
-				IntPtr thrown;
-				var c   = NativeMethods.java_interop_jnienv_find_class (info.EnvironmentPointer, out thrown, classname);
-				if (thrown == IntPtr.Zero) {
+#if FEATURE_JNIENVIRONMENT_JI_PINVOKES || FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				if (TryRawFindClass (info.EnvironmentPointer, classname, out var c, out var thrown)) {
 					var r   = new JniObjectReference (c, JniObjectReferenceType.Local);
 					JniEnvironment.LogCreateLocalRef (r);
 					return r;
 				}
-
-				// If the Java-side exception stack trace is *lost* a'la 89a5a229,
-				// change `false` to `true` and rebuild+re-run.
-#if false
-				NativeMethods.java_interop_jnienv_exception_describe (info.EnvironmentPointer);
-#endif
-
-				NativeMethods.java_interop_jnienv_exception_clear (info.EnvironmentPointer);
+				RawExceptionClear (info.EnvironmentPointer);
 
 				var findClassThrown     = new JniObjectReference (thrown, JniObjectReferenceType.Local);
 				LogCreateLocalRef (findClassThrown);
@@ -70,18 +62,18 @@ static unsafe JniObjectReference TryFindClass (string classname, bool throwOnErr
 					var __args  = stackalloc JniArgumentValue [1];
 					__args [0]  = new JniArgumentValue (java);
 
-					c = NativeMethods.java_interop_jnienv_call_object_method_a (info.EnvironmentPointer, out thrown, info.Runtime.ClassLoader.Handle, info.Runtime.ClassLoader_LoadClass.ID, (IntPtr) __args);
+					c = RawCallObjectMethodA (info.EnvironmentPointer, out thrown, info.Runtime.ClassLoader.Handle, info.Runtime.ClassLoader_LoadClass.ID, (IntPtr) __args);
 					JniObjectReference.Dispose (ref java);
 					if (thrown == IntPtr.Zero) {
 						(pendingException as IJavaPeerable)?.Dispose ();
 						var r = new JniObjectReference (c, JniObjectReferenceType.Local);
 						JniEnvironment.LogCreateLocalRef (r);
 						return r;
 					}
-					NativeMethods.java_interop_jnienv_exception_clear (info.EnvironmentPointer);
+					RawExceptionClear (info.EnvironmentPointer);
 
 					if (pendingException != null) {
-						NativeMethods.java_interop_jnienv_delete_local_ref (info.EnvironmentPointer, thrown);
+						JniEnvironment.References.RawDeleteLocalRef (info.EnvironmentPointer, thrown);
 					}
 					else {
 						var loadClassThrown = new JniObjectReference (thrown, JniObjectReferenceType.Local);
@@ -95,7 +87,7 @@ static unsafe JniObjectReference TryFindClass (string classname, bool throwOnErr
 					return default;
 				}
 				throw pendingException!;
-#endif  // !FEATURE_JNIENVIRONMENT_JI_PINVOKES
+#endif  // !(FEATURE_JNIENVIRONMENT_JI_PINVOKES || FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS)
 #if FEATURE_JNIOBJECTREFERENCE_SAFEHANDLES
 				var c       = info.Invoker.FindClass (info.EnvironmentPointer, classname);
 				var thrown  = info.Invoker.ExceptionOccurred (info.EnvironmentPointer);
@@ -137,6 +129,59 @@ static unsafe JniObjectReference TryFindClass (string classname, bool throwOnErr
 #endif  // !FEATURE_JNIOBJECTREFERENCE_SAFEHANDLES
 			}
 
+			static bool TryRawFindClass (IntPtr env, string classname, out IntPtr klass, out IntPtr thrown)
+			{
+#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
+				klass = NativeMethods.java_interop_jnienv_find_class (env, out thrown, classname);
+				if (thrown == IntPtr.Zero) {
+					return true;
+				}
+#endif  // !FEATURE_JNIENVIRONMENT_JI_PINVOKES
+#if FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				var _classname_ptr = Marshal.StringToCoTaskMemUTF8 (classname);
+				klass   = JniNativeMethods.FindClass (env, _classname_ptr);
+				thrown  = JniNativeMethods.ExceptionOccurred (env);
+				Marshal.ZeroFreeCoTaskMemUTF8 (_classname_ptr);
+				if (thrown == IntPtr.Zero) {
+					return true;
+				}
+#endif  // !FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				return false;
+			}
+
+			static void RawExceptionClear (IntPtr env)
+			{
+#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
+				// If the Java-side exception stack trace is *lost* a'la 89a5a229,
+				// change `false` to `true` and rebuild+re-run.
+#if false
+				NativeMethods.java_interop_jnienv_exception_describe (env);
+#endif  // FEATURE_JNIENVIRONMENT_JI_PINVOKES
+
+				NativeMethods.java_interop_jnienv_exception_clear (env);
+#elif FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				// If the Java-side exception stack trace is *lost* a'la 89a5a229,
+				// change `false` to `true` and rebuild+re-run.
+#if false
+				JniNativeMethods.ExceptionDescribe (env);
+#endif
+				JniNativeMethods.ExceptionClear (env);
+#endif  // FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+			}
+
+			static IntPtr RawCallObjectMethodA (IntPtr env, out IntPtr thrown, IntPtr instance, IntPtr jmethodID, IntPtr args)
+			{
+#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
+				return NativeMethods.java_interop_jnienv_call_object_method_a (env, out thrown, instance, jmethodID, args);
+#elif FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				var r   = JniNativeMethods.CallObjectMethodA (env, instance, jmethodID, args);
+				thrown  = JniNativeMethods.ExceptionOccurred (env);
+				return r;
+#else   // FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				return IntPtr.Zero;
+#endif  // FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+			}
+
 #if NET
 			public static bool TryFindClass (string classname, out JniObjectReference instance)
 			{

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

@@ -170,14 +170,34 @@ internal    static  void    LogCreateLocalRef (IntPtr value)
 		}
 #endif  // FEATURE_JNIOBJECTREFERENCE_INTPTRS
 
-#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
 		partial class References {
-			internal static int GetJavaVM (IntPtr jnienv, out IntPtr vm)
+
+			internal static unsafe int GetJavaVM (IntPtr jnienv, out IntPtr vm)
 			{
+#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
 				return NativeMethods.java_interop_jnienv_get_java_vm (jnienv, out vm);
+#elif FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				IntPtr _vm;
+				int r       = JniNativeMethods.GetJavaVM (jnienv, &_vm);
+				vm          = _vm;
+				return r;
+#else
+				Invoker     = CreateInvoker (environmentPointer);
+				var r       = Invoker.GetJavaVM (EnvironmentPointer, out vm);
+				return r;
+#endif
+			}
+
+			internal static void RawDeleteLocalRef (IntPtr env, IntPtr localRef)
+			{
+#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
+				NativeMethods.java_interop_jnienv_delete_local_ref (env, localRef);
+#elif FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
+				JniNativeMethods.DeleteLocalRef (env, localRef);
+#endif  // FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
 			}
 		}
-#endif  // !FEATURE_JNIENVIRONMENT_JI_PINVOKES
+
 	}
 
 	sealed class JniEnvironmentInfo : IDisposable {
@@ -206,13 +226,7 @@ public IntPtr                  EnvironmentPointer {
 
 				environmentPointer  = value;
 				IntPtr  vmh = IntPtr.Zero;
-				int     r   = 0;
-#if FEATURE_JNIENVIRONMENT_JI_PINVOKES
-				r           = JniEnvironment.References.GetJavaVM (EnvironmentPointer, out vmh);
-#else
-				Invoker     = CreateInvoker (environmentPointer);
-				r           = Invoker.GetJavaVM (EnvironmentPointer, out vmh);
-#endif  // #if !FEATURE_JNIENVIRONMENT_JI_PINVOKES
+				int     r   = JniEnvironment.References.GetJavaVM (EnvironmentPointer, out vmh);
 				if (r < 0)
 					throw new InvalidOperationException ("JNIEnv::GetJavaVM() returned: " + r.ToString ());
 
@@ -285,7 +299,7 @@ public void Dispose ()
 		};
 #endif  // FEATURE_JNIENVIRONMENT_SAFEHANDLES
 
-#if !FEATURE_JNIENVIRONMENT_JI_PINVOKES
+#if !FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS && !FEATURE_JNIENVIRONMENT_JI_PINVOKES
 		internal    JniEnvironmentInvoker   Invoker                 {get; private set;}
 
 		static unsafe JniEnvironmentInvoker CreateInvoker (IntPtr handle)

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

@@ -3,6 +3,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Reflection;
 using System.Runtime.InteropServices;
@@ -78,13 +79,6 @@ interface ISetRuntime {
 		}
 	}
 
-	partial class NativeMethods {
-		const string JavaInteropLibrary = "java-interop";
-
-		[DllImport (JavaInteropLibrary, CallingConvention=CallingConvention.Cdecl)]
-		internal static extern int java_interop_jvm_list ([Out] IntPtr[]? handles, int bufLen, out int nVMs);
-	}
-
 	public partial class JniRuntime : IDisposable
 	{
 		const   int     JNI_OK          = 0;
@@ -107,22 +101,14 @@ public static IEnumerable<JniRuntime> GetRegisteredRuntimes ()
 			}
 		}
 
-		internal static int GetCreatedJavaVMs (IntPtr[]? handles, int bufLen, out int nVMs)
-		{
-			return NativeMethods.java_interop_jvm_list (handles, bufLen, out nVMs);
-		}
-
+		[Obsolete ("Not sensible/usable at this level, and cannot work on e.g. Android.  " +
+				"Try Java.Interop.JreRuntime.GetAvailableInvocationPointers() in Java.Runtime.Environment.dll, " +
+				"or rethink your structure.", error: true)]
+		[SuppressMessage ("Design", "CA1024:Use properties where appropriate",
+				Justification = "ABI compatibility")]
 		public static IEnumerable<IntPtr> GetAvailableInvocationPointers ()
 		{
-			int nVMs;
-			int r = GetCreatedJavaVMs (null, 0, out nVMs);
-			if (r != 0)
-				throw new NotSupportedException ("JNI_GetCreatedJavaVMs() returned: " + r.ToString ());
-			var handles = new IntPtr [nVMs];
-			r = GetCreatedJavaVMs (handles, handles.Length, out nVMs);
-			if (r != 0)
-				throw new InvalidOperationException ("JNI_GetCreatedJavaVMs() [take 2!] returned: " + r.ToString ());
-			return handles;
+			throw new NotSupportedException ();
 		}
 
 		static JniRuntime? current;
@@ -143,17 +129,9 @@ public static JniRuntime CurrentRuntime {
 					return c!;
 				}
 				if (count > 1)
-					throw new NotSupportedException (string.Format ("Found {0} Java Runtimes. Don't know which to use. Use JniRuntime.SetCurrent().", count));
+					throw new NotSupportedException (string.Format ("Found {0} known Java Runtime instances. Don't know which to use. Use JniRuntime.SetCurrent().", count));
 				Debug.Assert (count == 0);
-				var available   = GetAvailableInvocationPointers ().FirstOrDefault ();
-				if (available == IntPtr.Zero)
-					throw new NotSupportedException ("No available Java runtime to attach to. Please create one.");
-				var options     = new CreationOptions () {
-					DestroyRuntimeOnDispose = false,
-					InvocationPointer       = available,
-				};
-				// Sets `current`
-				return new JniRuntime (options);
+				throw new NotSupportedException ("No available Java runtime to attach to. Please create one.");
 			}
 		}