[Java.Runtime.Environment] Partial support for .NET Core

Enable C#8 [Nullable Reference Types][0] for
`Java.Runtime.Environment.dll`.

Add partial support for a "non-bridged backend", so that a
`JniRuntime.JniValueManager` exists for .NET Core.  This new
"managed" backed is used if the Mono runtime is *not* used.

To work, `ManagedValueManager` holds *strong* references to
`IJavaPeerable` instances.  As such, tests which required the use of
GC integration are now "optional", conditional on the
`!NO_GC_BRIDGE_SUPPORT` define.

The `ManagedValueManager.CollectPeers()` method calls
`IJavaPeerable.Dispose()` on all currently referenced peers, then
stops referencing the managed peers.
This causes all GREFs to be dropped, allowing Java peers to be
collected, and then allows the .NET GC to collect the `IJavaPeerable`
values.

Any and all exceptions thrown by `IJavaPeerable.Dispose()` are
caught and re-thrown by an `AggregateException`.

Update `Java.Interop-Tests.csproj` to define `NO_GC_BRIDGE_SUPPORT`
and `NO_MARSHAL_MEMBER_BUILDER_SUPPORT` when building for .NET Core.
This excludes all currently "troublesome"/non-passing tests.

These changes allow all remaining `Java.Interop-Tests` unit tests to
execute under .NET Core:

	% dotnet test -v diag '--logger:trx;verbosity=detailed' bin/TestDebug-netcoreapp3.1/Java.Interop-Tests.dll
	Passed!  - Failed:     0, Passed:   617, Skipped:     1, Total:   618, Duration: 1 s

Other changes:

  * The attempt to retain useful Java-side exceptions in 89a5a22
    proved to be incomplete.  Add a comment to invoke
    [`JNIEnv::ExceptionDescribe()`][1].  We don't always want this
    to be present, but when we do want it…

  * While `NO_MARSHAL_MEMBER_BUILDER_SUPPORT` is set -- which means
    that `Java.Interop.Export`-related tests aren't run -- there are
    some fixes for `Java.Interop.Export` & related unit tests for
    .NET Core, to avoid the use of generic delegate types and to
    avoid a `Type.GetType()` which is no longer needed.

[0]: https://docs.microsoft.com/dotnet/csharp/nullable-references
[1]: https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/functions.html#ExceptionDescribe

build-tools/automation/azure-pipelines.yaml

@@ -21,6 +21,7 @@ variables:
   DotNetCoreVersion: 3.1.300
   HostedMacImage: macOS-10.15
   HostedWinVS2019: Hosted Windows 2019 with VS2019
+  NetCoreTargetFrameworkPathSuffix: -netcoreapp3.1
 
 jobs:
 - job: windows_build

build-tools/automation/templates/core-tests.yaml

@@ -67,6 +67,14 @@ steps:
     arguments: bin/Test$(Build.Configuration)/Java.Interop-Tests.dll
   continueOnError: true
 
+- task: DotNetCoreCLI@2
+  displayName: 'Tests: Java.Interop'
+  condition: eq('${{ parameters.runNativeTests }}', 'true')
+  inputs:
+    command: test
+    arguments: bin/Test$(Build.Configuration)$(NetCoreTargetFrameworkPathSuffix)/Java.Interop-Tests.dll
+  continueOnError: true
+
 - task: DotNetCoreCLI@2
   displayName: 'Tests: Java.Interop.Dynamic'
   condition: eq('${{ parameters.runNativeTests }}', 'true')

src/Java.Interop.Export/Java.Interop/MarshalMemberBuilder.cs

@@ -370,7 +370,12 @@ static Expression GetRuntime ()
 			return Expression.Property (null, typeof (JniEnvironment), "Runtime");
 		}
 
-		static  MethodInfo  FormatterServices_GetUninitializedObject    = Type.GetType ("System.Runtime.Serialization.FormatterServices", throwOnError: true)
+		static  MethodInfo  FormatterServices_GetUninitializedObject    =
+#if NETFRAMEWORK || NET_2_0
+			typeof (System.Runtime.Serialization.FormatterServices)
+#else   // !(NETFRAMEWORK || NET_2_0)
+			typeof (System.Runtime.CompilerServices.RuntimeHelpers)
+#endif  // NETFRAMEWORK || NET_2_0
 			.GetRuntimeMethod ("GetUninitializedObject", new[]{typeof (Type)});
 		static  MethodInfo  IJavaPeerable_SetPeerReference              = typeof (IJavaPeerable).GetRuntimeMethod ("SetPeerReference", new[]{typeof (JniObjectReference)});
 

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

@@ -1,6 +1,7 @@
 #nullable enable
 
 using System;
+using System.Diagnostics;
 using System.Collections.Generic;
 using System.Text;
 
@@ -47,6 +48,12 @@ public static unsafe JniObjectReference FindClass (string classname)
 					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);
 
 				var findClassThrown     = new JniObjectReference (thrown, JniObjectReferenceType.Local);
@@ -167,6 +174,17 @@ public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegi
 
 			public static void RegisterNatives (JniObjectReference type, JniNativeMethodRegistration [] methods, int numMethods)
 			{
+#if DEBUG && NETCOREAPP
+				foreach (var m in methods) {
+					if (m.Marshaler.GetType ().GenericTypeArguments.Length != 0) {
+						var method  = m.Marshaler.Method;
+						Debug.WriteLine ($"JNIEnv::RegisterNatives() given a generic delegate type.  .NET Core doesn't like this.");
+						Debug.WriteLine ($"  Java: {m.Name}{m.Signature}");
+						Debug.WriteLine ($"  Marshaler Type={m.Marshaler.GetType ().FullName} Method={method.DeclaringType.FullName}.{method.Name}");
+					}
+				}
+#endif  // DEBUG && NETCOREAPP
+
 				int r   = _RegisterNatives (type, methods, numMethods);
 
 				if (r != 0) {

src/Java.Runtime.Environment/Java.Interop/JreRuntime.cs

@@ -31,6 +31,9 @@ public class JreRuntimeOptions : JniRuntime.CreationOptions {
 
 		public  Collection<string>  ClassPath           {get; private set;}
 
+		public  TextWriter? JniGlobalReferenceLogWriter {get; set;}
+		public  TextWriter? JniLocalReferenceLogWriter  {get; set;}
+
 		public JreRuntimeOptions ()
 		{
 			JniVersion  = JniVersion.v1_2;
@@ -39,16 +42,6 @@ public JreRuntimeOptions ()
 					Path.GetDirectoryName (typeof (JreRuntimeOptions).Assembly.Location),
 					"java-interop.jar"),
 			};
-
-			bool onMono = Type.GetType ("Mono.Runtime", throwOnError: false) != null;
-			if (onMono) {
-				ValueManager            = ValueManager              ?? new MonoRuntimeValueManager ();
-				ObjectReferenceManager  = ObjectReferenceManager    ?? new MonoRuntimeObjectReferenceManager ();
-			}
-			else {
-				ValueManager            = ValueManager              ?? new DummyValueManager ();
-				ObjectReferenceManager  = ObjectReferenceManager    ?? new DummyObjectReferenceManager ();
-			}
 		}
 
 		public JreRuntimeOptions AddOption (string option)
@@ -87,12 +80,22 @@ static unsafe JreRuntimeOptions CreateJreVM (JreRuntimeOptions builder)
 			if (builder == null)
 				throw new ArgumentNullException ("builder");
 
+			bool onMono = Type.GetType ("Mono.Runtime", throwOnError: false) != null;
+			if (onMono) {
+				builder.ValueManager            = builder.ValueManager              ?? new MonoRuntimeValueManager ();
+				builder.ObjectReferenceManager  = builder.ObjectReferenceManager    ?? new MonoRuntimeObjectReferenceManager ();
+			}
+			else {
+				builder.ValueManager            = builder.ValueManager              ?? new ManagedValueManager ();
+				builder.ObjectReferenceManager  = builder.ObjectReferenceManager    ?? new ManagedObjectReferenceManager (builder.JniGlobalReferenceLogWriter, builder.JniLocalReferenceLogWriter);
+			}
+
 			if (builder.InvocationPointer != IntPtr.Zero)
 				return builder;
 
 			if (!string.IsNullOrEmpty (builder.JvmLibraryPath)) {
 				IntPtr errorPtr = IntPtr.Zero;
-				int r = NativeMethods.java_interop_jvm_load_with_error_message (builder.JvmLibraryPath, out errorPtr);
+				int r = NativeMethods.java_interop_jvm_load_with_error_message (builder.JvmLibraryPath!, out errorPtr);
 				if (r != 0) {
 					string error = Marshal.PtrToStringAnsi (errorPtr);
 					NativeMethods.java_interop_free (errorPtr);
@@ -166,52 +169,5 @@ partial class NativeMethods {
 		[DllImport (JavaInteropLib, CharSet=CharSet.Ansi, CallingConvention=CallingConvention.Cdecl)]
 		internal static extern int java_interop_jvm_create (out IntPtr javavm, out IntPtr jnienv, ref JavaVMInitArgs args);
 	}
-
-	class DummyValueManager : JniRuntime.JniValueManager {
-
-		public override void WaitForGCBridgeProcessing ()
-		{
-		}
-
-		public override void CollectPeers ()
-		{
-		}
-
-		public override void AddPeer (IJavaPeerable reference)
-		{
-		}
-
-		public override void RemovePeer (IJavaPeerable reference)
-		{
-		}
-
-		public override void FinalizePeer (IJavaPeerable reference)
-		{
-		}
-
-		public override List<JniSurfacedPeerInfo> GetSurfacedPeers ()
-		{
-			return null;
-		}
-
-		public override IJavaPeerable PeekPeer (global::Java.Interop.JniObjectReference reference)
-		{
-			return null;
-		}
-
-		public override void ActivatePeer (IJavaPeerable self, JniObjectReference reference, ConstructorInfo cinfo, object [] argumentValues)
-		{
-		}
-	}
-
-	class DummyObjectReferenceManager : JniRuntime.JniObjectReferenceManager {
-		public override int GlobalReferenceCount {
-			get {return 0;}
-		}
-
-		public override int WeakGlobalReferenceCount {
-			get {return 0;}
-		}
-	}
 }