[Java.Runtime.Environment] Support .NET Core

Fixes: dotnet#426

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

Add 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".

To make this work, update `JniRuntime.JniValueManager` to have the
following new abstract members:

	partial class JniRuntime {
	    partial class JniValueManager {
	        public abstract bool PeersRequireRelease {get;}
	        public abstract void ReleasePeers ();
	    }
	}

`PeersRequireRelease` shall be `true` when there is no GC bridge.
When this is true, `JniValueManager.CollectPeers()` is a no-op.
If an `IJavaPeerable` must have disposal logic performed, then
`.Dispose()` must be called on that instance.  There is no
finalization integration.

The new `JniValueManager.ReleasePeers()` method:

 1. Releases all GREFs for all held peers.

    This allows Java to collect the Java peers.

 2. Stops referencing all `IJavaPeerable` values.

    This allows the .NET GC to collect the `IJavaPeerable` values.

There is no notification to the `IJavaPeerable` instances that this
has happened.

Update `Java.Interop-Tests.csproj` to define
`NO_MARSHAL_MEMBER_BUILDER_SUPPORT` when building for .NET Core.

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

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

@@ -64,7 +64,7 @@ steps:
   condition: eq('${{ parameters.runNativeTests }}', 'true')
   inputs:
     command: test
-    arguments: bin/Test$(Build.Configuration)/Java.Interop-Tests.dll
+    arguments: bin/Test$(Build.Configuration)$(NetCoreTargetFrameworkPathSuffix)/Java.Interop-Tests.dll
   continueOnError: true
 
 - task: DotNetCoreCLI@2

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 +48,8 @@ public static unsafe JniObjectReference FindClass (string classname)
 					return r;
 				}
 
+				// NativeMethods.java_interop_jnienv_exception_describe (info.EnvironmentPointer);
+
 				NativeMethods.java_interop_jnienv_exception_clear (info.EnvironmentPointer);
 
 				var findClassThrown     = new JniObjectReference (thrown, JniObjectReferenceType.Local);
@@ -167,6 +170,17 @@ 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
+
 				int r   = _RegisterNatives (type, methods, numMethods);
 
 				if (r != 0) {

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

@@ -74,9 +74,12 @@ protected virtual void Dispose (bool disposing)
 				disposed = true;
 			}
 
+			public abstract bool PeersRequireRelease {get;}
+
 			public abstract void WaitForGCBridgeProcessing ();
 
 			public abstract void CollectPeers ();
+			public abstract void ReleasePeers ();
 
 			public abstract void AddPeer (IJavaPeerable value);
 

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;}
-		}
-	}
 }