Use Class.ForName(String, boolean, ClassLoader)

Context: dotnet/android#9931
Context: dotnet/android#9931 (comment)

The attempted integration test crashed!

	F mono-rt : [ERROR] FATAL UNHANDLED EXCEPTION: Java.Lang.ClassNotFoundException: Didn't find class "crc641855b07eca6dcc03.MyCb" on path: DexPathList[[directory "."],nativeLibraryDirectories=[/system/lib64, /system/product/lib64, /system/lib64, /system/product/lib64]]
	F mono-rt :    at Java.Interop.JniEnvironment.Types.TryFindClass(String classname, Boolean throwOnError)
	F mono-rt :    at Java.Interop.JniEnvironment.Types.FindClass(String classname)
	F mono-rt :    at Java.Interop.JniType..ctor(String classname)
	F mono-rt :    at Java.Interop.JniPeerMembers.JniInstanceMethods..ctor(Type declaringType)
	F mono-rt :    at Java.Interop.JniPeerMembers.JniInstanceMethods.GetConstructorsForType(Type declaringType)
	F mono-rt :    at Java.Interop.JniPeerMembers.JniInstanceMethods.StartCreateInstance(String constructorSignature, Type declaringType, JniArgumentValue* parameters)
	F mono-rt :    at Java.Lang.Object..ctor()
	F mono-rt :    at Java.InteropTests.MyCb..ctor()
	F mono-rt :    at Java.InteropTests.JnienvTest.<>c__DisplayClass29_0.<DoNotLeakWeakReferences>b__1()
	F mono-rt :    at System.Threading.Thread.StartHelper.Callback(Object state)
	F mono-rt :    at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
	F mono-rt : --- End of stack trace from previous location ---
	F mono-rt :    at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
	F mono-rt :   --- End of managed Java.Lang.ClassNotFoundException stack trace ---
	F mono-rt : java.lang.ClassNotFoundException: Didn't find class "crc641855b07eca6dcc03.MyCb" on path: DexPathList[[directory "."],nativeLibraryDirectories=[/system/lib64, /system/product/lib64, /system/lib64, /system/product/lib64]]
	F mono-rt :      at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:196)
	F mono-rt :      at java.lang.ClassLoader.loadClass(ClassLoader.java:379)
	F mono-rt :      at java.lang.ClassLoader.loadClass(ClassLoader.java:312)

Perhaps unsurprisingly, we need to provide the system ClassLoader
in order to load types from outside the default "classpath", which is
why we used `ClassLoader.loadClass()` in the first place.

Instead of `Class.forName()`, use the
[`Class.forName(String name, boolean initialize, ClassLoader loader)`][0]
overload, so that we can provide a `ClassLoader` instance.

This *should* fix the crash.

[0]: https://developer.android.com/reference/java/lang/Class#forName(java.lang.String,%20boolean,%20java.lang.ClassLoader)

Author: jonpryor

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

@@ -32,7 +32,7 @@ static Types ()
 				using (var t = new JniType ("java/lang/Class")) {
 					Class_reference = t.PeerReference.NewGlobalRef ();
 					Class_getName   = t.GetInstanceMethod ("getName", "()Ljava/lang/String;");
-					Class_forName   = t.GetStaticMethod ("forName", "(Ljava/lang/String;)Ljava/lang/Class;");
+					Class_forName   = t.GetStaticMethod ("forName", "(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;");
 				}
 			}
 
@@ -63,8 +63,10 @@ static unsafe JniObjectReference TryFindClass (string classname, bool throwOnErr
 
 				if (Class_forName.IsValid) {
 					var java    = info.ToJavaName (classname);
-					var __args  = stackalloc JniArgumentValue [1];
+					var __args  = stackalloc JniArgumentValue [3];
 					__args [0]  = new JniArgumentValue (java);
+					__args [1]  = new JniArgumentValue (true);  // initialize the class
+					__args [2]  = new JniArgumentValue (info.Runtime.ClassLoader);
 
 					c = RawCallStaticObjectMethodA (info.EnvironmentPointer, out thrown, Class_reference.Handle, Class_forName.ID, (IntPtr) __args);
 					JniObjectReference.Dispose (ref java);

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

@@ -58,7 +58,6 @@ public partial class CreationOptions {
 			public  IntPtr                      InvocationPointer           {get; set;}
 			public  IntPtr                      EnvironmentPointer          {get; set;}
 
-			[Obsolete ("No longer supported; Class.forName() is now used instead")]
 			public  JniObjectReference          ClassLoader                 {get; set;}
 			[Obsolete ("No longer supported; Class.forName() is now used instead")]
 			public  IntPtr                      ClassLoader_LoadClass_id    {get; set;}
@@ -154,7 +153,6 @@ public static void SetCurrent (JniRuntime newCurrent)
 		bool                                            DestroyRuntimeOnDispose;
 
 		internal    JniObjectReference                  ClassLoader;
-		internal    JniMethodInfo?                      ClassLoader_LoadClass;
 
 		public  IntPtr                                  InvocationPointer   {get; private set;}
 
@@ -207,6 +205,18 @@ protected JniRuntime (CreationOptions options)
 			var env     = new JniEnvironmentInfo (envp, this);
 			JniEnvironment.SetEnvironmentInfo (env);
 
+			ClassLoader = options.ClassLoader;
+			if (ClassLoader.IsValid) {
+				ClassLoader = ClassLoader.NewGlobalRef ();
+			} else {
+				using (var t = new JniType ("java/lang/ClassLoader")) {
+					var m       = t.GetStaticMethod ("getSystemClassLoader", "()Ljava/lang/ClassLoader;");
+					var loader  = JniEnvironment.StaticMethods.CallStaticObjectMethod (t.PeerReference, m);
+					ClassLoader = loader.NewGlobalRef ();
+					JniObjectReference.Dispose (ref loader);
+				}
+			}
+
 #if !XA_JI_EXCLUDE
 			ManagedPeer.Init ();
 #endif  // !XA_JI_EXCLUDE