[Java.Interop] Use Class.forName() as fallback to load Java classes (#1326)

Fixes: #23
Context: dotnet/android@aba2726
Context: b4d44e4
Context: https://github.com/xamarin/monodroid/commit/ed984a3a0bfbe71f6499a7855c647b5bc2b28466
Context: dotnet/android#7616
Commit b4d44e4 noted:
> Android is..."special", in that not all threads get the same
> ClassLoader behavior. Specifically, *managed* threads --
> System.Threading.Thread instances -- get a different ClassLoader than
> the main/UI thread on Android. (Untested, but the ClassLoader *may*
> behave sanely if you use a java.lang.Thread instance instead. But who
> wants to use java.lang.Thread instances...?)
dotnet/android#7616 provided additional context: `JNIEnv::FindClass()`
behavior *is* tied to the thread that calls it, and one of the knock-on
effects is that `Java.Lang.JavaSystem.LoadLibrary("MyLib")` doesn't
work properly when invoked from a new `System.Threading.Thread` thread.
(This is still the case, by the way.)
Which brings us to xamarin/mondroid@ed984a3a, which updated then
Xamarin.Android to use
[`Class.forName(String name, boolean initialize, ClassLoader loader)`][0]
instead of [`ClassLoader.loadClass(String)`][1], because the "real"
JDK cannot use `ClassLoader.loadClass(String)` to load array types:
Class c1 = Class.forName("[Ljava.lang.String;"); // works
Class c2 = ClassLoader.getSystemClassLoader()
.loadClass("[Ljava.lang.String;"); // throws java.lang.ClassNotFoundException
Class c3 = Class.forName("[I"); // works; array of int
Class c4 = ClassLoader.getSystemClassLoader()
.loadClass("[I"); // throws java.lang.ClassNotFoundException
Using `ClassLoader.loadClass(String)` to load array types works on
Android, presumably as an undocumented implementation detail, but as
xamarin/monodroid@ed984a3a was trying to get things working within
the (now dead) Android Designer -- which ran using a Desktop JDK --
Android-specific extensions were not available.
Update `JniEnvironment.Types` to use `Class.forName(String)` instead
of `ClassLoader.loadClass(String)` to load Java classes, as a fallback
for when `JNIEnv::FindClass()` fails to find the class. `[Obsolete]` the `JniRuntime.CreationOptions.ClassLoader_LoadClass_id`
property as it is no longer used.
[0]: https://developer.android.com/reference/java/lang/Class#forName(java.lang.String,%20boolean,%20java.lang.ClassLoader)
[1]: https://developer.android.com/reference/java/lang/ClassLoader#loadClass(java.lang.String)

Author: jonpryor

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

@@ -24,11 +24,15 @@ static partial class Types {
 };
 
 staticreadonlyJniMethodInfoClass_getName;
+staticreadonlyJniMethodInfoClass_forName;
+staticreadonlyJniObjectReferenceClass_reference;
 
 staticTypes()
 {
 using(vart=newJniType("java/lang/Class")){
+Class_reference=t.PeerReference.NewGlobalRef();
 Class_getName=t.GetInstanceMethod("getName","()Ljava/lang/String;");
+Class_forName=t.GetStaticMethod("forName","(Ljava/lang/String;ZLjava/lang/ClassLoader;)Ljava/lang/Class;");
 }
 }
 
@@ -57,12 +61,14 @@ static unsafe JniObjectReference TryFindClass (string classname, bool throwOnErr
 LogCreateLocalRef(findClassThrown);
 varpendingException=info.Runtime.GetExceptionForThrowable(reffindClassThrown,JniObjectReferenceOptions.CopyAndDispose);
 
-if(info.Runtime.ClassLoader_LoadClass!=null){
+if(Class_forName.IsValid){
 varjava=info.ToJavaName(classname);
-var__args=stackallocJniArgumentValue[1];
+var__args=stackallocJniArgumentValue[3];
 __args[0]=newJniArgumentValue(java);
+__args[1]=newJniArgumentValue(true);// initialize the class
+__args[2]=newJniArgumentValue(info.Runtime.ClassLoader);
 
-c=RawCallObjectMethodA(info.EnvironmentPointer,outthrown,info.Runtime.ClassLoader.Handle,info.Runtime.ClassLoader_LoadClass.ID,(IntPtr)__args);
+c=RawCallStaticObjectMethodA(info.EnvironmentPointer,outthrown,Class_reference.Handle,Class_forName.ID,(IntPtr)__args);
 JniObjectReference.Dispose(refjava);
 if(thrown==IntPtr.Zero){
 (pendingExceptionasIJavaPeerable)?.Dispose();
@@ -169,12 +175,12 @@ static void RawExceptionClear (IntPtr env)
 #endif // FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
 }
 
-staticIntPtrRawCallObjectMethodA(IntPtrenv,outIntPtrthrown,IntPtrinstance,IntPtrjmethodID,IntPtrargs)
+staticIntPtrRawCallStaticObjectMethodA(IntPtrenv,outIntPtrthrown,IntPtrclazz,IntPtrjmethodID,IntPtrargs)
 {
 #if FEATURE_JNIENVIRONMENT_JI_PINVOKES
-returnNativeMethods.java_interop_jnienv_call_object_method_a(env,outthrown,instance,jmethodID,args);
+returnNativeMethods.java_interop_jnienv_call_static_object_method_a(env,outthrown,clazz,instance,jmethodID,args);
 #elif FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS
-varr=JniNativeMethods.CallObjectMethodA(env,instance,jmethodID,args);
+varr=JniNativeMethods.CallStaticObjectMethodA(env,clazz,jmethodID,args);
 thrown=JniNativeMethods.ExceptionOccurred(env);
 returnr;
 #else // FEATURE_JNIENVIRONMENT_JI_FUNCTION_POINTERS

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

@@ -59,6 +59,7 @@ public partial class CreationOptions {
 publicIntPtrEnvironmentPointer{get;set;}
 
 publicJniObjectReferenceClassLoader{get;set;}
+[Obsolete("No longer supported; Class.forName() is now used instead")]
 publicIntPtrClassLoader_LoadClass_id{get;set;}
 
 publicJniObjectReferenceManager?ObjectReferenceManager{get;set;}
@@ -152,7 +153,6 @@ public static void SetCurrent (JniRuntime newCurrent)
 boolDestroyRuntimeOnDispose;
 
 internalJniObjectReferenceClassLoader;
-internalJniMethodInfo?ClassLoader_LoadClass;
 
 publicIntPtrInvocationPointer{get;privateset;}
 
@@ -206,25 +206,14 @@ protected JniRuntime (CreationOptions options)
 JniEnvironment.SetEnvironmentInfo(env);
 
 ClassLoader=options.ClassLoader;
-if(options.ClassLoader_LoadClass_id!=IntPtr.Zero){
-ClassLoader_LoadClass=newJniMethodInfo(options.ClassLoader_LoadClass_id,isStatic:false);
-}
-
 if(ClassLoader.IsValid){
 ClassLoader=ClassLoader.NewGlobalRef();
-}
-
-if(!ClassLoader.IsValid||ClassLoader_LoadClass==null){
+}else{
 using(vart=newJniType("java/lang/ClassLoader")){
-if(!ClassLoader.IsValid){
-varm=t.GetStaticMethod("getSystemClassLoader","()Ljava/lang/ClassLoader;");
-varloader=JniEnvironment.StaticMethods.CallStaticObjectMethod(t.PeerReference,m);
-ClassLoader=loader.NewGlobalRef();
-JniObjectReference.Dispose(refloader);
-}
-if(ClassLoader_LoadClass==null){
-ClassLoader_LoadClass=t.GetInstanceMethod("loadClass","(Ljava/lang/String;)Ljava/lang/Class;");
-}
+varm=t.GetStaticMethod("getSystemClassLoader","()Ljava/lang/ClassLoader;");
+varloader=JniEnvironment.StaticMethods.CallStaticObjectMethod(t.PeerReference,m);
+ClassLoader=loader.NewGlobalRef();
+JniObjectReference.Dispose(refloader);
 }
 }