Switch `JniEnvironment.FindClass` to use Class.forName instead of ClassLoader.loadClass

[garuma]

See equivalent commit in monodroid: https://github.com/xamarin/monodroid/commit/ed984a3a0bfbe71f6499a7855c647b5bc2b28466

[jonpryor]

The commit message of xamarin/monodroid@ed984a provides some useful context:

Previously, JNIEnv.FindClass(string) would defer to ClassLoader.loadClass() to load a Java type.

Because loading some classes on Java is a bit of an arcane art, you have to use a special syntax for primitive types. This syntax seemed to have been supported just fine on (Dalvik|ART) when using the loadClass method but on a desktop java like the Oracle VM this is not the case anymore.

Instead, passing by the Class.forName() static method seems to handle such case that involves primitive types so this commit supply the necessary handles to JNIEnv from native and switch the FindClass method to use them.

For instance, a type that used to trip the runtime beforehand with a ClassNotFoundException but doesn't
after this commit is [I (an array of primitive Java integer).

Which raises an interesting aside: JNIEnv::FindClass() is used to obtain the java.lang.Class for everything, including arrays of types such as "array of int" aka [I and "array of String" aka [Ljava/lang/String;. How does this work with Class.forName(), which doesn't accept JNI / syntax? (It's apparently always worked, I just didn't understand the array side of things.)

Simply replacing / with . is enough: Class.forName("[Ljava.lang.String;") works! This is likewise true for nested types: leave the $ along, replace / with .: Class.forName("[Ljava.lang.Thread$UncaughtExceptionHandler;") works!

[jonpryor]

This is a fun demo app:

class Example {
    public static void main(String[] args) throws Throwable {
        String[] types = {
            "java.lang.String", "java/lang/String",
            "[Ljava/lang/String;", "[Ljava.lang.String;", "[java.lang.String", "java.lang.String[]",
            "I", "int", "[I", "[int", "int[]",
            "[Ljava.lang.Thread$UncaughtExceptionHandler;",
            // "int", "long", "float", "double", "char", "byte", "short", "boolean"
        };
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        for (String type : types) {
            try {
                Class<?> c = Class.forName(type);
                System.out.println("Class.forName(" + type + ") found: " + c.getName()); // prints "int", "long", etc.
            } catch (ClassNotFoundException e) {
                System.out.println("error: Class.forName(" + type + ") failed: " + e.toString());
            }
            try {
                Class<?> c = cl.loadClass(type);
                System.out.println("ClassLoader.loadClass(" + type + ") found: " + c.getName()); // prints "int", "long", etc.
            } catch (ClassNotFoundException e) {
                System.out.println("error: ClassLoader.loadClass(" + type + ") failed: " + e.toString());
            }
        }
        Class as = String[].class;
        System.out.println("String[].class.getName()=" + as.getName()); // prints "[Ljava.lang.String;"
        System.out.println("Thread.UncaughtExceptionHandler[].class.getCanonicalName()=" + Thread.UncaughtExceptionHandler[].class.getName()); // prints "java.lang.String[]"
    }
}

output:

Class.forName(java.lang.String) found: java.lang.String
ClassLoader.loadClass(java.lang.String) found: java.lang.String
error: Class.forName(java/lang/String) failed: java.lang.ClassNotFoundException: java/lang/String
error: ClassLoader.loadClass(java/lang/String) failed: java.lang.ClassNotFoundException: java/lang/String
error: Class.forName([Ljava/lang/String;) failed: java.lang.ClassNotFoundException: [Ljava/lang/String;
error: ClassLoader.loadClass([Ljava/lang/String;) failed: java.lang.ClassNotFoundException: [Ljava/lang/String;
Class.forName([Ljava.lang.String;) found: [Ljava.lang.String;
error: ClassLoader.loadClass([Ljava.lang.String;) failed: java.lang.ClassNotFoundException: [Ljava.lang.String;
error: Class.forName([java.lang.String) failed: java.lang.ClassNotFoundException: [java/lang/String
error: ClassLoader.loadClass([java.lang.String) failed: java.lang.ClassNotFoundException: [java.lang.String
error: Class.forName(java.lang.String[]) failed: java.lang.ClassNotFoundException: java/lang/String[]
error: ClassLoader.loadClass(java.lang.String[]) failed: java.lang.ClassNotFoundException: java.lang.String[]
error: Class.forName(I) failed: java.lang.ClassNotFoundException: I
error: ClassLoader.loadClass(I) failed: java.lang.ClassNotFoundException: I
error: Class.forName(int) failed: java.lang.ClassNotFoundException: int
error: ClassLoader.loadClass(int) failed: java.lang.ClassNotFoundException: int
Class.forName([I) found: [I
error: ClassLoader.loadClass([I) failed: java.lang.ClassNotFoundException: [I
error: Class.forName([int) failed: java.lang.ClassNotFoundException: [int
error: ClassLoader.loadClass([int) failed: java.lang.ClassNotFoundException: [int
error: Class.forName(int[]) failed: java.lang.ClassNotFoundException: int[]
error: ClassLoader.loadClass(int[]) failed: java.lang.ClassNotFoundException: int[]
Class.forName([Ljava.lang.Thread$UncaughtExceptionHandler;) found: [Ljava.lang.Thread$UncaughtExceptionHandler;
error: ClassLoader.loadClass([Ljava.lang.Thread$UncaughtExceptionHandler;) failed: java.lang.ClassNotFoundException: [Ljava.lang.Thread$UncaughtExceptionHandler;
String[].class.getName()=[Ljava.lang.String;
Thread.UncaughtExceptionHandler[].class.getCanonicalName()=[Ljava.lang.Thread$UncaughtExceptionHandler;

The TL;DR:

ClassLoader.loadClass() doesn't like arrays at all, at least not with the same syntax that Class.forName() supports. Which is why xamarin/monodroid@ed984a3a was needed in the first place.