gistfile1.txt

public class TestClass {
    // So assumming internal classes are closed of with Java 16
    
    // For Java [9;15] 
    
    // This will work because all packages are open to the unnamed module (stuff on the classpath)
    // It will print a warning because you use method.setAccessible on a class in a package that will 
    // be closed of in Java 16
    public static void main1(String[] args) throws Exception {
        Module unnamed = TestClass.class.getModule();
        Class<?> cls = Class.forName("sun.nio.ch.NativeThread");
        java.lang.reflect.Method method = cls.getDeclaredMethod("signal", long.class);
        assert cls.getModule().isOpen("sun.nio.ch", unnamed); //true
        method.setAccessible(true);
        method.invoke(null, 123);
    }
     
    
    // This will work because all packages are open to the unnamed module (stuff on the classpath)
    // It will not print a warning, because warnings are only printed when you use reflection
    public static void main2(String[] args) throws Throwable {
        Module unnamed = TestClass.class.getModule();
        Class<?> cls = Class.forName("sun.nio.ch.NativeThread");
        java.lang.reflect.Method method = cls.getDeclaredMethod("signal", long.class);
        assert cls.getModule().isOpen("sun.nio.ch", unnamed); //true
        MethodHandle mh = MethodHandles.lookup().unreflect(method);
        mh.invoke(123);
    }
    
        
    // Java [16;], you can simulate this with --illegal-access=deny
    
    // This will fail with java.lang.reflect.InaccessibleObjectException
    // Because access to the sun.nui.ch package has been closed effectively with Java 16
    public static void main3(String[] args) throws Exception {
        Module unnamed = TestClass.class.getModule();
        Class<?> cls = Class.forName("sun.nio.ch.NativeThread");
        java.lang.reflect.Method method = cls.getDeclaredMethod("signal", long.class);
        assert !cls.getModule().isOpen("sun.nio.ch", unnamed); //false
        method.setAccessible(true);
    }

    // This will fail with java.lang.IllegalAccessException
    // Because access to the sun.nui.ch package has been closed effectively with Java 16
    public static void main4(String[] args) throws Exception {
        Module unnamed = TestClass.class.getModule();
        Class<?> cls = Class.forName("sun.nio.ch.NativeThread");
        java.lang.reflect.Method method = cls.getDeclaredMethod("signal", long.class);
        assert !cls.getModule().isOpen("sun.nio.ch", unnamed); //false
        MethodHandles.lookup().unreflect(method);
    }
    
    // Calling cls.getModule().addOpens("sun.nio.ch", TestClass.class.getModule());
    // or --add-opens java.base/sun.nio.ch=ALL-UNNAME on the first two examples.
    // Have no other program effect that disabling the reflection warning because 
    // all classes are opened by default to any class on the classpath.
    
    // If you use --add-opens java.base/sun.nio.ch=ALL-UNNAME on the last two examples
    // with Java16 they will no longer fail, because they are actually closedv
}