How to invoke Kotlin object method using reflection in Java?

Question

I want to invoke setApiHelper method in the below code using java reflection. How can I do so?

object PlayerUtils {
    private var apiHelper: String? = null
    fun setApiHelper(apiHelper: String) {
        this.apiHelper = apiHelper
        println(apiHelper)
    }

    fun getApiHelper(): String? {
        return this.apiHelper
    }
}

My Implementation

private static void testingPlayerUtils() {
        try {
            Class<?> cls = Class.forName("reflection.PlayerUtils");
            cls.newInstance();
            Method method = cls.getDeclaredMethod("setApiHelper");
            method.invoke(cls.newInstance(), "TESTING");
        } catch (ClassNotFoundException | NoSuchMethodException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        }
    }

this gives an error

java.lang.IllegalAccessException: Class TestingReflection2 can not access a member of class reflection.PlayerUtils with modifiers "private"
    at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:102)
    at java.lang.Class.newInstance(Class.java:436)
    at TestingReflection2.testingPlayerUtils(TestingReflection2.java:20)
    at TestingReflection2.main(TestingReflection2.java:14)

Answer 1

Usually when you want to access object declared in Kotlin using Java code, you can do like below code snippet:

PlayerUtils.INSTANCE.setApiHelper("");
//or
PlayerUtils.INSTANCE.getApiHelper();

Now that being said, in order to access methods of PlayerUtils in Java using reflection, you'll need to access it's static member called INSTANCE first.

And you can do that by using Field from Class declaration, something like below:

Class<?> cls = Class.forName("reflection.PlayerUtils");
Object instance = cls.getField("INSTANCE");
Method method = cls.getDeclaredMethod("setApiHelper");
method.invoke(instance, "TESTING");

Refer here for detailed info.

Answer 2

You need to set accessible true for the method as below -

 Class<?> cls = Class.forName("reflection.PlayerUtils");
 cls.newInstance();
 Method method = cls.getDeclaredMethod("setApiHelper");
 method.setAccessible(true);
 method.invoke(cls.newInstance(), "TESTING");