varargs and null argument

Question

So, I have a weird issue. This expression: method.invoke(target, null) throws java.lang.IllegalArgumentException: wrong number of arguments

The problem is that this expression is deep inside hibernate (BasicPropertyAccessor$BasicGetter to be precise) and I guess it should work fine (it did until recently)

I debugged this problem in eclipse and indeed, if I (in Display view) call method.invoke(target) or method.invoke(target, (Object[]) null) everything works fine. I'm sure my class has this getter and it has no arguments. So ... the question is, what the hell is going on?!

EDIT:

• method is an instance of java.lang.reflect.Method and points to the said getter
• target is an instance of the class with public X getX() getter
• JDK 1.6.0_31

• code in hibernate (BasicPropertyAccessor:143) :

  public Object get(Object target) throws HibernateException {
      try {
          return method.invoke(target, null);
      }
      catch{ ... }
  }
  

Answer 1

When you pass a null value to a varargs method, it could be interpreted as one of two things:

• A null array
• An array with one element which is null.

Java chooses the former unless the null is explicitly cast to the component type of the varargs method. (It is recommended that you explicitly cast either way for clarity, you'll also get an obnoxious warning in Eclipse if you don't.)

When invoking methods with no arguments, you can either pass Method.invoke an array with 0 elements, no extra arguments (which will result in an empty array), or a null array:

public class MethodInvoke {
    public static void noParams() {
        System.out.println("noParams called");
    }

    public static void main(String[] args) throws NoSuchMethodException,
            SecurityException, IllegalAccessException,
            IllegalArgumentException, InvocationTargetException {
        Method noParams = MethodInvoke.class.getMethod("noParams");
        Object target = null;
        noParams.invoke(target, new Object[0]);
        noParams.invoke(target);
        noParams.invoke(target, null);
        noParams.invoke(target, (Object[]) null);
        noParams.invoke(target, (Object) null); // wrong number of arguments
    }
}

What appears to be happening is that method.invoke(target, null) is being interpreted as method.invoke(target, (Object) null). That is inconsistent with how Java is supposed to behave.

Are you absolutely sure that your method has no parameters?

I also noticed that in newer version of hibernate they explicitly cast that null to Object[].