在受包保护的类的ctor上调用newInstance

Question

I have a package-protected (default) class

package a;    
class Foo {}

So implicitly, it'll have a package-protected constructor.

Now if I have a reference to Constructor<Foo> fooCtor = ... ;

Should I not get some kind of exception if I try to invoke fooCtor.newInstance() outside of package a ?

Answer 1

You can't create an instance of the class unless you change the constructor's accessibility. If your Foo class is as following:

package org.visib.a;
class Foo {
  @Override
  public String toString() {
    return "This is a foo";
  }
}

You can try instantiating a Foo but will get an IllegalAccessException , eg, try:

package org.visib.b;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public class FooCreator {
  public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InvocationTargetException, InstantiationException {
    Class<?> clazz = Class.forName("org.visib.a.Foo");
    for (Constructor<?> c : clazz.getDeclaredConstructors()) {
      System.out.println("Found constructor: " + c);
      Object foo = c.newInstance();
      System.out.println(foo);
    }
  }
}

However, you can change the access modifier of the constructor. Just add the following line in the for loop:

c.setAccessible(true);