Question

Spring newbie here.

I observed that Spring was able to instantiate a non-public class (i.e. a class with default visibility) that I had defined. Can anyone tell me how Spring achieves this? Why is this allowed ?

Answer 1

OK, here’s how they do it. Take this sample class:

package hidden;  

class YouCantInstantiateMe{

    private YouCantInstantiateMe(){
        System.out.println("Damn, you did it!!!");
    }

}

The above is a package-private class with a private constructor in a different package, but we’ll still instantiate it:

Code (run from a class in a different package):

public static void main(final String[] args) throws Exception{
    Class<?> clazz = Class.forName("hidden.YouCantInstantiateMe");
                                            // load class by name
    Constructor<?> defaultConstructor = clazz.getDeclaredConstructor();
    // getDeclaredConstructor(paramTypes) finds constructors with
    // all visibility levels, we supply no param types to get the default
    // constructor
    defaultConstructor.setAccessible(true); // set visibility to public
    defaultConstructor.newInstance();       // instantiate the class
}

Output:

Damn, you did it!!!

---

Of course what Spring does is much more complex, because they also deal with Constructor Injection etc., but this is how to instantiate invisible classes (or invisible constructors).