Java Reflection: Invoke function in a inner static class

Question

My simple School class looks like this:

public class School{
   ...
   public static class Student{
       public void register(){
          ...
       }
   }

}

As you see above, the School class has a inner public static class named Student. There is a method register() defined in Student.

In my main function, I am able to load the inner static Student class by:

Class<?> studentClass = Class.forName("School$Student");

I am wondering, with the above line of code, how could I invoke the register() function in java reflection way.

assylias · Accepted Answer · 2014-01-12 18:33:46Z

2

If you have a valid class, then you can access and invoke its methods with:

Method register = studentClass.getDeclaredMethod("register");
register.invoke(studentClass.newInstance());

answered Jan 12, 2014 at 18:33

assylias

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4 Comments

Mellon Over a year ago

But my Student class is a static class without an empty constructor. The "newInstance()" call will cause exception

assylias Over a year ago

@Mellon does it have any constructors?

Pshemo Over a year ago

@Mellon Every class has its default constructor (even nested/inner ones), unless you create separate one. Also default constructor will have same visibility as class so since Student is public class then you can have access to this constructor from anywhere.

Pshemo Over a year ago

@Mellon Based on example from your question Student doesn't have any constructors so you can use default one with studentClass.newInstance(). But in case it does have constructors you can select one of them with studentClass.getConstructor(parameterTypes) then just invoke its newInstance method with parameters you described in getConstructor like studentClass.getConstructor(int.class, String.class).newInstance(1,"abc");. This should solve your problem with lack of default constructor.

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