如何使用反射调用内部类的构造函数

Question

I want to create an object of inner class using reflection. This is my class:

//outer class
public final class EMSToCompMessages {
private EMSToCompMessages() {}
//inner class
public static final class CompTypeConfig extends
{
    private CompTypeConfig(com.google.protobuf.GeneratedMessage.Builder<?> builder) {
        super(builder);
        this.unknownFields = builder.getUnknownFields();
    }
    private CompTypeConfig(boolean noInit) { 
        this.unknownFields = com.google.protobuf.UnknownFieldSet.getDefaultInstance(); 
    }

    private static final CompTypeConfig defaultInstance;
    public static CompTypeConfig getDefaultInstance() {
        return defaultInstance;
    }
}

I have to invoke inner class CompTypeConfig using reflection. I am getting following exception:

java.lang.IllegalArgumentException: wrong number of arguments

What am I doing wrong?
I am using following code:

Class<?> loadedMyClass = Class.forName("EMSToCompMessages", true, loader);
        Constructor constructor = loadedMyClass.getDeclaredConstructor();
        constructor.setAccessible(true);
        Object obj = constructor.newInstance();
        Class[] innerClass = loadedMyClass.getDeclaredClasses();
        for (Class<?> getClass : innerClass) {
             Constructor ctor = getClass.getDeclaredConstructors()[0];
                System.out.println(ctor.getName());
                ctor.setAccessible(true);
                Object innerObj = ctor.newInstance(obj);// Exception is coming here
        }             

Answer 1

You try to create a object of CompTypeConfig type which has no a constructor without parameters. CompTypeConfig has two constructors with com.google.protobuf.GeneratedMessage.Builder<?> builder and boolean noInit params. You have to pass some param for them, when create object. Also as I understood, you pass incorrect parameter type for constructor.

Answer 2

You're apparently trying to call

Object innerObj = constructor.newInstance(obj); //you wrote constru, which isn't defined

constructor.newInstance(obj) will try to create an instance of EMSToCompMessages class, not an inner class.

Anyway, the Javadoc says (about the getConstructor method)

Returns a Constructor object that reflects the specified public constructor of the class represented by this Class object. The parameterTypes parameter is an array of Class objects that identify the constructor's formal parameter types, in declared order. If this Class object represents an inner class declared in a non-static context, the formal parameter types include the explicit enclosing instance as the first parameter.

So, you will first have to make your inner class constructors public. Calling setAccessible(true) is not enough in this case.