IllegalArgumentException: wrong number of arguments in Java Constructor.newInstance()

Question

Consider the following code,

public class StartUp {

    public StartUp(String[] test){}

    public static void main(String[] args) throws Exception{
        Constructor cd = StartUp.class.getConstructor(String[].class);
        System.out.println(cd.newInstance(new String[]{}).toString());
    }
}

What's wrong with it? I get the following Exception:

Exception in thread "main" java.lang.IllegalArgumentException: wrong number of arguments at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27) at java.lang.reflect.Constructor.newInstance(Constructor.java:513) at com.test.StartUp.main(StartUp.java:10)

Answer 1

Your String[] is being implicitly converted to Object[] and taken as an empty array of arguments, instead of as a single argument which is an empty array. Try this:

Object arg = new String[0];
System.out.println(cd.newInstance(arg).toString());

or

System.out.println(cd.newInstance(((Object)new String[0]).toString());

or even avoid the compiler having to create the array for you at all:

System.out.println(cd.newInstance(new Object[] { new String[0] }).toString());

Basically this is a mixture of varargs handling and array covariance :(

Answer 2

You could use dp4j verbose option to answer your question, and get the correct reflection code that you need:

$ vim ReflectedAcces.java
 class StartUp {

    private StartUp(String[] test){}

}

public class ReflectedAcces{

        @com.dp4j.InjectReflection
    public static void main(String[] args) throws Exception{
                StartUp su = new StartUp(new String[]{});
                System.out.println(su.toString());
    }

}
 

$ javac -cp dp4j-1.0-jar-with-dependencies.jar -Averbose=true ReflectedAcces.java 
...
ReflectedAcces.java:10: Note: 
class StartUp {
    
    private StartUp(String[] test) {
    }
}
public class ReflectedAcces {
    
    public ReflectedAcces() {
        super();
    }
    
    @com.dp4j.InjectReflection()
    public static void main(String[] args) java.lang.ClassNotFoundException, java.lang.NoSuchFieldException, java.lang.IllegalAccessException, java.lang.NoSuchMethodException, java.lang.reflect.InvocationTargetException, java.lang.InstantiationException {
        final java.lang.reflect.Constructor startUpConstructor = Class.forName("StartUp").getDeclaredConstructor(.java.lang.String[].class);
        startUpConstructor.setAccessible(true);
        StartUp su = (.StartUp)startUpConstructor.newInstance(new .java.lang.Object[1][]{new String[]{}});
        System.out.println(su.toString());
    }
}