Add final modifier at runtime in Java

Question

It might sound a bit strange, but is it possible to add the final modifier at runtime?

I have a variable which is marked as public static int/short. At certain point I want to prevent changing its value, and I want to keep its accessibility as a standard static value (ClassName.field).

public class Main {

    private static int a = 0;

    public static void addFinalMod(Field f) {

        Field modifiersField = null;
        try {
            modifiersField = Field.class.getDeclaredField("modifiers");
        }
        catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        modifiersField.setAccessible(true);
        try {
            modifiersField.setInt(f, f.getModifiers() & Modifier.FINAL);
        }
        catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {

        System.out.println(a);
        try {
            Field f = Main.class.getDeclaredField("a");
            addFinalMod(f);
        }
        catch (NoSuchFieldException e) {
            e.printStackTrace();
        }
        a = 10; //I was expecting error/exception here
        System.out.println(a);
    }

Output:

0
10

Answer 1

At certain point I want to prevent changing its value

Do that in application logic. Make the variable only accessible through methods. Keep a flag tracking any change to the variable. After you've applied the change or reached the certain point, raise the flag, and throw an exception for any further attempts at changing the variable.

final is a language keyword/feature for writing your source code and preventing re-assignment of a variable in source code. At runtime, it does (almost) nothing.