Cannot change static final field using java reflection?

You can avoid compiler inlining by making the value a result of a method call, even a dummy one.

public class Main {
    // value is not known at compile time, so not inlined
    public static final boolean FLAG = Boolean.parseBoolean("false");

    static void setFinalStatic(Class clazz, String fieldName, Object newValue) throws NoSuchFieldException, IllegalAccessException {
        Field field = clazz.getDeclaredField(fieldName);
        field.setAccessible(true);
        Field modifiers = field.getClass().getDeclaredField("modifiers");
        modifiers.setAccessible(true);
        modifiers.setInt(field, field.getModifiers() & ~Modifier.FINAL);
        field.set(null, newValue);
    }

    public static void main(String... args) throws Exception {
        System.out.printf("Everything is %s%n", FLAG);
        setFinalStatic(Main.class, "FLAG", true);
        System.out.printf("Everything is %s%n", FLAG);
    }
}

prints

Everything is false
Everything is true

When accessing primitive static final fields, the Java compiler will assume that the value is a constant and inline the value instead of generating code that accesses the field. This means that the compiler will replace with the reference to the FALSE field with the value false. If you use reflection to access the field, you will see that the value of the field has actually changed.

This will not work for non-primitive fields, as the value of an object reference can not be inlined at compile time.