public class User {
    private String name;
    public User() {
        
    }
}

  protected final void injectField(Object source, String fieldName, Object fieldValue) {
        try {
            Field field = source.getClass().getDeclaredField(fieldName);
            field.setAccessible(true);
            field.set(source, fieldValue);
        } catch (Exception e) {
            throw new IllegalStateException(e);
        }
    }