goja-web/src/main/java/goja/mvc/Controller.java
protected <U extends Model> Optional<AppUser<U>> getPrincipal() {
        if (Securitys.isLogin()) {
            final AppUser<U> appUser = Securitys.getLogin();
            return Optional.of(appUser);
        }
        return Optional.absent();
    }
protected <U extends Model> Optional<AppUser<U>> getPrincipal() {
        if (Securitys.isLogin()) {
            final AppUser<U> appUser = Securitys.getLogin();
            return Optional.of(appUser);
        }
        return Optional.absent();
    }
The current Shiro login user. <p/> If it opens the secruity function can call this method to
obtain the logged in user.

@param <U> Generic parameter.
@return Shiro login user.