doFilter抛出空指针异常

Question

我有简单的过滤器SimpleFilter

public void doFilter(ServletRequest arg0, ServletResponse arg1,
        FilterChain arg2) throws IOException, ServletException {

    HttpServletRequest request = (HttpServletRequest) arg0;
    HttpServletResponse response = (HttpServletResponse) arg1;
    HttpSession session = request.getSession();

    Object o = null;
    o = session.getAttribute("enterAttr");

    if (o == null) {
        response.sendRedirect("/reservation/main.jsp");
        return;
    }

    boolean bool = (boolean) o;

    if (bool == true)
        arg2.doFilter(arg0, arg1);
    else
        response.sendRedirect("/reservation/main.jsp");
}

当会话属性main.jsp为false或null时，我想将用户重定向到enterAttr。 但我总是得到NullPointerException

java.lang.NullPointerException
com.project.reservation.web.filter.SimpleFilter.doFilter(SimpleFilter.java:48)

有什么问题？我认为当对象o为空时它不会继续。但它仍然引发了这个例外。