1.给后台加了登录过滤器
2. 登录后出现异常：
java.lang.IllegalStateException: Cannot forward after response has been committed
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:328)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:318)
at com.zuikc.bookstore.admin.web.filter.AdminFilter.doFilter(AdminFilter.java:32)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:240)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:212)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:94)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:141
这个异常意味response已经被提交，不能再跳转了。这个错误的出现是因为response多次提交或在跳转到页面后仍有跳转请求。
3.好像过滤器有问题：
异常信息中有这么一句： at com.zuikc.bookstore.admin.web.filter.AdminFilter.doFilter(AdminFilter.java:32)
4.过滤器代码如下：
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
/*
* 从session中获取admin
* 如果admin!=null,放行
* 如果为null，保存异常信息到request域，转发到login.jsp
*/
HttpServletRequest httpRequest = (HttpServletRequest) request;
Admin admin = (Admin) httpRequest.getSession().getAttribute("session_admin");
if(admin != null)chain.doFilter(request, response);
request.setAttribute("msg", "未登录用户不能访问访问此页面，请先登录");
httpRequest.getRequestDispatcher("/adminjsps/login.jsp").forward(httpRequest, response);
}
5.解决方法：
if判断这里出了问题，上面的代码导致放行后后面的代码还是执行了，即又放行又转发。所有才会提示：Cannot forward after response has been committed
修改代码如下：
或者：
