session过期，ajax请求处理

session会话过期，如果是请求。可以直接定位到页面。如果是ajax请求。无法跳转到页面。

可以通过如下方式实现，在Response的输出流里面，向前端写一段html代码来实现。

public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object arg2) throws Exception {

String url=request.getRequestURL().toString();

System.out.println("当前访问地址："+url);

//登录页面不用检测，不然会出现Cannot forward after response has been committed（request多次提交）
if(url.indexOf("userLogin.do")>=0){
return true;
}

HttpSession session=request.getSession();
SessionInfo sessionInfo=(SessionInfo)session.getAttribute("sessionInfo");

if(sessionInfo!=null){
return true;
}

toAlert(response);

return false;
}

//前台弹出alert框
public void toAlert( HttpServletResponse response){

try {
response.setContentType("text/html;charset=UTF-8");
response.setCharacterEncoding("UTF-8");

OutputStreamWriter out=new OutputStreamWriter(response.getOutputStream());

String msg="由于您长时间没有操作，session已过期，请重新登录！";
msg=new String(msg.getBytes("UTF-8"));

out.write("<meta http-equiv='Content-Type' content='text/html';charset='UTF-8'>");
out.write("<script>");
out.write("alert('"+msg+"');");
out.write("top.location.href = '/EasyUI/login.jsp'; ");
out.write("</script>");
out.flush();
out.close();

} catch (IOException e) {
e.printStackTrace();
}
}