HTTP Status 500 - Cannot create a session after the response has been committed

[alleni123]

我想通过filter获取session的值来让用户不随意登入网页，

但是当我打开不存在的页面的时候， 就会出现这个错误，
下面是我的代码

filter.java

	public void doFilter(ServletRequest req, ServletResponse res,
			FilterChain chain) throws IOException, ServletException
	{
		// TODO Auto-generated method stub
		System.out.println("before filter");
		
		req.setCharacterEncoding("utf-8");
		
		chain.doFilter(req, res);
	 
	
		req=(HttpServletRequest)req;
		HttpServletRequest request= (HttpServletRequest)req;
		
		HttpSession hs=request.getSession();
		
		Object obj_username= hs.getAttribute("username");
		System.out.println("username="+obj_username);
		System.out.println("requestURI"+request.getRequestURI());
		String reqURI=request.getRequestURI();
		if(obj_username==null&&!reqURI.equals("/ajax1.3/filter/login.jsp")&&!reqURI.equals("/ajax1.3/login.do")){
			//String username=(String)obj_username;
			System.out.println("非法登入");
			req.getRequestDispatcher("/filter/login.jsp");
			 
		}
	 
		 
		
		//req.getRequestDispatcher("filter/filter1.jsp").forward(req, res);
	}

[花谢尊前不敢香]

public void doFilter(ServletRequest req, ServletResponse res,
        FilterChain chain) throws IOException, ServletException
{
    // TODO Auto-generated method stub
    System.out.println("before filter");
     
    req.setCharacterEncoding("utf-8");
     
    chain.doFilter(req, res);
  
 
    req=(HttpServletRequest)req;
    HttpServletRequest request= (HttpServletRequest)req;
     
    HttpSession hs=request.getSession();
     
    Object obj_username= hs.getAttribute("username");
    System.out.println("username="+obj_username);
    System.out.println("requestURI"+request.getRequestURI());
    String reqURI=request.getRequestURI();
    if(obj_username==null&&!reqURI.equals("/ajax1.3/filter/login.jsp")&&!reqURI.equals("/ajax1.3/login.do")){
        //String username=(String)obj_username;
        System.out.println("非法登入");
        req.getRequestDispatcher("/filter/login.jsp");
        return;
          
    }
  
      
     
    //req.getRequestDispatcher("filter/filter1.jsp").forward(req, res);
}

[iLemon]

Filter的配置贴上来啊

[alleni123]

<filter>
  	<filter-name>CharEncodingFilter</filter-name>
  	<filter-class>com.filter.CharEncodingFilter</filter-class>
  </filter>
  <filter-mapping>
  	<filter-name>CharEncodingFilter</filter-name>
  	<url-pattern>/*</url-pattern>
  </filter-mapping>

zhege?

[alleni123]

能不能给我讲解一下啊， 为什么这里加个return就不报错了，

[花谢尊前不敢香]

安全，你可以看下api，经验之谈。

[chengxu2011]

你不加return 你把chain.doFilter(req, res); 去掉试试看报错么