HttpServletRequest和ServletRequest都是接口
HttpServletRequest繼承自ServletRequest 
HttpServletRequest比ServletRequest多了一些針對于Http協議的方法。 例如：
getHeader()， getMethod() ， getsession() 
	@Override	public void doFilter(ServletRequest request, ServletResponse response,			FilterChain filterChain) throws IOException, ServletException {		// 過濾用戶請求，判斷是否登錄		HttpServletRequest httpServletRequest = (HttpServletRequest)request;		HttpServletResponse httpServletResponse = (HttpServletResponse)response;		httpServletResponse .setContentType("text/html;charset=utf-8");		httpServletRequest.setCharacterEncoding("utf-8");		httpServletResponse.setCharacterEncoding("utf-8");				String username = (String)httpServletRequest.getSession().getAttribute("username");		if (username == null) {			String path = httpServletRequest.getContextPath();			httpServletResponse.sendRedirect(path+"/index.jsp");		}		filterChain.doFilter(httpServletRequest, httpServletResponse);	}