@WebFilter("/*")//訪問所有資源之前，都會(huì)執(zhí)行該過濾器
public class FilterDemo1 implements Filter {
       @Override
       public void init(FilterConfig filterConfig) throws ServletException {
            //過濾器初始化的方法，一般用于申請(qǐng)資源
   // init:在服務(wù)器啟動(dòng)后，會(huì)創(chuàng)建Filter對(duì)象，然后調(diào)用init方法。只執(zhí)行一次。用于加載資源
       }
            
       @Override
        public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
   //執(zhí)行攔截的動(dòng)作，具體的攔截邏輯寫在這里  
   //doFilter:每一次請(qǐng)求被攔截資源時(shí)，會(huì)執(zhí)行。執(zhí)行多次         
   System.out.println("filterDemo1被執(zhí)行了....");
            
           //放行
           filterChain.doFilter(servletRequest,servletResponse);
            
       }
            
       @Override
       public void destroy() {
            //過濾器銷毀的方法，一般用于釋放資源
   //destroy:在服務(wù)器關(guān)閉后，F(xiàn)ilter對(duì)象被銷毀。如果服務(wù)器是正常關(guān)閉，則會(huì)執(zhí)行destroy方法。只執(zhí)行一次。用于釋放資源
       }
}

* 方法：
  * void contextDestroyed(ServletContextEvent sce) ：ServletContext對(duì)象被銷毀之前會(huì)調(diào)用該方法
  * void contextInitialized(ServletContextEvent sce) ：ServletContext對(duì)象創(chuàng)建后會(huì)調(diào)用該方法

@WebFilter("*.jsp")
public class FilterTest1 implements Filter {
 public void destroy() {
 }
 
 public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
  //判斷是否已經(jīng)登錄了，如果沒有登錄就需要跳轉(zhuǎn)到登錄界面，但是如果訪問的資源路徑本來
  //就是登錄頁面，那么就不需要攔截了，這里需要進(jìn)行判斷排除
 
  //記得強(qiáng)制轉(zhuǎn)型
  HttpServletRequest request = (HttpServletRequest)req ;
 
  //獲取session對(duì)象，看看對(duì)應(yīng)的屬性是否有值
  HttpSession session = request.getSession();
  Object login = session.getAttribute("login");
 
  //獲取url路徑，看看里面是否是登錄資源相關(guān)的
  String requestURI = request.getRequestURI();
 
  //在過濾掉登錄相關(guān)時(shí)，一定要排除一些js，css文件
  if (login != null || requestURI.contains("/longin.jsp") || requestURI.contains("/css/") ||
    requestURI.contains("/js/") || requestURI.contains("/fonts/") ) {
   chain.doFilter(req, resp);
  } else {
   request.getRequestDispatcher("/longin.jsp").forward(request, resp);
  }
 }
 
 public void init(FilterConfig config) throws ServletException {
 
 }
 
}