<filter>
    <!--設(shè)置filter的別名-->
    <filter-name>LoggingFilter</filter-name>
    <!--filter的字節(jié)碼路徑-->
    <filter-class>com.example.LoggingFilter</filter-class>
</filter>

<filter-mapping>
  	<!--使用filter別名所對應(yīng)的過濾路徑，可以有多個-->
    <filter-name>LoggingFilter</filter-name>
     <!--/*表示對所有路徑進行過濾-->
    <url-pattern>/*</url-pattern>
    <!--所要過濾的servlet的別名-->
    <servlet-name>servlet1</servlet-name>
</filter-mapping>

@WebFilter(
        filterName = "loggingFilter",
        urlPatterns = {"/servlet1","*.html"},
        servletNames = {"servlet1","Servlet2"}
)

@WebFilter("/*")
public class CheckLoginFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        //向下轉(zhuǎn)型，實現(xiàn)重定向，獲得session等功能
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;

        // 檢查用戶是否已登錄
        String user = (String) httpRequest.getSession().getAttribute("username");
        if (user == null) {
            // 用戶未登錄，重定向到登錄頁面
            httpResponse.sendRedirect(httpRequest.getContextPath() + "/login");
        } else {
            // 用戶已登錄，放行請求，注意是調(diào)用的FilterChain中的doFilter方法??！
            chain.doFilter(request, response);
        }
    }
}