关于过滤器Filter的介绍和使用详解

<filter>
    <!--设置filter的别名-->
    <filter-name>LoggingFilter</filter-name>
    <!--filter的字节码路径-->
    <filter-class>com.example.LoggingFilter</filter-class>
</filter>

<filter-mapping>
  	<!--使用filter别名所对应的过滤路径，可以有多个-->
    <filter-name>LoggingFilter</filter-name>
     <!--/*表示对所有路径进行过滤-->
    <url-pattern>/*</url-pattern>
    <!--所要过滤的servlet的别名-->
    <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 {
        //向下转型，实现重定向，获得session等功能
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;

        // 检查用户是否已登录
        String user = (String) httpRequest.getSession().getAttribute("username");
        if (user == null) {
            // 用户未登录，重定向到登录页面
            httpResponse.sendRedirect(httpRequest.getContextPath() + "/login");
        } else {
            // 用户已登录，放行请求，注意是调用的FilterChain中的doFilter方法！！
            chain.doFilter(request, response);
        }
    }
}