springboot+监听器+过滤器+拦截器之回顾

监听器：listener是servlet规范中定义的一种特殊类。用于监听servletContext、HttpSession和servletRequest等域对象的创建和销毁事件。监听域对象的属性发生修改的事件。用于在事件发生前、发生后做一些必要的处理。其主要可用于以下方面：1、统计在线人数和在线用户2、系统启动时加载初始化信息3、统计网站访问量4、记录用户访问路径。

过滤器：Filter是Servlet技术中最实用的技术，Web开发人员通过Filter技术，对web服务器管理的所有web资源：例如Jsp, Servlet, 静态图片文件或静态 html 文件等进行拦截，从而实现一些特殊的功能。例如实现URL级别的权限访问控制、过滤敏感词汇、压缩响应信息等一些高级功能。它主要用于对用户请求进行预处理，也可以对HttpServletResponse进行后处理。使用Filter的完整流程：Filter对用户请求进行预处理，接着将请求交给Servlet进行处理并生成响应，最后Filter再对服务器响应进行后处理。

拦截器：Interceptor 在AOP（Aspect-Oriented Programming）中用于在某个方法或字段被访问之前，进行拦截然后在之前或之后加入某些操作。比如日志，安全等。一般拦截器方法都是通过动态代理的方式实现。可以通过它来进行权限验证，或者判断用户是否登陆，或者是像12306 判断当前时间是否是购票时间。

代码示例：

public class DemoFilter implements Filter {

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println(servletRequest.getParameter("name"));
        HttpServletRequest httpServletRequest = (HttpServletRequest)servletRequest;
        HttpServletResponseWrapper wrapper = new HttpServletResponseWrapper((HttpServletResponse) servletResponse);
        if(httpServletRequest.getRequestURI().indexOf("/index") != -1 ||
                httpServletRequest.getRequestURI().indexOf("/asd") != -1 ||
                httpServletRequest.getRequestURI().indexOf("/online") != -1 ||
                httpServletRequest.getRequestURI().indexOf("/login") != -1
        ) {
            filterChain.doFilter(servletRequest, servletResponse);
        }else {
            wrapper.sendRedirect("/login");
        }
    }

    @Override
    public void destroy() {
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
    }

}

public class DemoHttpSessionListener implements HttpSessionListener {

    public static int online = 0;

    @Override
    public void sessionCreated(HttpSessionEvent se) {
        System.out.println("listener------------------创建session");
        online ++;
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        System.out.println("listener------------------销毁session");

    }

}

public class DemoInterceptor implements HandlerInterceptor {

    //在请求处理之前进行调用
    @Override
    public boolean preHandle(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Object o) throws Exception {
        System.out.println("preHandle被调用");
        Map map =(Map)httpServletRequest.getAttribute(HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE);
        System.out.println(map.get("name"));
        System.out.println(httpServletRequest.getParameter("username"));
        if(map.get("name").equals("zhangsan")) {
            return true;    //如果false，停止流程，api被拦截
        }else {
            PrintWriter printWriter = httpServletResponse.getWriter();
            printWriter.write("please login again!");
            return false;
        }
    }

    @Override
    public void postHandle(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Object o, ModelAndView modelAndView) throws Exception {
        System.out.println("postHandle被调用");
    }

    @Override
    public void afterCompletion(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Object o, Exception e) throws Exception {
        System.out.println("afterCompletion被调用");
    }

}

@Configuration
public class WebMvcConfig implements WebMvcConfigurer {

    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addViewController("/zxc/foo").setViewName("foo");
    }
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new DemoInterceptor())
                .addPathPatterns("/asd/**");
        System.out.println("添加----------------------------interceptor");
    }
    @Bean
    public FilterRegistrationBean filterRegist() {
        FilterRegistrationBean frBean = new FilterRegistrationBean();
        frBean.setFilter(new DemoFilter());
        frBean.addUrlPatterns("/*");
        System.out.println("添加----------------------------filter");
        return frBean;
    }
    @Bean
    public ServletListenerRegistrationBean listenerRegist() {
        ServletListenerRegistrationBean srb = new ServletListenerRegistrationBean();
        srb.setListener(new DemoHttpSessionListener());
        System.out.println("添加----------------------------listener");
        return srb;
    }

}

或通过注解的方式添加，如：

过滤器所在类添加@WebFilter(urlPatterns = "/*", filterName = "reqResFilter")
启动类添加@ServletComponentScan

如：

@WebListener//标识这是servelt的监听器
启动类添加
@ServletComponentScan("com.hzt")//servlet监听器的使用

在springboot 应用中,使用监听器有servlet容器的,有spring的。servlet是：public class DemoServletListener implements ServletContextListener

spring是：public class DemoSpringListener implements ApplicationListener<ApplicationEvent>

看springboot启动时,监听自己的事件,servlet监听事件早于spring的.