Filter学习

Filter功能

拦截jsp、静态图片文件、静态html资源文件
实现URL级别的权限访问控制
过滤敏感词汇
压缩相应信息

Filter的工作原理

看源码可以看出来，Filter接口中有3个方法：init、doFilter、destory
在这里插入图片描述

init方法，初始化Filter，Filter在服务器启动阶段被实例化。并且调用init方法，init方法只执行一次，因为Filter只被实例化一次。

destory方法也好理解，就是销毁Filter，释放内存，在运行结束时调用此方法当。长时间没有用户访问，或者项目重新部署，或者服务器重新启动的时候，会销毁所有的Filter对象，销毁之前会先调用Filter对象的destroy方法，完成销毁前的准备工作。也是只执行一次。

doFilter方法提供给开发者重写过滤逻辑。

多个Filter的实现

public class AFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("输出filter2的init方法");

    }
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        System.out.println("输出filter2的doFilter方法之前");
        filterChain.doFilter(request, response);
        System.out.println("输出filter2的doFilter方法之后");

    }
    @Override
    public void destroy() {
        System.out.println("输出filter2的destroy方法");
    }
}

public class BFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        System.out.println("输出filter1的init方法");

    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletResponse response = (HttpServletResponse) servletResponse;
        HttpServletRequest request = (HttpServletRequest) servletRequest;
        System.out.println("输出filter1的doFilter方法之前");
        filterChain.doFilter(request, response);
        System.out.println("输出filter1的doFilter方法之后");

    }

    @Override
    public void destroy() {
        System.out.println("输出filter1的destroy方法");
    }
}

配置Filter的过滤顺序，通过setOrder方法执行FilterChain的顺序，数字越小，优先级越高。

@Configuration
public class WebConfig {
    @Bean
    public FilterRegistrationBean filter1() {

        FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean();
        AFilter aFilter = new AFilter();

        filterRegistrationBean.setFilter(aFilter);
        filterRegistrationBean.setOrder(1);

        return filterRegistrationBean;

    }

    @Bean
    public FilterRegistrationBean filter2() {

        FilterRegistrationBean filterRegistrationBean = new FilterRegistrationBean();
        BFilter bFilter = new BFilter();

        filterRegistrationBean.setFilter(bFilter);
        filterRegistrationBean.setOrder(2);

        return filterRegistrationBean;

    }
}