为什么说过滤器Filter由回调实现

回调(个人理解):类A中任意方法中调用了类B的方法,而B方法又调用了A中某回调方法;

加了接口的说法:回调接口的实现类A里调用了B的方法，而Ｂ的方法里又调用了回调接口的回调方法,具体执行方法已经由实现类Ａ重写(加接口是为了不把代码写死,方便复用)

现实场景下:A约B一起吃饭,B有事要做,等B事情做完,让B通知一下A(回调方法就是去吃饭)

回调推荐https://blog.youkuaiyun.com/lemon_tree12138/article/details/51231841
过滤器：推荐文章https://blog.youkuaiyun.com/beliefer/article/details/51894747

    FilterChain是回调接口,doFilter(request,response)是回调方法,ApplicationFilterChain是实现类,里面能得到实现了Filter接口的实现类xxxFilter（平常我们自己写过滤器就是实现Filte接口并重写doFilter）,在doFilter(request,response)中执行中了某个Filter实现类的doFilter(request, response, this)(this指的当前ApplicationFilterChain类)方法,在这个方法执行某些处理后需要回调ApplicationFilterChain.doFilter(request,response),这个回调会执行filter链中的下一个循环到结束

public interface FilterChain {
    public void doFilter(ServletRequest request, ServletResponse response);
}

public final class ApplicationFilterChain implements FilterChain {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response) {
            ...//省略
            internalDoFilter(request,response);
    }

    private void internalDoFilter(ServletRequest request, ServletResponse response){
    if (pos < n) {
            //获取第pos个filter    
            ApplicationFilterConfig filterConfig = filters[pos++];        
            Filter filter = filterConfig.getFilter();
            ...
            filter.doFilter(request, response, this);
        }
    }

}

public class xxFilter implements Filter{
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain){
        ...
        filterChain.doFilter(request, response);
    }
}

因为ApplicationFilterChain是在回调方法里调用的各个filter的doFilter方法,而各个filter里又回调了ApplicationFilterChain的回调方法,所以会循环执行;有些文章是把Filter当回调接口,ApplicationFilterChain调用xxFilter的doFilter()当回调,理论上应该不算错(存疑),但实际场景是先执行ApplicationFilterChain,所以以此为主方法