doFilter和doFilterInternal OncePerRequestFilter 顾名思义，它能够确保在一次请求中只通过一次filter，而不会重复执行，是由Spring提供的抽象类，重写该接口的dofilterinternal方法，用来记录每次请求ip mac sessionid和返回报文日志

OncePerRequestFilter
顾名思义，它能够确保在一次请求中只通过一次filter，而不会重复执行
是由Spring提供的抽象类
https://www.cnblogs.com/sjxbg/p/13475387.html

摘录自：https://www.jianshu.com/p/3d9b4cfe1a62

doFilter是整个过滤器最底层的概念Filter接口中的方法
所有过滤器都要实现Filter

package javax.servlet;
import java.io.IOException;
public interface Filter {

void init(FilterConfig var1) throws ServletException;
</span><span style="color: rgba(0, 0, 255, 1)">void</span> doFilter(ServletRequest var1, ServletResponse var2, FilterChain var3) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> IOException, ServletException;

</span><span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> destroy();

}

而doFilterInternal是OncePerRequestFilter 中的一个抽象方法
阅读源码可知：
OncePerRequestFilter.doFilter方法中通过request.getAttribute判断当前过滤器是否已执行
若未执行过，则调用doFilterInternal方法，交由其子类实现

OncePerRequestFilter
顾名思义，它能够确保在一次请求中只通过一次filter，而不会重复执行
是由Spring提供的抽象类

package org.springframework.session.web.http;
import java.io.IOException;

import javax.servlet.Filter;

import javax.servlet.FilterChain;

import javax.servlet.FilterConfig;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.ServletResponse;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;
abstract class OncePerRequestFilter implements Filter {

public static final String ALREADY_FILTERED_SUFFIX = ".FILTERED";

private String alreadyFilteredAttributeName = this.getClass().getName().concat(".FILTERED");
OncePerRequestFilter() {
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">final</span> <span style="color: rgba(0, 0, 255, 1)">void</span> doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> ServletException, IOException {
    </span><span style="color: rgba(0, 0, 255, 1)">if</span> (request <span style="color: rgba(0, 0, 255, 1)">instanceof</span> HttpServletRequest &amp;&amp; response <span style="color: rgba(0, 0, 255, 1)">instanceof</span><span style="color: rgba(0, 0, 0, 1)"> HttpServletResponse) {
        HttpServletRequest httpRequest </span>=<span style="color: rgba(0, 0, 0, 1)"> (HttpServletRequest)request;
        HttpServletResponse httpResponse </span>=<span style="color: rgba(0, 0, 0, 1)"> (HttpServletResponse)response;
        </span><span style="color: rgba(0, 0, 255, 1)">boolean</span> hasAlreadyFilteredAttribute = request.getAttribute(<span style="color: rgba(0, 0, 255, 1)">this</span>.alreadyFilteredAttributeName) != <span style="color: rgba(0, 0, 255, 1)">null</span><span style="color: rgba(0, 0, 0, 1)">;
        </span><span style="color: rgba(0, 0, 255, 1)">if</span><span style="color: rgba(0, 0, 0, 1)"> (hasAlreadyFilteredAttribute) {
            filterChain.doFilter(request, response);
        } </span><span style="color: rgba(0, 0, 255, 1)">else</span><span style="color: rgba(0, 0, 0, 1)"> {
            request.setAttribute(</span><span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.alreadyFilteredAttributeName, Boolean.TRUE);

            </span><span style="color: rgba(0, 0, 255, 1)">try</span><span style="color: rgba(0, 0, 0, 1)"> {
                </span><span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.doFilterInternal(httpRequest, httpResponse, filterChain);
            } </span><span style="color: rgba(0, 0, 255, 1)">finally</span><span style="color: rgba(0, 0, 0, 1)"> {
                request.removeAttribute(</span><span style="color: rgba(0, 0, 255, 1)">this</span><span style="color: rgba(0, 0, 0, 1)">.alreadyFilteredAttributeName);
            }
        }

    } </span><span style="color: rgba(0, 0, 255, 1)">else</span><span style="color: rgba(0, 0, 0, 1)"> {
        </span><span style="color: rgba(0, 0, 255, 1)">throw</span> <span style="color: rgba(0, 0, 255, 1)">new</span> ServletException("OncePerRequestFilter just supports HTTP requests"<span style="color: rgba(0, 0, 0, 1)">);
    }
}

</span><span style="color: rgba(0, 0, 255, 1)">protected</span> <span style="color: rgba(0, 0, 255, 1)">abstract</span> <span style="color: rgba(0, 0, 255, 1)">void</span> doFilterInternal(HttpServletRequest var1, HttpServletResponse var2, FilterChain var3) <span style="color: rgba(0, 0, 255, 1)">throws</span><span style="color: rgba(0, 0, 0, 1)"> ServletException, IOException;

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> init(FilterConfig config) {
}

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> destroy() {
}

}

常识上都认为，一次请求本来就只filter一次，为什么还要由此特别限定呢？
往往我们的常识和实际的实现并不真的一样，经过一番资料的查阅，此方法是为了兼容不同的web container
也就是说并不是所有的container都入我们期望的只过滤一次，servlet版本不同，执行过程也不同
因此，为了兼容各种不同运行环境和版本，默认filter继承OncePerRequestFilter是一个比较稳妥的选择。