doFilter和doFilterInternal

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;

    void doFilter(ServletRequest var1, ServletResponse var2, FilterChain var3) throws IOException, ServletException;

    void 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() {
    }

    public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws ServletException, IOException {
        if (request instanceof HttpServletRequest && response instanceof HttpServletResponse) {
            HttpServletRequest httpRequest = (HttpServletRequest)request;
            HttpServletResponse httpResponse = (HttpServletResponse)response;
            boolean hasAlreadyFilteredAttribute = request.getAttribute(this.alreadyFilteredAttributeName) != null;
            if (hasAlreadyFilteredAttribute) {
                filterChain.doFilter(request, response);
            } else {
                request.setAttribute(this.alreadyFilteredAttributeName, Boolean.TRUE);

                try {
                    this.doFilterInternal(httpRequest, httpResponse, filterChain);
                } finally {
                    request.removeAttribute(this.alreadyFilteredAttributeName);
                }
            }

        } else {
            throw new ServletException("OncePerRequestFilter just supports HTTP requests");
        }
    }

    protected abstract void doFilterInternal(HttpServletRequest var1, HttpServletResponse var2, FilterChain var3) throws ServletException, IOException;

    public void init(FilterConfig config) {
    }

    public void 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。