doFilter和doFilterInternal

摘錄自：https://www.jianshu.com/p/3d9b4cfe1a62

doFilter是整個過濾器最底層的概念Filter接口中的方法
所有過濾器都要實現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 中的一個抽象方法
閱讀源碼可知：
OncePerRequestFilter.doFilter方法中通過request.getAttribute判斷當前過濾器是否已執行
若未執行過，則調用doFilterInternal方法，交由其子類實現

OncePerRequestFilter
顧名思義，它能夠確保在一次請求中只通過一次filter，而不會重復執行
是由Spring提供的抽象類

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() {
    }
}

常識上都認為，一次請求本來就只filter一次，為什么還要由此特別限定呢？ 往往我們的常識和實際的實現並不真的一樣，經過一番資料的查閱，此方法是為了兼容不同的web container 也就是說並不是所有的container都入我們期望的只過濾一次，servlet版本不同，執行過程也不同 因此，為了兼容各種不同運行環境和版本，默認filter繼承OncePerRequestFilter是一個比較穩妥的選擇。