在spring中，filter都默认继承OncePerRequestFilter，但为什么要这样呢？

OncePerRequestFilter顾名思义，他能够确保在一次请求只通过一次filter，而不需要重复执行。

• AccessLogFilter 核心代码

public class AccessLogFilter extends OncePerRequestFilter {
        private static final Logger LOGGER = LoggerFactory.getLogger(AccessLogFilter .class);


    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
                                    FilterChain filterChain) throws ServletException, IOException {

        Long startTime = System.currentTimeMillis();
        ContentCachingRequestWrapper requestWrapper = new ContentCachingRequestWrapper(request);
        ContentCachingResponseWrapper responseWrapper = new ContentCachingResponseWrapper(response);

        filterChain.doFilter(requestWrapper, responseWrapper);

        String requestPayload = new String(requestWrapper.getContentAsByteArray(),
                request.getCharacterEncoding());
        String responsePayload = IOUtils.toString(responseWrapper.getContentInputStream(), responseWrapper.getCharacterEncoding());
        responseWrapper.copyBodyToResponse();
       
    }

}

其核心思想是实现了OncePerRequestFilter这个虚基类，而这个虚基类是实现了Filter接口，并且要基于SpringMVC框架的，因此这个方法只能适用于SpringMVC工程

核心方法是doFilterInternal

protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
                                FilterChain filterChain) throws ServletException, IOException {

    Long startTime = System.currentTimeMillis();
    ContentCachingRequestWrapper requestWrapper = new ContentCachingRequestWrapper(request);
    ContentCachingResponseWrapper responseWrapper = new ContentCachingResponseWrapper(response);

    filterChain.doFilter(requestWrapper, responseWrapper);

    String requestPayload = getPayLoad(requestWrapper.getContentAsByteArray(),
            request.getCharacterEncoding());
    String responsePayload = getPayLoad(responseWrapper.getContentAsByteArray(),
            response.getCharacterEncoding());
    responseWrapper.copyBodyToResponse();
    StringBuilder logResult = generateResultLogger(logMap);
      if (LOGGER.isInfoEnabled()){
           LOGGER.info(logResult.toString());
      }
}

首先通过

ContentCachingRequestWrapper requestWrapper = new ContentCachingRequestWrapper(request);
ContentCachingResponseWrapper responseWrapper = new ContentCachingResponseWrapper(response);

这两个方式对请求和返回进行包装（让body可以被缓存），这样来解决inputstream不能多次读取的问题

调用

filterChain.doFilter(requestWrapper, responseWrapper);

来执行其他的filter之后

可以通过

    String requestPayload = new String(requestWrapper.getContentAsByteArray(),
                request.getCharacterEncoding());
        String responsePayload = IOUtils.toString(responseWrapper.getContentInputStream(), responseWrapper.getCharacterEncoding());

这种方式取出request和response的内容，最后一定不要忘记

    responseWrapper.copyBodyToResponse();

重新写入response

最后打印出整个log