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一個系統上線，肯定會或多或少的存在異常情況。為了更快更好的排雷，記錄請求參數和響應結果是非常必要的。所以，Nginx 和 Tomcat 之類的 web 服務器，都提供了訪問日志，可以幫助我們記錄一些請求信息。

本文是在我們的應用中，定義一個Filter來實現記錄請求參數和響應結果的功能。

有一定經驗的都知道，如果我們在Filter中讀取了HttpServletRequest或者HttpServletResponse的流，就沒有辦法再次讀取了，這樣就會造成請求異常。所以，我們需要借助 Spring 提供的ContentCachingRequestWrapper和ContentCachingRequestWrapper實現數據流的重復讀取。

定義 Filter

通常來說，我們自定義的Filter是實現Filter接口，然后寫一些邏輯，但是既然是在 Spring 中，那就借助 Spring 的一些特性。在我們的實現中，要繼承OncePerRequestFilter實現我們的自定義實現。

從類名上推斷，OncePerRequestFilter是每次請求只執行一次，但是，難道Filter在一次請求中還會執行多次嗎？Spring 官方也是給出定義這個類的原因：

Filter base class that aims to guarantee a single execution per request dispatch, on any servlet container. It provides a doFilterInternal(javax.servlet.http.HttpServletRequest, javax.servlet.http.HttpServletResponse, javax.servlet.FilterChain) method with HttpServletRequest and HttpServletResponse arguments.

As of Servlet 3.0, a filter may be invoked as part of a REQUEST or ASYNC dispatches that occur in separate threads. A filter can be configured in web.xml whether it should be involved in async dispatches. However, in some cases servlet containers assume different default configuration. Therefore sub-classes can override the method shouldNotFilterAsyncDispatch() to declare statically if they should indeed be invoked, once, during both types of dispatches in order to provide thread initialization, logging, security, and so on. This mechanism complements and does not replace the need to configure a filter in web.xml with dispatcher types.

Subclasses may use isAsyncDispatch(HttpServletRequest) to determine when a filter is invoked as part of an async dispatch, and use isAsyncStarted(HttpServletRequest) to determine when the request has been placed in async mode and therefore the current dispatch won't be the last one for the given request.

Yet another dispatch type that also occurs in its own thread is ERROR. Subclasses can override shouldNotFilterErrorDispatch() if they wish to declare statically if they should be invoked once during error dispatches.

也就是說，Spring 是為了兼容不同的 Web 容器，所以定義了只會執行一次的OncePerRequestFilter。

接下來開始定義我們的Filter類：

public class AccessLogFilter extends OncePerRequestFilter {
  //... 這里有一些必要的屬性

  @Override
  protected void doFilterInternal(final HttpServletRequest request,
                                  final HttpServletResponse response,
                                  final FilterChain filterChain)
          throws ServletException, IOException {
      // 如果是被排除的 uri，不記錄 access_log
      if (matchExclude(request.getRequestURI())) {
          filterChain.doFilter(request, response);
          return;
      }

      final String requestMethod = request.getMethod();
      final boolean shouldWrapMethod = StringUtils.equalsIgnoreCase(requestMethod, HttpMethod.PUT.name())
              || StringUtils.equalsIgnoreCase(requestMethod, HttpMethod.POST.name());

      final boolean isFirstRequest = !isAsyncDispatch(request);

      final boolean shouldWrapRequest = isFirstRequest && !(request instanceof ContentCachingRequestWrapper) && shouldWrapMethod;
      final HttpServletRequest requestToUse = shouldWrapRequest ? new ContentCachingRequestWrapper(request) : request;

      final boolean shouldWrapResponse = !(response instanceof ContentCachingResponseWrapper) && shouldWrapMethod;
      final HttpServletResponse responseToUse = shouldWrapResponse ? new ContentCachingResponseWrapper(response) : response;

      final long startTime = System.currentTimeMillis();
      Throwable t = null;
      try {
          filterChain.doFilter(requestToUse, responseToUse);
      } catch (Exception e) {
          t = e;
          throw e;
      } finally {
          doSaveAccessLog(requestToUse, responseToUse, System.currentTimeMillis() - startTime, t);
      }
  }

  // ... 這里是一些必要的方法

這段代碼就是整個邏輯的核心所在，其他的內容從源碼中找到。

分析

這個代碼中，整體的邏輯沒有特別復雜的地方，只需要注意幾個關鍵點就可以了。

默認的HttpServletRequest和HttpServletResponse中的流被讀取一次之后，再次讀取會失敗，所以要使用ContentCachingRequestWrapper和ContentCachingResponseWrapper進行包裝，實現重復讀取。
既然我們可以自定義Filter，那我們依賴的組件中也可能會自定義Filter，更有可能已經對請求和響應對象進行過封裝，所以，一定要先進行一步判斷。也就是request instanceof ContentCachingRequestWrapper和response instanceof ContentCachingResponseWrapper。

只要注意了這兩點，剩下的都是這個邏輯的細化實現。

運行

接下來我們就運行一遍，看看結果。先定義幾種不同的請求：普通 get 請求、普通 post 請求、上傳文件、下載文件，這四個接口幾乎可以覆蓋絕大部分場景。（因為都是比較簡單的寫法，源碼就不贅述了，可以從文末的源碼中找到）

先啟動項目，然后借助 IDEA 的 http 請求工具：

###普通 get 請求
GET http://localhost:8080/index/get?name=howard

###普通 post 請求
POST http://localhost:8080/index/post
Content-Type: application/json

{"name":"howard"}

###上傳文件
POST http://localhost:8080/index/upload
Content-Type: multipart/form-data; boundary=WebAppBoundary

--WebAppBoundary
Content-Disposition: form-data; name="file"; filename="history.txt"
Content-Type: multipart/form-data
</Users/liuxh/history.txt
--WebAppBoundary--

###下載文件
GET http://localhost:8080/index/download

再看看打印的日志：

2021-04-29 19:44:57.495 INFO 83448 --- [nio-8080-exec-1] c.h.d.s.filter.AccessLogFilter : time=44ms,ip=127.0.0.1,uri=/index/get,headers=[host:localhost:8080,connection:Keep-Alive,user-agent:Apache-HttpClient/4.5.12 (Java/11.0.7),accept-encoding:gzip,deflate],status=200,requestContentType=null,responseContentType=text/plain;charset=UTF-8,params=name=howard,request=,response=
2021-04-29 19:44:57.551 INFO 83448 --- [nio-8080-exec-2] c.h.d.s.filter.AccessLogFilter : time=36ms,ip=127.0.0.1,uri=/index/post,headers=[content-type:application/json,content-length:17,host:localhost:8080,connection:Keep-Alive,user-agent:Apache-HttpClient/4.5.12 (Java/11.0.7),accept-encoding:gzip,deflate],status=200,requestContentType=application/json,responseContentType=application/json,params=,request={"name":"howard"},response={"name":"howard","timestamp":"1619696697540"}
2021-04-29 19:44:57.585 INFO 83448 --- [nio-8080-exec-3] c.h.d.s.filter.AccessLogFilter : time=20ms,ip=127.0.0.1,uri=/index/upload,headers=[content-type:multipart/form-data; boundary=WebAppBoundary,content-length:232,host:localhost:8080,connection:Keep-Alive,user-agent:Apache-HttpClient/4.5.12 (Java/11.0.7),accept-encoding:gzip,deflate],status=200,requestContentType=multipart/form-data; boundary=WebAppBoundary,responseContentType=application/json,params=,request=,response={"contentLength":"0","contentType":"multipart/form-data"}
2021-04-29 19:44:57.626 INFO 83448 --- [nio-8080-exec-4] c.h.d.s.filter.AccessLogFilter : time=27ms,ip=127.0.0.1,uri=/index/download,headers=[host:localhost:8080,connection:Keep-Alive,user-agent:Apache-HttpClient/4.5.12 (Java/11.0.7),accept-encoding:gzip,deflate],status=200,requestContentType=null,responseContentType=application/octet-stream;charset=utf-8,params=,request=,response=