Why Does My Java Filter Log Requests Twice? The Hidden Implicit Registration Explained

While building the DCS_FunTester distributed performance‑testing framework, a custom WrappingFilter was created to log HTTP request parameters and response bodies. After migrating the project, the developer noticed that every request appeared twice in the console output.

Observed Duplicate Logs

The console showed pairs of identical log lines such as:

INFO com.funtester.master.common.wapper.WrappingFilter:65 请求:/m/infos,耗时:1 ms,参数:,响应:{"code":0,"data":{}},来源:0:0:0:0:0:0:0:1

Because the local tests responded within 1–2 ms, the duplication was initially thought to be a manual repeat, but further inspection confirmed a systematic double logging.

Filter Implementation

@Component
@WebFilter(urlPatterns = "/*", filterName = "WrappingFilter")
public class WrappingFilter implements Filter {
    private static Logger logger = LoggerFactory.getLogger(MethodHandles.lookup().lookupClass());

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest req = (HttpServletRequest) request;
        String ipAddress = getIpAddress(req);
        HttpServletResponse resp = (HttpServletResponse) response;
        ResponseWrapper responseWrapper = new ResponseWrapper(resp);
        RequestWrapper requestWrapper = new RequestWrapper(req);
        String url = requestWrapper.getRequestURI();
        String queryArgs = requestWrapper.getQueryString();
        queryArgs = queryArgs == null ? DecodeEncode.unicodeToString(requestWrapper.getBody()) : queryArgs;
        String method = requestWrapper.getMethod();
        long start = Time.getTimeStamp();
        chain.doFilter(requestWrapper == null ? request : requestWrapper, responseWrapper);
        long end = Time.getTimeStamp();
        byte[] bytes = responseWrapper.getContent();
        String respContent = new String(bytes, Constant.UTF_8);
        logger.info("请求:{},耗时:{} ms,参数:{},响应:{},来源:{}", url, end - start, queryArgs, respContent, ipAddress);
        response.getOutputStream().write(respContent.getBytes(Constant.UTF_8));
    }
}

Root Cause Investigation

The developer first suspected the log4j2.xml configuration, but after verifying the file the issue persisted. Repeated service restarts and GET requests still produced paired log entries, with only the response time differing.

Hidden Implicit Filter Registration

Spring Boot automatically registers a filter bean using the class name with a lowercase first letter (e.g., wrappingFilter) when @WebFilter is present. Because the explicit registration used filterName = "WrappingFilter", the framework ended up creating two filter instances: the explicit one and the implicit one. Both processed each request, resulting in duplicate log statements.

Solution

Override the implicit registration by setting the filter name to the lowercase bean name: filterName = "wrappingFilter" Doing so ensures only a single filter instance handles the request, eliminating the duplicated logs.

Additional Note

If the implicit filter is not overridden, its urlPatterns default to /*, meaning every endpoint is processed by the hidden filter as well, which can unintentionally broaden the scope of logging or other filter logic.