问题表现

使用AWS SDK for Java 1.x访问S3，已经确认文件存在，且具有权限，仍然出现403 Forbidden应答。

解决方法

升级到AWS SDK for Java 2.x。

问题原因

AWS签名机制严格依赖请求的精确路径格式，任何URI的差异（如 // 与 /%2F ）都会导致签名校验失败。AWS SDK for Java 1.x版本中，当资源路径 resourcePath 以斜杠开头时（如 /foo/... ），与 endpoint 拼接后会产生双斜杠 // 。SDK内部会将其转义为 /%2F ，导致实际请求路径与签名计算的路径不一致，触发 SignatureDoesNotMatch 错误。

关键代码分析

在AWS SDK for Java 1.x版本中，当调用 httpRequestFactory.create(request, options) 方法时，URL的生成过程涉及路径拼接逻辑与双斜杠转义机制，具体流程如下：

@Override
   public HttpRequestBase create(final Request<?> request,
                                 final HttpClientSettings settings)
           throws
           FakeIOException {
       URI endpoint = request.getEndpoint();

       String uri;
       // skipAppendUriPath is set for APIs making requests with presigned urls. Otherwise
       // a slash will be appended at the end and the request will fail
       if (request.getOriginalRequest().getRequestClientOptions().isSkipAppendUriPath()) {
           uri = endpoint.toString();
       } else {
           /*
            * HttpClient cannot handle url in pattern of "http://host//path", so we
            * have to escape the double-slash between endpoint and resource-path
            * into "/%2F"
            */
           uri = SdkHttpUtils.appendUri(endpoint.toString(), request.getResourcePath(), true);
       }

       String encodedParams = SdkHttpUtils.encodeParameters(request);

       /*
        * For all non-POST requests, and any POST requests that already have a
        * payload, we put the encoded params directly in the URI, otherwise,
        * we'll put them in the POST request's payload.
        */
       boolean requestHasNoPayload = request.getContent() != null;
       boolean requestIsPost = request.getHttpMethod() == HttpMethodName.POST;
       boolean putParamsInUri = !requestIsPost || requestHasNoPayload;
       if (encodedParams != null && putParamsInUri) {
           uri += "?" + encodedParams;
       }

       final HttpRequestBase base = createApacheRequest(request, uri, encodedParams);
       addHeadersToRequest(base, request);
       addRequestConfig(base, request, settings);

       return base;
   }

假设原始请求参数为

Endpoint: http://127.0.0.1/mybucket
ResourcePath: /foo/bar/... (以斜杠开头)

SdkHttpUtils.appendUri() 将 endpoint 与 resourcePath 拼接为：

http://127.0.0.1/mybucket//foo/bar/...

注意中间的 // 双斜杠。由于第三个参数 escapeDoubleSlash=true ，SDK会将双斜杠转义为 /%2F ：

http://172.24.152.73:80/mybucket/%2Ffoo/bar/...

生成的URI变为转义后的路径，而计算签名时使用的路径是未经转义的原始路径 /mybucket//foo/bar/... ，导致签名不匹配。