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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：
@Overridepublic HttpRequestBase create(final Request<?> request,final HttpClientSettings settings)throwsFakeIOException {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 failif (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。