http为什么对我的Java GET请求的响应的头比Curl的相同URL的头少

Question

我在玩OANDA API。文档说明，对所有成功GET请求的响应都将包含一个ETag头：http://developer.oanda.com/rest-live/development-guide/#etag

我已经确认，当我成功向Curl提出请求时，情况就是这样：

$ curl -i "https://api-sandbox.oanda.com/v1/prices?instruments=EUR_USD"
HTTP/1.1 200 OK
Server: openresty/1.7.0.1
Date: Fri, 26 Dec 2014 17:27:01 GMT
Content-Type: application/json
Content-Length: 139
Connection: keep-alive
ETag: "6a6e222a3f90df8f333ff2a6edfb603e4a354905"

{
        "prices" : [
                {
                        "instrument" : "EUR_USD",
                        "time" : "2014-12-26T17:26:12.336249Z",
                        "bid" : 1.24105,
                        "ask" : 1.24119
                }
        ]
}

但是，当我在HttpClient org.apache.httpcomponents 4.3上有一个成功的请求时，不包括内容长度和ETag头

2014-12-26 17:27:54 INFO  RestClient:91 - Executing request: GET https://api-sandbox.oanda.com/v1/prices?instruments=EUR_USD HTTP/1.1
2014-12-26 17:27:55 INFO  RestClient:103 - Response OK, processing...
2014-12-26 17:27:55 INFO  RestClient:108 - {
    "prices" : [
        {
            "instrument" : "EUR_USD",
            "time" : "2014-12-26T17:28:01.935790Z",
            "bid" : 1.241,
            "ask" : 1.24117
        }
    ]
}
2014-12-26 17:27:55 INFO  RestClient:110 - Headers: [Server: openresty/1.7.0.1, Date: Fri, 26 Dec 2014 17:28:14 GMT, Content-Type: application/json, Transfer-Encoding: chunked, Connection: keep-alive]

当我记录响应Java GET请求收到的所有头时，我缺少ETag和Content-Length头

我想在我的Java代码中使用ETag头，所以请您帮助我理解为什么不返回它

Answer 1

默认情况下，HttpClient启用了压缩，并且在启用压缩时，OANDA API不支持ETag：

Note: ETags can not be used in conjunction with compressed responses.