httpclient使用了連接池，如果沒有設置keep-alive策略，PoolingHttpClientConnectionManager會默認使用永久連接。

最近在調用京東api時，發現一個請求開始是可以獲取到數據的，但隔了兩分鐘後再請求就會出現read timeout異常。對比請求成功和請求失敗的日誌後發現，請求成功的有以下日誌“Connection: keep-alive”，“Connection can be kept alive indefinitely”；但請求失敗的卻打印“Shutdown connection”，“Connection discarded”。每次失敗後再請求都會成功。因此推測中應該是對方服務器端禁止長連接，當連接到達一定時間會就會斷開。後來上網找到keep-alive策略的代碼，添加策略後，問題解決。

 1 ConnectionKeepAliveStrategy keepAliveStrategy = new ConnectionKeepAliveStrategy() {
 2 
 3             @Override
 4             public long getKeepAliveDuration(HttpResponse response, HttpContext context) {
 5                 HeaderElementIterator it = new BasicHeaderElementIterator(response.headerIterator(HTTP.CONN_KEEP_ALIVE));
 
 6                 while (it.hasNext()) {
 7                     HeaderElement he = it.nextElement();
 8                     String param = he.getName();
 9                     String value = he.getValue();
10                     if (value != null && param.equalsIgnoreCase("timeout")) {
 
11                         try {
12                             return Long.parseLong(value) * 1000;
13                         }
14                         catch (NumberFormatException ignore) {
15 
16                         }
17                     }
18                 }
19                 HttpHost target = (HttpHost) context.getAttribute(HttpClientContext.HTTP_TARGET_HOST);
20                 if ("bizapi.jd.com ".equalsIgnoreCase(target.getHostName())) {
21                     return 60 * 1000;
22                 }
23                 else {
24                     return 300 * 1000;
25                 }
26 
27    CloseableHttpClient httpClient = httpClientBuilder.setConnectionManager(pollingConnectionManager)
28                 .setKeepAliveStrategy(keepAliveStrategy).setDefaultRequestConfig(defaultRequestConfig).build();

解決httpclient因為保持永久長連接造成連接吊死的問題