解決httpclient因為保持永久長連接造成連接吊死的問題

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，但隔了兩分鍾后再請求就會出現read timeout異常。對比請求成功和請求失敗的日志后發現，請求成功的有以下日志“Connection: keep-alive”，“Connection can be kept alive indefinitely”；但請求失敗的卻打印“Shutdown connection”，“Connection discarded”。每次失敗后再請求都會成功。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。后來上網找到keep-alive策略的代碼，添加策略后，問題解決。

ConnectionKeepAliveStrategy keepAliveStrategy = new ConnectionKeepAliveStrategy() {

            @Override
            public long getKeepAliveDuration(HttpResponse response, HttpContext context) {
                HeaderElementIterator it = new BasicHeaderElementIterator(response.headerIterator(HTTP.CONN_KEEP_ALIVE));
                while (it.hasNext()) {
                    HeaderElement he = it.nextElement();
                    String param = he.getName();
                    String value = he.getValue();
                    if (value != null && param.equalsIgnoreCase("timeout")) {
                        try {
                            return Long.parseLong(value) * 1000;
                        }
                        catch (NumberFormatException ignore) {

                        }
                    }
                }
                HttpHost target = (HttpHost) context.getAttribute(HttpClientContext.HTTP_TARGET_HOST);
                if ("bizapi.jd.com ".equalsIgnoreCase(target.getHostName())) {
                    return 60 * 1000;
                }
                else {
                    return 300 * 1000;
                }

   CloseableHttpClient httpClient = httpClientBuilder.setConnectionManager(pollingConnectionManager)
                .setKeepAliveStrategy(keepAliveStrategy).setDefaultRequestConfig(defaultRequestConfig).build();