解决httpclient因为保持永久长连接造成连接吊死的问题

httpclient使用了连接池，如果没有设置keep-alive策略，PoolingHttpClientConnectionManager会默认使用永久连接。

最近在调用京东api时，发现一个请求开始是可以获取到数据的，但隔了两分钟后再请求就会出现read timeout异常。对比请求成功和请求失败的日志后发现，请求成功的有以下日志“Connection: keep-alive”，“Connection can be kept alive indefinitely”；但请求失败的却打印“Shutdown connection”，“Connection discarded”。每次失败后再请求都会成功。因此推测中应该是对方服务器端禁止长连接，当连接到达一定时间会就会断开。后来上网找到keep-alive策略的代码，添加策略后，问题解决。

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();