解决httpclient的NoHttpResponseException异常

httpclient版本：4.5.2

在项目实际运行中，偶发异常：org.apache.http.NoHttpResponseException。

官网解释是：

In some circumstances, usually when under heavy load, the web server may be able to receive requests but unable to process them. A lack of sufficient resources like worker threads is a good example. This may cause the server to drop the connection to the client without giving any response. HttpClient throws NoHttpResponseException when it encounters such a condition. In most cases it is safe to retry a method that failed with NoHttpResponseException.

意思就是 当服务器端由于负载过大等情况发生时，可能会导致在收到请求后无法处理(比如没有足够的线程资源)，会直接丢弃链接而不进行处理。此时客户端就回报错：NoHttpResponseException。
官方建议出现这种情况时，可以选择重试。但是重试一定要限制重试次数，避免雪崩。

修改方案：

CloseableHttpClient httpClient = HttpClients.custom()
    .setDefaultRequestConfig(config)
    .setConnectionManager(getPoolManager())

    //不使用这种方式，不方便看日志，使用下面自定义的retry
//  .setRetryHandler(new DefaultHttpRequestRetryHandler(3,true))

    .setRetryHandler((exception, executionCount, context) -> {
 if (executionCount > 3) {
 log.warn("Maximum tries reached for client http pool ");
 return false;
 }

 if (exception instanceof NoHttpResponseException //NoHttpResponseException 重试
 || exception instanceof ConnectTimeoutException //连接超时重试
// || exception instanceof SocketTimeoutException //响应超时不重试，避免造成业务数据不一致
 ) {
 log.warn("NoHttpResponseException on " + executionCount + " call");
 return true;
 }
 return false;
 })
 .build();

参考资料：

官方文档
httpclient4.4 出现NoHttpResponseException的异常解决
[get NoHttpResponseException for load testing

](https://stackoverflow.com/questions/10570672/get-nohttpresponseexception-for-load-testing/10680629#10680629)