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Apache HttpClient是由Apache软件基金会维护的一款开源HTTP客户端库，对比最基础的 HttpURLConnection 而言,它的优势时支持连接池管理，拦截器（Interceptor）机制，同步/异步请求支持等能力。
在使用这个组件时，需要格外注意连接池相关的配置，否则容易踩坑。
踩坑案例
问题
一个对外转发请求的项目,部分渠道的对接使用了HttpClient来实现的，由于业务访问量不大，上线后只部署了几台服务，前段时间三方平台曝光量增加，导致业务量比平时多了一倍，随后这个服务出现了问题：
从APM监控上看，上游调用该服务的请求有大量的超时，但是该服务只是请求转发而已，从http组件监控看该服务调三方接口的请求的RT也有明显增大，还有一部分请求出现了超时。但是该服务的CPU JVM资源指标都比较正常，而且项目中有多个平台的对接业务，目前只有这个平台的请求是有问题的。
排查过程
起初怀疑是网络抖动，但是找运维看了说网络延迟是正常的没有抖动，即便如此，还是觉得是网络不好导致请求hold住了(从apm看请求超时报错时间都比较久应该是配置的不太合理)，顺着思路想着先扩容试试吧，扩容后发现起初是有效果的，但是过了一会儿又开始出现超时的请求了。
查到这感觉不像是网络原因了，只能翻代码了,随后翻了一下代码现状和关于HttpClient的连接池配置资料，找到了问题的原因......
问题1：HttpClient的连接池只设置了全局最大连接MaxTotal，但是未设置单路由的最大连接defaultMaxPerRoute(默认只有2)。在并发情况下，同路由下的没有空闲连接就会导致一直阻塞等待，直到获取到连接才能进行请求，所以超时的请求其实是在等待获取连接，并不是等待三方响应超时。
问题2：只有这个渠道的对接用了HttpClient，其他渠道直接用RestTemplate实现的。这就可以解释为什么只有这个平台的请求是有问题了
下面整理一下httpClient相关的配置，避免以后踩坑。
import lombok.extern.slf4j.Slf4j;
import org.apache.commons.lang3.StringUtils;
import org.apache.http.HttpEntity;
import org.apache.http.client.ResponseHandler;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.methods.HttpUriRequest;
import org.apache.http.config.Registry;
import org.apache.http.config.RegistryBuilder;
import org.apache.http.conn.socket.ConnectionSocketFactory;
import org.apache.http.conn.socket.PlainConnectionSocketFactory;
import org.apache.http.conn.ssl.NoopHostnameVerifier;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.conn.ssl.TrustStrategy;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.ssl.SSLContextBuilder;
import org.apache.http.util.EntityUtils;
import tech.yummy.common.caja.tools.utils.BizThreadPoolUtils;import java.io.IOException;
import java.security.KeyManagementException;
import java.security.KeyStoreException;
import java.security.NoSuchAlgorithmException;
import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;
import java.util.HashMap;
import java.util.Map;@Slf4j
public class HttpClientUtils {private static CloseableHttpClient httpClient;private static PoolingHttpClientConnectionManager clientConnectionManager;private static SSLConnectionSocketFactory sslConnectionSocketFactory;private static SSLContextBuilder sslContextBuilder;private static String encoding;private static final String HTTP = "http";private static final String HTTPS = "https";static {try {encoding = "UTF-8";Registry<ConnectionSocketFactory> registry = initConnectionSocketFactoryRegistry();clientConnectionManager = new PoolingHttpClientConnectionManager(registry);// 创建连接池（默认 maxTotal=20, defaultMaxPerRoute=2 validateAfterInactivity=2000）clientConnectionManager = new PoolingHttpClientConnectionManager(registry);//覆盖默认配置 全局最大连接数 500clientConnectionManager.setMaxTotal(500);//覆盖默认配置 每路由默认连接数 50clientConnectionManager.setDefaultMaxPerRoute(50);//覆盖默认配置 连接在池中闲置多久后需要验证其有效性 5秒clientConnectionManager.setValidateAfterInactivity(5000);RequestConfig requestConfig = RequestConfig.custom()//从连接池获取连接的超时时间 - 连接池满时会阻塞等待,超时拿不到链接会抛 ConnectionPoolTimeoutException.setConnectionRequestTimeout(2000)//建立TCP连接的超时时间（握手）.setConnectTimeout(1000)//数据传输的间隔超时时间.setSocketTimeout(3000).build();httpClient = HttpClientBuilder.create().useSystemProperties().setConnectionManager(clientConnectionManager).setDefaultRequestConfig(requestConfig).build();} catch (Exception e) {log.error("初始化httpclient 配置执行异常", e);}}private static Registry<ConnectionSocketFactory> initConnectionSocketFactoryRegistry() throws NoSuchAlgorithmException, KeyStoreException, KeyManagementException {sslContextBuilder = new SSLContextBuilder();// 全部信任 不做身份鉴定sslContextBuilder.loadTrustMaterial(null, new TrustStrategy() {@Overridepublic boolean isTrusted(X509Certificate[] x509Certificates, String s) throws CertificateException {return true;}});sslConnectionSocketFactory = new SSLConnectionSocketFactory(sslContextBuilder.build(),null,null,NoopHostnameVerifier.INSTANCE);Registry<ConnectionSocketFactory> registry = RegistryBuilder.<ConnectionSocketFactory>create().register(HTTP, new PlainConnectionSocketFactory()).register(HTTPS, sslConnectionSocketFactory).build();return registry;}private static String request(HttpUriRequest request) throws IOException {ResponseHandler<String> responseHandler = response -> {int status = response.getStatusLine().getStatusCode();log.debug("response status:{}", status);HttpEntity entity = response.getEntity();return entity != null ? EntityUtils.toString(entity, encoding) : null;};log.debug("httpClient request:{} {}", request.getMethod(), request.getURI());String responseBody = httpClient.execute(request, responseHandler);log.debug("httpClient response:{}", responseBody);return responseBody;}//======================================================GET Start====================================================================public String get(String url, String params) throws IOException {if (!StringUtils.isBlank(params)) {url = url.concat("?").concat(params);}HttpGet httpGet = new HttpGet(url);return request(httpGet);}public static String get(String url, String params, Map<String, String> headers) throws IOException {if (!StringUtils.isBlank(params)) {url = url.concat("?").concat(params);}HttpGet httpGet = new HttpGet(url);if (headers != null) {headers.forEach(httpGet::setHeader);}return request(httpGet);}//======================================================GET End====================================================================//======================================================POST Start====================================================================/*** POST -> JSON 通用*/public static String post(String url, String rawContents, Map<String, String> headers) throws IOException {HttpPost httpPost = new HttpPost(url);HttpEntity entity = new StringEntity(rawContents, encoding);httpPost.setEntity(entity);if (headers != null) {headers.forEach(httpPost::setHeader);}return request(httpPost);}//======================================================POST End====================================================================public static void main(String[] args) throws IOException, InterruptedException {String url = "https://www.test.com";Map<String, String> headers = new HashMap<>();headers.put("Content-Type", "application/json");headers.put("charset", "UTF-8");headers.put("token", "ST-10384-3D0AQqHag-QqmKby4Upyu6YdB4f45fcc9-9qjdd");//参数String postParam = "{\"pageNum\":1,\"pageSize\":10}";for(int i = 0;i < 7;i++){String finalUrl = url;BizThreadPoolUtils.submit(() ->{long start = System.currentTimeMillis();try {String postResponse = HttpClientUtils.post(finalUrl, postParam, headers);log.info("请求耗时:{},返回值：{}",(System.currentTimeMillis() - start),postResponse);} catch (Exception e) {log.error("耗时：" + (System.currentTimeMillis() - start) + ",异常：" + e);}});}Thread.sleep(10000);String params = "page=1&size=10";String getResponse = HttpClientUtils.get(url, params, headers);System.out.println(getResponse);}}