springboot使用httpclient在高并发的情况下会出现Timeout waiting for connection from pool，经测试是因为和有些银行接口建立链接时会超时，有些则不会，可能和双方网络有关系，或者对方服务器限制了连接数，需要设长从连接池中获取到连接的最长时间http.connectionRequestTimeout=10000，之前是500,单位毫秒，另外并发数
http.defaultMaxPerRoute =200，需要设置大一点，之前是20。完整的配置文件参考如下：

#httpclient config
#最大连接数
http.maxTotal = 1000
#并发数
http.defaultMaxPerRoute = 200
#创建连接的最长时间,单位毫秒
http.connectTimeout=10000
#从连接池中获取到连接的最长时间,单位毫秒
http.connectionRequestTimeout=10000
#数据传输的最长时间,单位毫秒
http.socketTimeout=10000
#提交请求前测试连接是否可用
http.staleConnectionCheckEnabled=false

工具类HttpClientUtils.java也给出供参考

/**
 * 
 */
package com.chinapay.utils;

import java.net.URLDecoder;
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.ArrayList;
import java.util.List;
import java.util.Map;

import javax.annotation.PostConstruct;
import javax.net.ssl.SSLContext;

import org.apache.http.NameValuePair;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.client.utils.URIBuilder;
import org.apache.http.conn.ssl.SSLConnectionSocketFactory;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.ssl.SSLContextBuilder;
import org.apache.http.ssl.TrustStrategy;
import org.apache.http.util.EntityUtils;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

import com.figo.util.log.MonitorLogger;

/**
 * @author figo .
 *
 */
@Component
public class HttpClientUtils {
		/**
		 * logger .
		 */
		protected MonitorLogger logger = MonitorLogger.getLogger(getClass());
	    /**
	     * httpClient .
	     */
	    @Autowired
	    private CloseableHttpClient httpClient;
	    /**
	     * config .
	     */
	    @Autowired
	    private RequestConfig config;
	    /**
	     * 信任所有https请求，避免有些银行使用自己的CA生成证书,请求时会发生握手失败的情况 .
	     */
	    @PostConstruct
	    public void init() {
	        try {
	            SSLContext sslContext = new SSLContextBuilder().loadTrustMaterial(null, new TrustStrategy(){
		            //信任所有
		            public boolean isTrusted(X509Certificate[] chain, String authType) throws CertificateException {
		                return true;
		            }
		        }).build();
		        SSLConnectionSocketFactory sslsf = new SSLConnectionSocketFactory(sslContext);
		        httpClient=HttpClients.custom().setSSLSocketFactory(sslsf).build();
			} catch (Exception e) {
				// TODO: handle exception
			}
	    }
	    
	    /**
	     * 不带参数的get请求，如果状态码为200，则返回body，如果不为200，则返回null .
	     * 
	     * @param url .
	     * @return .
	     * @throws Exception .
	     */
	    public String doGet(String url) throws Exception {
	    	logger.info("doGet请求url="+url);
	        // 声明 http get 请求
	        HttpGet httpGet = new HttpGet(url);
	 
	        // 装载配置信息
	        httpGet.setConfig(config);
	 
	        // 发起请求
	        CloseableHttpResponse response = this.httpClient.execute(httpGet);
	 
	        // 判断状态码是否为200
	        if (response.getStatusLine().getStatusCode() == 200) {
	            // 返回响应体的内容
	            return EntityUtils.toString(response.getEntity(), "UTF-8");
	        }
	        return null;
	    }
	 
	    /**
	     * 带参数的get请求，如果状态码为200，则返回body，如果不为200，则返回null .
	     * 
	     * @param url .
	     * @param map .
	     * @return .
	     * @throws Exception .
	     */
	    public String doGet(String url, Map<String, Object> map) throws Exception {
	        URIBuilder uriBuilder = new URIBuilder(url);
	 
	        if (map != null) {
	            // 遍历map,拼接请求参数
	            for (Map.Entry<String, Object> entry : map.entrySet()) {
	                uriBuilder.setParameter(entry.getKey(), entry.getValue().toString());
	            }
	        }
	 
	        // 调用不带参数的get请求
	        return this.doGet(uriBuilder.build().toString());
	 
	    }
	 
	    /**
	     * 带参数的post请求 .
	     * 
	     * @param url .
	     * @param map .
	     * @return .
	     * @throws Exception .
	     */
	    public HttpResult doPost(String url, Map<String, Object> map) throws Exception {
	    	logger.info("doPost请求url="+url);
	        // 声明httpPost请求
	        HttpPost httpPost = new HttpPost(url);
	        // 加入配置信息
	        httpPost.setConfig(config);
	 
	        // 判断map是否为空，不为空则进行遍历，封装from表单对象
	        if (map != null) {
	            List<NameValuePair> list = new ArrayList<NameValuePair>();
	            for (Map.Entry<String, Object> entry : map.entrySet()) {
	                list.add(new BasicNameValuePair(entry.getKey(), (String)entry.getValue()));
	            }
	            // 构造from表单对象
	            UrlEncodedFormEntity urlEncodedFormEntity = new UrlEncodedFormEntity(list, "UTF-8");
	 
	            // 把表单放到post里
	            httpPost.setEntity(urlEncodedFormEntity);
	        }
	 
	        // 发起请求
	        CloseableHttpResponse response = this.httpClient.execute(httpPost);
	        String respBody=EntityUtils.toString(response.getEntity(), "UTF-8");
	        //2判断是否做了urlEncode如果做了，urldecode一下
	        if(UrlEncoderUtils.hasUrlEncoded(respBody))
	        {
	        	logger.info("返回数据做了urlEncode,respBody="+respBody);
	        	respBody=URLDecoder.decode(respBody, "UTF-8");
	        	logger.info("urlDecode之后,respBody="+respBody);
	        }
        	logger.info("返回数据未做urlEncode,respBody="+respBody);

	        return new HttpResult(response.getStatusLine().getStatusCode(), respBody);
	    }
	 
	    /**
	     * 不带参数post请求 .
	     * 
	     * @param url .
	     * @return .
	     * @throws Exception .
	     */
	    public HttpResult doPost(String url) throws Exception {
	        return this.doPost(url, null);
	    }
	   
	    
}