依赖

        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpcore</artifactId>
            <version>4.4.15</version>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpclient</artifactId>
            <version>4.5.13</version>
        </dependency>
        <dependency>
            <groupId>org.apache.httpcomponents</groupId>
            <artifactId>httpmime</artifactId>
            <version>4.5.13</version>
        </dependency>

HttpClientUtil.java

package ppp;

import lombok.extern.slf4j.Slf4j; 
import org.apache.http.HeaderElement;
import org.apache.http.HeaderElementIterator;
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.ConnectionKeepAliveStrategy;
import org.apache.http.entity.StringEntity;
import org.apache.http.impl.client.CloseableHttpClient; 
import org.apache.http.impl.client.HttpClients;
import org.apache.http.impl.conn.PoolingHttpClientConnectionManager;
import org.apache.http.message.BasicHeaderElementIterator;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.protocol.HTTP;
import org.apache.http.util.EntityUtils; 

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;

/**
 * 引入单例连接池
 */
@Slf4j
public class HttpClientUtil { 

    private static CloseableHttpClient httpClient;
    private static RequestConfig config;

    private static class Holder {
        private static final HttpClientUtil INSTANCE = new HttpClientUtil();
    }

    public static HttpClientUtil me() {
        return Holder.INSTANCE;
    }

    private HttpClientUtil() {
        PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();
        //最大连接数
        connectionManager.setMaxTotal(200);
        // 并发数
        connectionManager.setDefaultMaxPerRoute(10);
        // 检查间隔为5秒
        connectionManager.setValidateAfterInactivity(5000);
        // 关闭空闲连接的时间为60S
        connectionManager.closeIdleConnections(60, TimeUnit.SECONDS);

        config = RequestConfig.custom()
                // 从连接池中取连接超时时间
                .setConnectionRequestTimeout(5000)
                // 建立链接超时时间
                .setConnectTimeout(5000)
                // 等待读取数据时间
                .setSocketTimeout(20000)
                .build();

        httpClient = HttpClients.custom()
                // 关闭自动重试
                .disableAutomaticRetries()
                //长连接
               // .setKeepAliveStrategy( keepAliveStrategy() )
                // 设置连接池
                .setConnectionManager(connectionManager)
                // 设置连接自动回收时间 连接池配置了,这个就不需要配置了
                //.evictIdleConnections(60, TimeUnit.SECONDS)
                // 设置超时时间
                .setDefaultRequestConfig( config )
                .build();
    }
   //长连接
   private ConnectionKeepAliveStrategy genKeepAliveStrategy() {
        return (response, context) -> {
            HeaderElementIterator it = new BasicHeaderElementIterator(response.headerIterator(HTTP.CONN_KEEP_ALIVE));
            while (it.hasNext()) {
                HeaderElement he = it.nextElement();
                if ("timeout".equalsIgnoreCase(he.getName())) {
                    return Long.parseLong(he.getValue()) * 1000;
                }
            }
            return 60 * 1000; // 默认保持活动时间为60秒
        };
    }
    /**
     * 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
     */
    public String doGet(String url) {
        log.info("httpGet_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
        try {
            CloseableHttpResponse response = 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
                String result = EntityUtils.toString(response.getEntity(), "UTF-8");
                log.info("httpGet_result==>{}", result);
                return result;
            }
        } catch (Exception e) {
            log.error("httpGet_error==>", e);
            return null;
        }
        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
     */
    public String doGet(String url, Map<String, Object> map) {
        URIBuilder uriBuilder = null;
        try {
            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());
                }
            }
            return doGet(uriBuilder.build().toString());
        } catch (Exception e) {
            return null;
        }
    }

    /**
     * 不带参数post请求
     *
     * @param url
     * @return string json串
     * @throws Exception
     */
    public String doPost(String url) throws Exception {
        return doPost(url, null, null);
    }

    public String doPost(String url, Map<String, Object> map) throws Exception {
        return doPost(url, map, null);
    }

    /**
     * 带map参数的post请求
     *
     * @param url
     * @param map
     * @return string json串
     * @throws Exception
     */
    public String doPost(String url, Map<String, Object> map, Map<String, String> headers) throws Exception {
        log.info("httpPost_url==>{},params==>{}", url, map);
        // 声明httpPost请求
        HttpPost httpPost = new HttpPost(url);
        // 加入配置信息
        httpPost.setConfig( config);
        if (headers != null) {
            for (Map.Entry<String, String> entry : headers.entrySet()) {
                httpPost.setHeader(entry.getKey(), entry.getValue());
            }
        }
        // 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<>();
            for (Map.Entry<String, Object> entry : map.entrySet()) {
                list.add(new BasicNameValuePair(entry.getKey(), entry.getValue().toString()));
            }
            // 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
        try (CloseableHttpResponse response = httpClient.execute(httpPost)) {
            if (response.getStatusLine().getStatusCode() == 200) {
                // 返回响应体的内容
                String result = EntityUtils.toString(response.getEntity(), "UTF-8");
                log.info("httpPost_result==>{}", result);
                return result;
            }
        }
        return null;
    }

    /**
     * 带json参数的post请求,用的最多,因为map可以转为json
     *
     * @param url  请求地址host+path
     * @param json json字符串
     * @return string json串
     */
    public String doPost(String url, String json) {
        log.info("httpPost_url==>{},json==>{}", url, json);
        //设置请求路径,请求格式,配置信息
        HttpPost httpPost = new HttpPost(url);
        httpPost.setHeader("Content-Type", "application/json;charset=UTF-8");
        httpPost.setConfig(config);
        // 发起请求
        try {
            //格式化请求数据并设值
            StringEntity se = new StringEntity(json, "UTF-8");
            se.setContentType("text/json");
            httpPost.setEntity(se);
            CloseableHttpResponse response = httpClient.execute(httpPost);
            // 判断状态码是否为200
            if (response.getStatusLine().getStatusCode() == 200) {
                // 返回响应体的内容
                String result = EntityUtils.toString(response.getEntity(), "UTF-8");
                log.info("httpPost_result==>{}", result);
                return result;
            }
        } catch (Exception e) {
            log.error("httpPost_error==>", e);
            return null;
        }
        return null;
    }
}

如果需要支持https

Registry<ConnectionSocketFactory> registry = RegistryBuilder.<ConnectionSocketFactory>create()
                .register("http", PlainConnectionSocketFactory.getSocketFactory())
                .register("https", sslConnectionSocketFactory())
                .build();
PoolingHttpClientConnectionManager httpClientConnectionManager = new PoolingHttpClientConnectionManager(registry);
//最大连接数
httpClientConnectionManager.setMaxTotal(100);
//并发数
httpClientConnectionManager.setDefaultMaxPerRoute(10);


/**
 * 支持SSL 
 */
private SSLConnectionSocketFactory sslConnectionSocketFactory() {
        try {
            return new SSLConnectionSocketFactory(
                    SSLContexts.custom().loadTrustMaterial(null, new TrustSelfSignedStrategy()).build(),
                    NoopHostnameVerifier.INSTANCE);
        } catch (Exception e) {
            return SSLConnectionSocketFactory.getSocketFactory();
        }
 }

如果需要修改请求重试

                    httpClient = HttpClientBuilder.create()
                            // 关闭自动重试
                            // .disableAutomaticRetries()
                             // 自定义自动重试
                            .setRetryHandler(httpRequestRetryHandler())
                            // 设置连接池
                            .setConnectionManager(connectionManager)
                            // setConnectionTimeToLive(2, TimeUnit.MINUTES) 设置链接最大存活时间 此选项无效
                            // 设置连接自动回收时间
                            .evictIdleConnections(1, TimeUnit.MINUTES)
                            // 设置超时时间
                            .setDefaultRequestConfig(requestConfig).build();
                            
 /**
     * 自动重试
     */
    private HttpRequestRetryHandler httpRequestRetryHandler() {
        return (exception, executionCount, context) -> {
            if (executionCount >= 5) {// 如果已经重试了5次,就放弃
                return false;
            }
            if (exception instanceof NoHttpResponseException) {// 如果服务器丢掉了连接,那么就重试
                return true;
            }
            if (exception instanceof SSLHandshakeException) {// 不要重试SSL握手异常
                return false;
            }
            if (exception instanceof InterruptedIOException) {// 超时
                return false;
            }
            if (exception instanceof UnknownHostException) {// 目标服务器不可达
                return false;
            }
            if (exception instanceof SSLException) {// SSL握手异常
                return false;
            }

            HttpClientContext clientContext = HttpClientContext
                    .adapt(context);
            HttpRequest request = clientContext.getRequest();
            // 如果请求是幂等的,就再次尝试
            return !(request instanceof HttpEntityEnclosingRequest);
        };
    }