java调用get请求

import lombok.extern.slf4j.Slf4j;
import org.apache.http.HttpStatus;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpPost;
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.util.EntityUtils;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
import java.nio.charset.StandardCharsets;
import java.util.List;
import java.util.Map;
import java.util.concurrent.TimeUnit;

@Slf4j
public class HttpRequest {
    private static final PoolingHttpClientConnectionManager POOL_CONN_MANAGER = new PoolingHttpClientConnectionManager();
    private volatile static CloseableHttpClient closeableHttpClient;

    static {
        POOL_CONN_MANAGER.setMaxTotal(500);
        POOL_CONN_MANAGER.setDefaultMaxPerRoute(200);
    }

    private HttpRequest() {}

    private static CloseableHttpClient getHttpClient() {
        if (null == closeableHttpClient) {
            synchronized (HttpRequest.class) {
                if (null == closeableHttpClient) {
                    closeableHttpClient = HttpClients.custom()
                            .setConnectionManager(POOL_CONN_MANAGER)
                            .evictExpiredConnections()
                            .disableAutomaticRetries()
                            .evictIdleConnections(30, TimeUnit.SECONDS)
                            .build();
                }
            }
        }
        return closeableHttpClient;
    }

    /**
     * 向指定URL发送GET方法的请求
     * @param url
     * 发送请求的URL
     * @param param
     * 请求参数,请求参数应该是 name1=value1&name2=value2 的形式。
     */
    public static String sendGet(String url, String param) {
        String result = "";
        BufferedReader in = null;
        try {
            String urlNameString = url + "?" + param;
            System.out.println(urlNameString);
            URL realUrl = new URL(urlNameString);
            // 打开和URL之间的连接
            URLConnection connection = realUrl.openConnection();
            // 设置通用的请求属性
            connection.setRequestProperty("accept", "*/*");
            connection.setRequestProperty("connection", "Keep-Alive");
            connection.setRequestProperty("user-agent",
                    "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1)");
            // 建立实际的连接
            connection.connect();
            // 获取所有响应头字段
            Map<String, List<String>> map = connection.getHeaderFields();
            // 遍历所有的响应头字段
            for (String key : map.keySet()) {
                System.out.println(key + "--->" + map.get(key));
            }
            // 定义 BufferedReader输入流来读取URL的响应
            in = new BufferedReader(new InputStreamReader(
                    connection.getInputStream()));
            String line;
            while ((line = in.readLine()) != null) {
                result += line;
            }
        } catch (Exception e) {
            System.out.println("发送GET请求出现异常！" + e);
            e.printStackTrace();
        }
        // 使用finally块来关闭输入流
        finally {
            try {
                if (in != null) {
                    in.close();
                }
            } catch (Exception e2) {
                e2.printStackTrace();
            }
        }
        return result;
    }