下面是一个使用线程池执行多线程请求的示例代码，该示例使用PoolingHttpClientConnectionManager来管理连接池：

import org.apache.http.HttpEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
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.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class HttpClientExample {
public static void main(String[] args) {
// 创建连接池管理器
PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();
// 设置最大连接数
connectionManager.setMaxTotal(10);
// 设置每个路由的最大连接数
connectionManager.setDefaultMaxPerRoute(5);

// 创建HttpClient对象，使用连接池管理连接
CloseableHttpClient httpClient = HttpClients.custom()
.setConnectionManager(connectionManager)
.build();

// 创建线程池
ExecutorService executorService = Executors.newFixedThreadPool(5);

try {
// 提交多个任务给线程池执行
for (int i = 0; i < 5; i++) {
RequestTask task = new RequestTask(httpClient);
executorService.submit(task);
}

// 等待所有任务完成
executorService.shutdown();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
// 关闭HttpClient连接
httpClient.close();
} catch (Exception e) {
e.printStackTrace();
}
// 关闭连接池
connectionManager.close();
}
}

static class RequestTask implements Runnable {
private final CloseableHttpClient httpClient;

public RequestTask(CloseableHttpClient httpClient) {
this.httpClient = httpClient;
}

@Override
public void run() {
try {
// 创建HttpGet请求
HttpGet httpGet = new HttpGet("https://api.example.com");

// 发送请求并获取响应
CloseableHttpResponse response = httpClient.execute(httpGet);

try {
// 获取响应实体
HttpEntity entity = response.getEntity();
// 处理响应内容
if (entity != null) {
String responseBody = EntityUtils.toString(entity);
System.out.println(Thread.currentThread().getName() + ": " + responseBody);
}
} finally {
// 关闭响应
response.close();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
在上述示例中，我们创建了一个PoolingHttpClientConnectionManager作为连接池管理器，并使用它来构建CloseableHttpClient对象。然后，我们创建了一个固定大小的线程池，最多同时执行5个任务。