要正确测量Apache HttpClient的响应时间，可以使用以下方法和代码示例：

1. 使用Apache HttpClient发送HTTP请求并计算响应时间的代码示例：

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;

public class HttpClientExample {
    public static void main(String[] args) throws Exception {
        // 创建HttpClient实例
        CloseableHttpClient httpClient = HttpClients.createDefault();
        
        // 创建HttpGet请求
        HttpGet httpGet = new HttpGet("http://example.com");
        
        // 发送请求并记录开始时间
        long startTime = System.currentTimeMillis();
        CloseableHttpResponse response = httpClient.execute(httpGet);
        
        // 计算响应时间
        long elapsedTime = System.currentTimeMillis() - startTime;
        
        // 处理响应
        try {
            // 获取响应实体
            HttpEntity entity = response.getEntity();
            
            // 打印响应内容
            if (entity != null) {
                System.out.println("Response content length: " + entity.getContentLength());
            }
        } finally {
            response.close();
        }
        
        // 打印响应时间
        System.out.println("Response time: " + elapsedTime + " milliseconds");
        
        // 关闭HttpClient实例
        httpClient.close();
    }
}

2. 如果需要多次测量响应时间，可以将上述代码放入一个循环中，并计算平均响应时间：

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;

public class HttpClientExample {
    public static void main(String[] args) throws Exception {
        // 创建HttpClient实例
        CloseableHttpClient httpClient = HttpClients.createDefault();
        
        // 创建HttpGet请求
        HttpGet httpGet = new HttpGet("http://example.com");
        
        // 运行多次以计算平均响应时间
        int numRequests = 10;
        long totalElapsedTime = 0;
        
        for (int i = 0; i < numRequests; i++) {
            // 发送请求并记录开始时间
            long startTime = System.currentTimeMillis();
            CloseableHttpResponse response = httpClient.execute(httpGet);
            
            // 计算响应时间
            long elapsedTime = System.currentTimeMillis() - startTime;
            totalElapsedTime += elapsedTime;
            
            // 处理响应
            try {
                // 获取响应实体
                HttpEntity entity = response.getEntity();
                
                // 打印响应内容
                if (entity != null) {
                    System.out.println("Response content length: " + entity.getContentLength());
                }
            } finally {
                response.close();
            }
            
            // 打印每次请求的响应时间
            System.out.println("Response time for request " + (i + 1) + ": " + elapsedTime + " milliseconds");
        }
        
        // 计算平均响应时间
        long averageElapsedTime = totalElapsedTime / numRequests;
        
        // 打印平均响应时间
        System.out.println("Average response time: " + averageElapsedTime + " milliseconds");
        
        // 关闭HttpClient实例
        httpClient.close();
    }
}

以上代码示例使用Apache HttpClient发送GET请求，并计算每次请求的响应时间。在循环中运行多次请求，然后计算平均响应时间。