Apache HttpAsyncClient是一个基于事件驱动的HTTP客户端库，可以异步发送HTTP请求并处理响应。CountDownLatch是Java中的一个同步辅助类，可以用于控制线程的执行顺序。

下面是一个使用Apache HttpAsyncClient和CountDownLatch的代码示例：

import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.nio.client.CloseableHttpAsyncClient;
import org.apache.http.impl.nio.client.HttpAsyncClients;
import org.apache.http.util.EntityUtils;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Future;

public class HttpAsyncClientExample {
    public static void main(String[] args) throws Exception {
        // 创建HttpAsyncClient
        CloseableHttpAsyncClient httpAsyncClient = HttpAsyncClients.createDefault();
        
        // 创建CountDownLatch，用于控制线程的执行顺序
        CountDownLatch latch = new CountDownLatch(1);

        try {
            // 启动HttpAsyncClient
            httpAsyncClient.start();

            // 创建HttpGet请求
            HttpGet request = new HttpGet("https://api.example.com/data");

            // 发送异步请求
            Future future = httpAsyncClient.execute(request, null);

            // 注册回调函数，处理响应
            future.get().completed(new ResponseHandler(latch)).failed(new ErrorHandler(latch));

            // 等待请求完成
            latch.await();
        } finally {
            // 关闭HttpAsyncClient
            httpAsyncClient.close();
        }
    }

    // 自定义响应回调函数
    static class ResponseHandler implements org.apache.http.concurrent.FutureCallback {
        private CountDownLatch latch;

        public ResponseHandler(CountDownLatch latch) {
            this.latch = latch;
        }

        @Override
        public void completed(HttpResponse response) {
            try {
                // 处理响应
                String responseBody = EntityUtils.toString(response.getEntity());
                System.out.println("Response: " + responseBody);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                // 通知CountDownLatch请求完成
                latch.countDown();
            }
        }

        @Override
        public void failed(Exception ex) {
            ex.printStackTrace();
            latch.countDown();
        }

        @Override
        public void cancelled() {
            System.out.println("Request cancelled");
            latch.countDown();
        }
    }

    // 自定义错误处理函数
    static class ErrorHandler implements org.apache.http.concurrent.FutureCallback {
        private CountDownLatch latch;

        public ErrorHandler(CountDownLatch latch) {
            this.latch = latch;
        }

        @Override
        public void completed(HttpResponse result) {
            // 通知CountDownLatch请求完成
            latch.countDown();
        }

        @Override
        public void failed(Exception ex) {
            ex.printStackTrace();
            latch.countDown();
        }

        @Override
        public void cancelled() {
            System.out.println("Request cancelled");
            latch.countDown();
        }
    }
}

在这个示例中，首先创建了一个HttpAsyncClient对象，并启动它。然后创建一个CountDownLatch对象，并将其传递给响应回调函数和错误处理函数。接下来，创建一个HttpGet请求对象，并使用HttpAsyncClient发送异步请求。使用future.get()方法获取响应对象，并通过回调函数处理响应。最后，调用latch.await()方法等待请求完成，然后关闭HttpAsyncClient。