java 使用Apache的框架，写一个http异步调用的案例

好的，下面是一个使用Apache的HttpAsyncClient框架进行异步调用的Java案例：

import java.io.IOException;
import java.util.concurrent.CompletableFuture;

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

public class HttpAsyncClientExample {
    
    public static void main(String[] args) throws IOException, InterruptedException {
        CloseableHttpAsyncClient httpclient = HttpAsyncClients.createDefault();
        httpclient.start();
        
        String url = "https://www.example.com";
        
        HttpGet request = new HttpGet(url);
        
        CompletableFuture<String> future = new CompletableFuture<>();
        httpclient.execute(request, new FutureCallback<HttpResponse>() {

            @Override
            public void completed(HttpResponse result) {
                HttpEntity entity = result.getEntity();
                try {
                    String body = EntityUtils.toString(entity);
                    future.complete(body);
                } catch (IOException e) {
                    future.completeExceptionally(e);
                }
            }

            @Override
            public void failed(Exception ex) {
                future.completeExceptionally(ex);
            }

            @Override
            public void cancelled() {
                future.cancel(true);
            }
            
        });
        
        String response = future.get();
        System.out.println(response);
        
        httpclient.close();
    }

}

这个案例使用了HttpAsyncClient的异步调用方式，通过FutureCallback回调函数来处理异步响应。首先创建一个CloseableHttpAsyncClient对象，调用start()方法启动客户端。然后创建一个HttpGet请求对象，并以此为参数调用execute方法，该方法会异步地发送请求。execute方法的第二个参数是一个FutureCallback对象，当请求完成时，会回调这个对象的completed方法，可以在completed方法中获取到响应的实体，并将其转化为字符串。如果请求失败，则回调failed方法，如果请求被取消，则回调cancelled方法。最后通过CompletableFuture的get方法来获取异步调用的结果，并输出结果。最后调用close方法关闭客户端。