如何发送并行 GET 请求并等待结果响应？

Question

我在spring mvc 3.2.2中使用apache http客户端同步发送5个get请求，如图所示。

如何异步（并行）发送所有这些内容并等待请求返回，以便从所有 GET 请求返回已解析的有效负载字符串？

public String myMVCControllerGETdataMethod()
{
   // Send 1st request 
   HttpClient httpclient = new DefaultHttpClient();
   HttpGet httpget = new HttpGet("http://api/data?type=1");   
   ResponseHandler<String> responseHandler = new BasicResponseHandler();
   String responseBody = httpclient.execute(httpget, responseHandler);

   // Send 2st request 
   HttpClient httpclient2 = new DefaultHttpClient();
   HttpGet httpget2 = new HttpGet("http://api/data?type=2");   
   ResponseHandler2<String> responseHandler2 = new BasicResponseHandler();
   String responseBody2 = httpclient.execute(httpget, responseHandler2);

   // o o o more gets here

   // Perform some work here...and wait for all requests to return
   // Parse info out of multiple requests and return
   String results = doWorkwithMultipleDataReturned();

   model.addAttribute(results, results);
   return "index";

}

Answer 1

将请求代码移动到单独的方法：

private String executeGet(String url){
   HttpClient httpclient = new DefaultHttpClient();
   HttpGet httpget = new HttpGet(url);   
   ResponseHandler<String> responseHandler = new BasicResponseHandler();
   return httpclient.execute(httpget, responseHandler);
}

并将它们提交给ExecutorService：

ExecutorService executorService = Executors.newCachedThreadPool();

Future<String> firstCallFuture = executorService.submit(() -> executeGet(url1));
Future<String> secondCallFuture = executorService.submit(() -> executeGet(url2));

String firstResponse = firstCallFuture.get();
String secondResponse = secondCallFuture.get();

executorService.shutdown();

或者

Future<String> firstCallFuture = CompletableFuture.supplyAsync(() -> executeGet(url1));
Future<String> secondCallFuture = CompletableFuture.supplyAsync(() -> executeGet(url2));

String firstResponse = firstCallFuture.get();
String secondResponse = secondCallFuture.get();

还是像《如何使用Spring WebClient同时进行多个调用》中描述的那样使用RestTemplate？

Answer 2

您应该使用AsyncHttpClient。你可以提出任意数量的请求，当它得到响应时，它会给你回电。您可以配置它可以创建多少个连接。所有线程都由库处理，所以这比自己管理线程要容易得多。

看看下面的例子：https://github.com/AsyncHttpClient/async-http-client

Answer 3

一般来说，您需要将您的工作单元封装在Runnable或java.util.concurrent.Callable中，并通过java.util.concurrent.Executor（或org.springframework.core.task.TaskExecutor）执行它们。这允许每个工作单元单独执行，通常以异步方式（取决于执行器的实现）。

因此，对于您的具体问题，您可以这样做：

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.Executor;
import java.util.concurrent.FutureTask;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.BasicResponseHandler;
import org.apache.http.impl.client.DefaultHttpClient;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;

@Controller
public class MyController {
    //inject this
    private Executor executor;

    @RequestMapping("/your/path/here")
    public String myMVCControllerGETdataMethod(Model model) {
        //define all async requests and give them to injected Executor
        List<GetRequestTask> tasks = new ArrayList<GetRequestTask>();
        tasks.add(new GetRequestTask("http://api/data?type=1", this.executor));
        tasks.add(new GetRequestTask("http://api/data?type=2", this.executor));
        //...
        //do other work here
        //...
        //now wait for all async tasks to complete
        while(!tasks.isEmpty()) {
            for(Iterator<GetRequestTask> it = tasks.iterator(); it.hasNext();) {
                GetRequestTask task = it.next();
                if(task.isDone()) {
                    String request = task.getRequest();
                    String response = task.getResponse();
                    //PUT YOUR CODE HERE
                    //possibly aggregate request and response in Map<String,String>
                    //or do something else with request and response
                    it.remove();
                }
            }
            //avoid tight loop in "main" thread
            if(!tasks.isEmpty()) Thread.sleep(100);
        }
        //now you have all responses for all async requests

        //the following from your original code
        //note: you should probably pass the responses from above
        //to this next method (to keep your controller stateless)
        String results = doWorkwithMultipleDataReturned();
        model.addAttribute(results, results);
        return "index";
    }

    //abstraction to wrap Callable and Future
    class GetRequestTask {
        private GetRequestWork work;
        private FutureTask<String> task;
        public GetRequestTask(String url, Executor executor) {
            this.work = new GetRequestWork(url);
            this.task = new FutureTask<String>(work);
            executor.execute(this.task);
        }
        public String getRequest() {
            return this.work.getUrl();
        }
        public boolean isDone() {
            return this.task.isDone();
        }
        public String getResponse() {
            try {
                return this.task.get();
            } catch(Exception e) {
                throw new RuntimeException(e);
            }
        }
    }

    //Callable representing actual HTTP GET request
    class GetRequestWork implements Callable<String> {
        private final String url;
        public GetRequestWork(String url) {
            this.url = url;
        }
        public String getUrl() {
            return this.url;
        }
        public String call() throws Exception {
            return new DefaultHttpClient().execute(new HttpGet(getUrl()), new BasicResponseHandler());
        }
    }
}

请注意，此代码尚未经过测试。

对于您的Exector实现，请查看Spring的TaskExecitor和任务：执行器命名空间。您可能希望此用例有一个可重用的线程池（而不是每次都创建一个新线程）。