java - 多个线程的并行执行和终止

Question

我有一个简单的应用程序，我在一个类中创建 3 个线程来 ping 3 个不同的网站，并记下执行此操作所需的时间。

我希望通过查看 3 个线程中的哪一个首先成功执行并终止另外两个来增强它。

JDK 的哪个类对此有帮助？以及如何？

ping 网站的示例代码:

public static boolean pingUrl(final String address) {

 try {

  final URL url = new URL("http://" + address);

  final HttpURLConnection urlConn = (HttpURLConnection) url.openConnection();
  urlConn.setConnectTimeout(1000 * 10); // mTimeout is in seconds

  final long startTime = System.currentTimeMillis();

  urlConn.connect();

  final long endTime = System.currentTimeMillis();

  if (urlConn.getResponseCode() == HttpURLConnection.HTTP_OK) {

   System.out.println("Time (ms) : " + (endTime - startTime));
   System.out.println("Ping to "+address +" was success");

   return true;
  }
 } catch (final MalformedURLException e1) {
  e1.printStackTrace();
 } catch (final IOException e) {
  e.printStackTrace();
 }
 return false;
}

Answer 1

I wish to enhance it by seeing which thread out of the 3 executes successfully first and terminating the other two .

我将结合使用 ExecutorService 和 ExecutorCompletionService。然后，当第一个任务完成时从完成服务返回第一个 Future 时，您将在 ExecutorService 上调用 shutdownNow()。

javadocs for ExecutorCompletionService非常好，并展示了如何使用它。

// maybe you want 10 threads working on your tasks
ExecutorService threadPool = Executors.newFixedThreadPool(10); 
CompletionService<Result> ecs
     = new ExecutorCompletionService<Result>(threadPool);
for (Callable<Result> task : tasks) {
    // submit your tasks to the completion service, they run in the thread-pool
    ecs.submit(task);
}
// once you get one result
Future<Result> future = ecs.take();
// kill the rest of the tasks
threadPool.shutdownNow();
Result result = future.get();
// probably will need to close the thread connections, see below
// maybe call threadPool.awaitShutdown(...) here to wait for the others to die

此机制的唯一问题是这只会中断线程。在您的情况下，它们将被困在不可中断的 urlConn.connect(); 中。一旦 ecs.take() 返回，您将必须重新运行您的任务并在 HttpURLConnection 上调用 disconnect()仍在进行中。即使如此，我也不确定它是否会停止当前正在进行的连接。如果这不起作用，那么您可能需要改用 Apache HttpClient 或其他一些可以关闭的类，以阻止线程等待更长时间。

for (Callable<Result> task : tasks) {
   // you'll need to do something like this
   task.closeConnection();
}

就您而言，您的任务可能类似于:

public class MyPingTask implements Callable<Boolean> {
    private String address;
    public MyPingTask(String address) {
        this.address = address;
    }
    public Boolean call() throws Exception {
         // obviously the pingUrl code could go right here
         return pingUrl(address);
    }
}

这是Java tutorial on ExecutorService以及相关的类。