:::tips 用在一个线程等待另一个线程的执行结果 ::: image.png

要点

  • 有一个结果需要从一个线程传递到另一个线程，让他们关联同一个 GuardedObject
  • 如果有结果不断从一个线程到另一个线程那么可以使用消息队列（见生产者/消费者）
  • JDK 中，join 的实现、Future 的实现，采用的就是此模式
  • 因为要等待另一方的结果，因此归类到同步模式
  1. public class TestGuardedObject {
  2.     public static void main(String[] args) {
  3.         GuardedObject guardedObject = new GuardedObject();
  4.         new Thread(() -> {
  5.             try {
  6.                 List<String> response = download();
  7.                 log.debug("download complete...");
  8.                 guardedObject.complete(response);
  9.             } catch (IOException e) {
  10.                 e.printStackTrace();
  11.             }
  12.         }).start();
  14.         log.debug("waiting...");
  15.         Object response = guardedObject.get();
  16.         log.debug("get response: [{}] lines", ((List<String>) response).size());
  18.     }
  19. }
  21. class GuardedObject {
  23.     private Object response;
  24.     private final Object lock = new Object();
  26.     public Object get() {
  27.         synchronized (lock) {
  28.             // 条件不满足则等待
  29.             while (response == null) {
  30.                 try {
  31.                     lock.wait();
  32.                 } catch (InterruptedException e) {
  33.                     e.printStackTrace();
  34.                 }
  35.             }
  36.             return response;
  37.         }
  38.     }
  40.     public void complete(Object response) {
  41.         synchronized (lock) {
  42.             // 条件满足，通知等待线程
  43.             this.response = response;
  44.             lock.notifyAll();
  45.         }
  46.     }
  47. }
  50. public class Downloader {
  51.     public static List<String> download() throws IOException {
  52.         HttpURLConnection conn = (HttpURLConnection) new URL("https://www.baidu.com/").openConnection();
  53.         List<String> lines = new ArrayList<>();
  54.         try (BufferedReader reader =
  55.                      new BufferedReader(new InputStreamReader(conn.getInputStream(), StandardCharsets.UTF_8))) {
  56.             String line;
  57.             while ((line = reader.readLine()) != null) {
  58.                 lines.add(line);
  59.             }
  60.         }
  61.         return lines;
  62.     }
  63. }