java - 当主线程仍在运行时，线程如何通知主线程

Question

我是 Java 初学者。我想通过生成一个线程来实现计时器。我想要运行的方式是主线程即使在生成线程后也应该继续工作。但是，一旦线程完成任务，它应该通知主线程任务已完成。我查看了 Thread.join() 和 Thread.sleep()。对于这两种方法，主线程都会等待线程完成任务。但是我希望主线程继续运行。

我在下面写了一段代码，其中主线程调用一个线程，该线程执行任务10s。但是我无法通知主线程任务完成情况。请帮助我。

public class MainThreadtest {
   public static void main(String[] args) {
      dispthread2 dt2 = new dispthread2();
      dt2.start();
      System.out.println("Thread dt2 has started");
   }
}

二等

public class dispthread2 extends Thread {
   public void run() {
      long endTimemillis = System.currentTimeMillis() + 10000;
      while (System.currentTimeMillis() < endTimemillis) {
      }
      System.out.println("The task is done");
   }
}

Answer 1

解决此问题的一种方法不需要使用 Thread#join，从而允许两个线程独立运行:

1. 使用观察者模式监听器接口(interface)，例如可以通过 Java Beans PropertyChangeSupport 和 PropertyChangeListener 获取该接口(interface)。
2. 不要扩展 Thread，而是实现 Runnable。

<小时/>

例如:

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;

import javax.swing.event.SwingPropertyChangeSupport;

public class MainThreadTest {
   public static void main(String[] args) {
      DispThread2 dt2 = new DispThread2();
      dt2.addPropertyChangeListener(DispThread2.STATE, new PropertyChangeListener() {

         @Override
         public void propertyChange(PropertyChangeEvent evt) {
            System.out.println("DT2 Thread state (from Main Thread): " + evt.getNewValue());
            if (evt.getNewValue() == DispThreadState.DONE) {
               System.out.println("Thread dt2 is now done (from Main Thread)");
            }
         }
      });
      new Thread(dt2, "DT2").start();
      System.out.println("Thread dt2 has started");
   }
}

class DispThread2 implements Runnable {
   public static final String STATE = "state";
   private volatile DispThreadState state = DispThreadState.PENDING;
   private PropertyChangeSupport support = new SwingPropertyChangeSupport(this);

   public void addPropertyChangeListener(PropertyChangeListener listener) {
      support.addPropertyChangeListener(listener);
   }

   public void addPropertyChangeListener(String propertyName, PropertyChangeListener listener) {
      support.addPropertyChangeListener(propertyName, listener);
   }

   public void removePropertyChangeListener(PropertyChangeListener listener) {
      support.removePropertyChangeListener(listener);
   }

   public void run() {
      setState(DispThreadState.RUNNING);
      long endTimemillis = System.currentTimeMillis() + 10000;
      while (System.currentTimeMillis() < endTimemillis) {
      }
      System.out.println("The task is done");
      setState(DispThreadState.DONE);
   }

   public DispThreadState getState() {
      return state;
   }

   public void setState(DispThreadState state) {
      DispThreadState oldValue = this.state;
      DispThreadState newValue = state;
      this.state = state;
      support.firePropertyChange(STATE, oldValue, newValue);
   }
}

enum DispThreadState {
   PENDING, RUNNING, DONE
}