爲什麼這個線程示例沒有工作？這一切等待（）的

Question

下面的代碼是模擬我正在使用的機器人模擬器。我不完全確定爲什麼這不起作用 - 我對線程不是很熟悉，即使我今天嘗試了大量的閱讀，但我似乎沒有取得進展。問題是，一旦調用pauseDistanceSensor（），它永遠不會醒來。

import java.util.Random; 

public class TestThreads 
{ 
    private DistanceSensor dist; 
    private Thread distanceThread; 
    public TestThreads() 
    { 
     this.dist = new DistanceSensor(); 
     this.distanceThread = new Thread(this.dist); 
     this.distanceThread.start(); 
    } 

    public int getDistance() 
    { 
     return this.dist.getMeasurement(); 
    } 

    public void pauseDistanceSensor() 
    { 
     synchronized(this.dist) 
     { 
      try { 
       this.dist.wait(); 
      } catch (InterruptedException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
     } 
    } 

    public void resumeDistanceSensor() 
    { 
     synchronized(this.dist) 
     { 
      this.dist.notify(); 
     } 
    } 

    public static void main(String[] args) 
    { 
     TestThreads test = new TestThreads(); 
     long timestamp = System.currentTimeMillis(); 
     System.out.println("Starting at "+timestamp); 
     System.out.println("1: "+test.getDistance()); 
     try { 
      Thread.sleep(1000); 
     } catch (InterruptedException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     System.out.println("2: "+test.getDistance()); 
     try { 
      Thread.sleep(1000); 
     } catch (InterruptedException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     System.out.println("waiting distance sensor and making 3 getDistance calls then sleeping main thread for 1 second - all 3 getDistance calls should be printed when the sleep ends"); 
     test.pauseDistanceSensor(); 
     System.out.println("3: "+test.getDistance()); 
     System.out.println("4: "+test.getDistance()); 
     System.out.println("5: "+test.getDistance()); 
     try { 
      Thread.sleep(1000); 
     } catch (InterruptedException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     System.out.println("Alive! Notifying the thread"); 
     test.resumeDistanceSensor(); 
     System.out.println("Done at "+System.currentTimeMillis()); 
    } 
} 

class DistanceSensor implements Runnable 
{ 
    private final Random gen = new Random(54); 
    private int currentVal; 
    public DistanceSensor() 
    { 
     this.currentVal = this.gen.nextInt(1500); 
    } 

    public void run() 
    { 
     this.currentVal = this.gen.nextInt(1500); 
    } 

    public int getMeasurement() 
    { 
     return this.currentVal; 
    } 
} 

Answer 1

您對'pauseDistanceSensor'的調用會阻塞等待調用的主線程。

此外，您的傳感器運行方法只設置一次傳感器值; run方法應該循環，每次都要設置這個值。

您的暫停方法應改爲使用布爾標誌或類似方法調用掛起傳感器運行循環的同步方法。

import java.util.Random; 

public class TestThreads 
{ 
    private DistanceSensor dist; 
    private Thread distanceThread; 
    public TestThreads() 
    { 
     this.dist = new DistanceSensor(); 
     this.distanceThread = new Thread(this.dist); 
     this.distanceThread.start(); 
    } 

    public int getDistance() 
    { 
     return this.dist.getMeasurement(); 
    } 

    public void pauseDistanceSensor() 
    { 
     dist.setPaused(true); 
    } 

    public void resumeDistanceSensor() 
    { 
     dist.setPaused(false); 
    } 

    public void finish() { 
     dist.setDone(); 
    } 

    public static void main(String[] args) 
    { 
     TestThreads test = new TestThreads(); 
     long timestamp = System.currentTimeMillis(); 
     System.out.println("Starting at "+timestamp); 
     System.out.println("1: "+test.getDistance()); 
     try { 
      Thread.sleep(1000); 
     } catch (InterruptedException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     System.out.println("2: "+test.getDistance()); 
     try { 
      Thread.sleep(1000); 
     } catch (InterruptedException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     System.out.println("waiting distance sensor and making 3 getDistance calls then sleeping main thread for 1 second - all 3 getDistance calls should be printed when the sleep ends"); 
     test.pauseDistanceSensor(); 
     System.out.println("3: "+test.getDistance()); 
     System.out.println("4: "+test.getDistance()); 
     System.out.println("5: "+test.getDistance()); 
     try { 
      Thread.sleep(1000); 
     } catch (InterruptedException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     System.out.println("Alive! Notifying the thread"); 
     test.resumeDistanceSensor(); 
     System.out.println("Done at "+System.currentTimeMillis()); 
     test.finish(); 
    } 
} 

class DistanceSensor implements Runnable 
{ 
    private final Random gen = new Random(54); 
    private int currentVal; 
    private boolean done = false, paused = false; 
    public DistanceSensor() 
    { 
     this.currentVal = this.gen.nextInt(1500); 
    } 

    public void run() 
    { 
     while(!getDone()) { 
      if(!getPaused()) synchronized(this) {this.currentVal = this.gen.nextInt(1500);} 
      synchronized(this) { 
      try { 
       wait(500); 
      } catch(InterruptedException ex) { 
       ex.printStackTrace(); 
      } 
      } 
     } 
    } 

    public synchronized int getMeasurement() 
    { 
     return this.currentVal; 
    } 

    public synchronized boolean getPaused() {return paused;} 
    public synchronized boolean getDone() {return done;} 
    public synchronized void setPaused(boolean p) {paused = p;} 
    public synchronized void setDone() {done = true;notify();} 
}