在PropertyListener中殺死一個線程（JavaFX8）

Question

我知道Java的實際模型是用於協作線程的，它強制線程死亡不會發生。

由於Thread.stop()已棄用（基於上述原因）。我試圖通過一個BooleanProperty監聽器來停止線程。

這裏是MCVE：

TestStopMethod.java

package javatest; 
import javafx.beans.property.BooleanProperty; 
import javafx.beans.property.SimpleBooleanProperty; 
import javafx.beans.value.ObservableValue; 
public class TestStopMethod extends Thread { 
    private BooleanProperty amIdead = new SimpleBooleanProperty(false); 
    public void setDeath() { 
     this.amIdead.set(true); 
    } 

    @Override 
    public void run() { 
     amIdead.addListener((ObservableValue<? extends Boolean> observable, Boolean oldValue, Boolean newValue) -> { 
      System.out.println("I'm dead!!!"); 
      throw new ThreadDeath(); 
     }); 
     for(;;); 
    } 
} 

WatchDog.java

package javatest; 

import java.util.TimerTask; 

public class Watchdog extends TimerTask { 
    TestStopMethod watched; 
    public Watchdog(TestStopMethod target) { 
     watched = target; 
    } 
    @Override 
    public void run() { 
     watched.setDeath(); 
     //watched.stop(); <- Works but this is exactly what I am trying to avoid 
     System.out.println("You're dead!"); 
    } 

} 

Driver.java

package javatest; 

import java.util.*; 
import java.util.logging.Level; 
import java.util.logging.Logger; 

public class Driver { 

    public static void main(String[] args) { 
     try { 
      TestStopMethod mythread = new TestStopMethod(); 
      Timer t = new Timer(); 
      Watchdog w = new Watchdog(mythread); 
      t.schedule(w, 1000); 
      mythread.start(); 
      mythread.join(); 
      t.cancel(); 
      System.out.println("End of story"); 
     } catch (InterruptedException ex) { 
      Logger.getLogger(Driver.class.getName()).log(Level.SEVERE, null, ex); 
     } 

    } 
} 

Answer 1

如果更改屬性值，則會在屬性發生更改的相同線程上調用偵聽器（請考慮如何實現屬性類）。因此，在您的示例中，ThreadDeath錯誤是從支持Timer實例的線程拋出的，這不是您真正想要的。

從外部（到該線程）終止線程的正確方法是設置一個標誌，然後在線程的實現中定期輪詢該標誌。這實際上比聽起來更棘手，因爲標誌必須從多個線程訪問，所以訪問它必須正確同步。

幸運的是，有一些工具類可以幫助實現這一點。例如，FutureTask包裝Runnable或Callable並提供cancel()和isCancelled()方法。如果您使用的是JavaFX，那麼javafx.concurrent API提供了一些實現Callable和Runnable的實現，並且還提供了專門用於在FX應用程序線程上執行代碼的功能。有些例子可以看看documentation for javafx.concurrent.Task。

因此，舉例來說，你可以這樣做：

package javatest; 
public class TestStopMethod implements Runnable { 

    @Override 
    public void run() { 
     try { 
      synchronized(this) { 
       for(;;) { 
        wait(1); 
       } 
      } 
     } catch (InterruptedException exc) { 
      System.out.println("Interrupted"); 
     } 
    } 
} 

Watchdog.java：

package javatest; 

import java.util.TimerTask; 
import java.util.concurrent.Future; 

public class Watchdog extends TimerTask { 
    Future<Void> watched; 
    public Watchdog(Future<Void> target) { 
     watched = target; 
    } 
    @Override 
    public void run() { 
     watched.cancel(true); 
     //watched.stop(); <- Works but this is exactly what I am trying to avoid 
     System.out.println("You're dead!"); 
    } 
} 

Driver.java：

package javatest; 

import java.util.*; 
import java.util.concurrent.FutureTask; 
import java.util.logging.Level; 
import java.util.logging.Logger; 

public class Driver { 

    public static void main(String[] args) { 
     try { 
      FutureTask<Void> myTask = new FutureTask<>(new TestStopMethod(), null); 
      Timer t = new Timer(); 
      Watchdog w = new Watchdog(myTask); 
      t.schedule(w, 1000); 
      Thread mythread = new Thread(myTask); 
      mythread.start(); 
      mythread.join(); 
      t.cancel(); 
      System.out.println("End of story"); 
     } catch (InterruptedException ex) { 
      Logger.getLogger(Driver.class.getName()).log(Level.SEVERE, null, ex); 
     } 

    } 
} 

在JavaFX應用程序，你可能會做它是這樣的。請注意，如果嘗試在沒有運行FX應用程序線程的情況下執行此操作，情況會變糟，因爲必須在該線程上更新FX Task中的cancelled標誌。

package javatest; 

import javafx.concurrent.Task; 

public class TestStopMethod extends Task<Void> { 

    @Override 
    public Void call() { 
     System.out.println("Calling"); 
     while (true) { 
      if (isCancelled()) { 
       System.out.println("Cancelled"); 
       break ; 
      } 
     } 
     System.out.println("Exiting"); 
     return null ; 
    } 
} 

Watchdog.java：

package javatest; 

import java.util.TimerTask; 

import javafx.concurrent.Task; 

public class Watchdog extends TimerTask { 
    Task<Void> watched; 
    public Watchdog(Task<Void> target) { 
     watched = target; 
    } 
    @Override 
    public void run() { 
     watched.cancel(); 
     //watched.stop(); <- Works but this is exactly what I am trying to avoid 
     System.out.println("You're dead!"); 
    } 

} 

Driver.java

package javatest; 

import java.util.Timer; 
import java.util.logging.Level; 
import java.util.logging.Logger; 

import javafx.application.Application; 
import javafx.concurrent.Task; 
import javafx.concurrent.Worker; 
import javafx.scene.Scene; 
import javafx.scene.control.TextArea; 
import javafx.scene.layout.BorderPane; 
import javafx.stage.Stage; 

public class Driver extends Application { 

    @Override 
    public void start(Stage primaryStage) { 
     try { 

      TextArea console = new TextArea(); 
      BorderPane root = new BorderPane(console); 
      Scene scene = new Scene(root, 600, 400); 
      primaryStage.setScene(scene); 
      primaryStage.show(); 

      Task<Void> myTask = new TestStopMethod(); 
      Timer t = new Timer(); 
      Watchdog w = new Watchdog(myTask); 
      t.schedule(w, 1000); 
      Thread mythread = new Thread(myTask); 
      mythread.setDaemon(true); 

      myTask.stateProperty().addListener((obs, oldState, newState) -> { 
       console.appendText("State change "+oldState+" -> "+newState+"\n"); 
       if (oldState == Worker.State.RUNNING) { 
        t.cancel(); 
        console.appendText("End of Story\n"); 
       } 
      }); 
      mythread.start(); 

     } catch (Exception ex) { 
      Logger.getLogger(Driver.class.getName()).log(Level.SEVERE, null, ex); 
     } 

    } 
}