同步方法似乎中斷

Question

我正在寫一個多線程的小程序。我遇到了一些奇怪的問題，我已經將它們跟蹤到了下面的課程。這是整個代碼，沒有剪輯。

public class BoundingBox { 
    private volatile int top; 
    private volatile int left; 
    private volatile int bottom; 
    private volatile int right; 

    public static final int IGNORE = 0; 
    public static final int TOP  = -2; 
    public static final int LEFT = -1; 
    public static final int BOTTOM = 2; 
    public static final int RIGHT = 1; 

    BoundingBox(int left, int top, int width, int height) { 
     this.top = top; 
     this.left = left; 
     this.right = left + width; 
     this.bottom = top + height; 
    } 

    public synchronized int top()  { return top;   } 
    public synchronized int left()  { return left;   } 
    public synchronized int bottom() { return bottom;  } 
    public synchronized int right()  { return right;   } 

    public synchronized int width()  { return right - left; } 
    public synchronized int height() { return bottom - top; } 

    public synchronized void translate(Vector2D vector) { 
     left += vector.getX(); 
     right += vector.getX(); 
     top += vector.getY(); 
     bottom += vector.getY(); 
    } 

    public synchronized void alignTo(Point2D point, int hAlign, int vAlign) { 
     if ((hAlign != IGNORE && hAlign != LEFT && hAlign != RIGHT) 
      || (vAlign != IGNORE && vAlign != TOP && vAlign != BOTTOM)) 
      throw new IllegalArgumentException(); 

     /// START DEBUG CODE /// 
     if (right - left != width()) 
      System.out.println("X"); 
     if (bottom - top != height()) 
      System.out.println("Y"); 
     /// END DEBUG CODE /// 

     int width = width(); 
     int height = height(); 

     if (hAlign != IGNORE) { 
      left = point.getX(); 
      if (hAlign == RIGHT) 
       left -= width; 
      right = left + width; 
     } 

     if (vAlign != IGNORE) { 
      top = point.getY(); 
      if (vAlign == BOTTOM) 
       top -= height; 
      bottom = top + height; 
     } 
    } 
} 

X和Y有時會打印。如您所見，width()定義爲right - left，但它仍然發生這兩個不相等（與height()相同）。這四個字段是私人的，方法是同步的，所以沒有什麼應該打斷alignTo，對吧？儘管如此，我覺得還是有些事情會發生。

這段代碼有什麼問題？

Answer 1

聲明頂部，左側，底部，右側，因爲揮發性不足以使它們以您需要的方式保持同步。問題是，這些變量正在使用translate方法在不同線程上進行修改，因此在執行alignTo過程中會發生變化。您需要在alignTo持續時間內鎖定這些變量，或將它們緩存到局部變量。