這個循環爲什麼不執行1000次？

Question

我希望編譯器爲每個線程運行循環1000次，但輸出是12 12 12 12.爲什麼會發生這種情況？

public class Runy implements Runnable { 
    int x, y; 

    public void run() { 
     for (int i = 0; i < 1000; i++) 
      synchronized (this) { 
       x = 12; 
       y = 12; 
      } 
     System.out.print(x + " " + y + " "); 
    } 

    public static void main(String args[]) { 
     Runy run = new Runy(); 
     Thread t1 = new Thread(run); 
     Thread t2 = new Thread(run); 
     t1.start(); 
     t2.start(); 
    } 
} 

Answer 1

問題是與你的for-loop ...

for (int i = 0; i < 1000; i++) 
    synchronized (this) { 
     x = 12; 
     y = 12; 
    } 
System.out.print(x + " " + y + " "); 

這是一樣的

for (int i = 0; i < 1000; i++) { 
    synchronized (this) { 
     x = 12; 
     y = 12; 
    } 
} 
System.out.print(x + " " + y + " "); 

現在應該突出問題。本質上，如果沒有{...}塊，則循環僅執行synchornized塊，一次執行1000次。

喜歡的東西...

for (int i = 0; i < 1000; i++) { 
    synchronized (this) { 
     x = 12; 
     y = 12; 
    } 
    System.out.print(x + " " + y + " "); 
} 

應爲您提供4000 12的