檢視 SCJP 1.6版考題 244 的原始碼

Given:
<br>1. &nbsp; &nbsp;public class TestSeven extends Thread{
<br>2. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; private static int x;
<br>3. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; public synchronized void doThings(){
<br>4. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; int current = x;
<br>5. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; current++;
<br>6. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; x = current;
<br>7. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }
<br>8. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;public void run(){
<br>9. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; doThings();
<br>10. &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;}<br>11. &nbsp;}





Which statement is true?<br>A. Compilation fails.<br>B. An exception is thrown at runtime.<br>C. Synchronizing the run() method would make the class thread-safe.<br>D. The data in variable "x" are protected from concurrent access problems.<br>E. Declaring the doThings() method as static would make the class thread-safe.<br>F. Wrapping the statements within doThings() in a synchronized(new Object()){} block would make the class thread-safe.


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<span style="font-size:medium;">解答</span>

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<span style="font-size: medium;">Ans: E   </span>

<span style="font-size:medium;"> 解說:
E是說若宣告doThings方法為靜態的方法將使得該類別為"執行緒安全"。
<br>
&nbsp; &nbsp;&nbsp; &nbsp; &nbsp;&nbsp;thread-safe是說不管執行緒如何執行，都不會破壞掉類別資料完整性。
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&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;若方法是物件的成員，則多個執行緒就可以執行多個版本的doThings方法，好像多人一起操作一個共同的整數資料x(因為x是靜態的/類別的，只有一個)，
<br>
&nbsp; &nbsp;&nbsp; &nbsp;&nbsp; &nbsp;若是把doThings方法變成類別，那麼也就只有一個版本的doThings方法，加上此方法又是同步的，同一時間只有此方法可以對x進行操作，不會因為多人方法的存取x而造成x的破壞。

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{{SCJP 1.6版考題講解}}