Re: Is 'new' operator thread-safe?

From:
Patricia Shanahan <pats@acm.org>
Newsgroups:
comp.lang.java.programmer
Date:
Tue, 05 Dec 2006 16:39:47 GMT
Message-ID:
<nhhdh.7054$tM1.25@newsread1.news.pas.earthlink.net>
xie bo wrote:

    Reference? Oh, I dunno, but I'd be willing to
bet you'll find something in the JLS.


I checked JLS and following is citation.
-------
Section 17.5 Final Field Semantics
...
An object is considered to be completely initialized when its
constructor finishes. A thread that can only see a reference to an
object after that object has been completely initialized is guaranteed
to see the correctly initialized values for that object's final fields.
-------

Is the above reference mean '"new" operator is thread-safe for Java'?

Thanks!


Not necessarily. It depends, as Eric pointed out, on what the
constructor does:

public class SynchTest {
   static int count1 = 0;

   static int count2 = 0;

   public static void main(String[] args) {
     Runnable myRun = new Runnable() {
       public void run() {
           new SynchTest();
       }
     };

     Thread[] allThreads = new Thread[10];
     for(int i=0; i<allThreads.length; i++){
       allThreads[i] = new Thread(myRun);
     }
     for(Thread t : allThreads){
       t.start();
     }
     for(Thread t : allThreads){
       try {
         t.join();
       } catch (InterruptedException e) {
         // IGNORE INTERRUPT
       }
     }
     System.out.printf(
       "Actual=%d count1=%d count2=%d%n",
       allThreads.length, count1, count2
     );
   }

   public SynchTest() {
     badIncrement();
     goodIncrement();
   }

   private void goodIncrement() {
     synchronized (this.getClass()) {
       count1++;
     }
   }

   private void badIncrement() {
     int x = count2;
     try {
       Thread.sleep(1000);
     } catch (InterruptedException e) {
       // IGNORE INTERRUPTS
     }
     x++;
     count2 = x;
   }
}

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