Re: Autoboxing and Performance ?

From:
Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 21 Aug 2013 07:19:06 -0400
Message-ID:
<kv27ne$p03$1@dont-email.me>
On 8/21/2013 4:05 AM, Joerg Meier wrote:

On Tue, 20 Aug 2013 21:19:36 -0400, Eric Sosman wrote:

      Nonetheless, I'm still of the opinion that autoboxing is Satan's
work. Another thread, somewhat further back, wondered why this
apparently thread-safe code wasn't thread-safe at all:

    class Whatever {
        private static Integer counter;
        void someMethod() {
            synchronized(counter) {
                ++counter;
                ...
            }
        }
        ...
    }

I still don't see how that wouldn't be thread-safe, could you elaborate ?
Or are we to assume other code touches counter without being in a
synchronized(counter) block itself in this example ?


     The `counter' variable is not an Integer object, but a
reference to an Integer object. The `++counter' line brings
a second Integer into the picture, and makes `counter' refer
to that new Integer instead of to the original. However, the
block is still synchronized on the first one; it doesn't
somehow magically re-synchronize on the second.

     Assume `counter' points to Integer.valueOf(0) initially
(I ought to have showed the initialization in the snippet;
sorry). Then with threads T1 and T2 executing:

    T1: synchronized(counter) // locks the 0 object
    T1: ... now in critical section ...
    T1: ++counter; // counter now points to the 1 object
    T1: ... still in critical section ...
    T2: synchronized(counter) // locks the *1* object
    T2: ... now in critical section ...

Both T1 and T2 are now in the synchronized block simultaneously,
holding the locks of two different Integer objects.

--
Eric Sosman
esosman@comcast-dot-net.invalid

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