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Newsgroups: comp.lang.java.programmer
Date: Wed, 21 Aug 2013 08:36:15 -0700 (PDT)
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Subject: Re: Autoboxing and Performance ?
From: Lew <lewbloch@gmail.com>
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Xref: csiph.com comp.lang.java.programmer:25092

Joerg Meier wrote:
> 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 ?

A monitor has to be the same object between threads. Holding a "lock" on two 
different objects actually doesn't lock anything.

> Or are we to assume other code touches counter without being in a

If 'counter' changes to point to a different object, as in the sample code 
here, then it no longer points to the object holding the monitor, which 
makes it useless as a lock.

> synchronized(counter) block itself in this example ?

The finger is not the moon.

That means a pointer is not the same thing as the object to which it points.

Locks are taken on objects, not pointers.

-- 
Lew