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Re: Safety Of Non-Synchronized Collections

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On 1/10/2013 10:14 AM, Jukka Lahtinen wrote:
> Eric Sosman <esosman@comcast-dot-net.invalid> writes:
>> On 1/10/2013 6:37 AM, Jukka Lahtinen wrote:
>>> Daniel Pitts <newsgroup.nospam@virtualinfinity.net> writes:
>>>> On 1/9/13 2:51 PM, Lew wrote:
>
>>>>> 'StringBuffer' is no more thread safe than any other class with
>>>>> synchronized methods.
>
>>>> Which is more safe than other classes without synchronized methods.
>>>> They are thread-safe to the point that each method call is atomic. What
>>>> else could you ask for? They didn't lie.
>
>>> Whenever thread safety is needed, you mostly need to synchronize not
>>> only the single method call to an instance of StringBuffer or some other
>>> class of the jdk, but also some context around it.
>
>>      This is probably not the case, because another thread might
>> call an (unsynchronized) StringBuilder method while you're in
>> the middle of your synchronized block:
>
> If you use the same instance in many places, potentially in different
> threads, you should of course synchronize all of them using the same
> lock.
>
>> 	StringBuilder sb = ...;
>> 	// Thread T1:
>> 	synchronized(sb) {
>> 	    if (sb.charAt(sb.length() - 1) == '\n') {
>> 	        sb.deleteCharAt(sb.length() - 1);
>> 	    }
>> 	}
>
> This is an example of what I wrote above. You need that synchronization
> to prevent others from modifying sb between the charAt, length and
> deletCharAt calls anyway.
> Using StringBuffer instead of StringBuilder does NOT accomplish that.
>
>> 	// Thread T2:
>> 	sb.append("Gotcha!");
>
> And so, this should be
>
> synchronized(sb) {
>      sb.append("Gotcha?");
> }
>
>> The synchronization in T1's code is no protection against
>> interference from T2.  If `sb' were changed from a StringBuilder
>> to a StringBuffer, the race condition would disappear.
>
> Neither is using StringBuffer.
> Like I said, you could be in trouble if T2 makes that call somewhere
> between the calls to the first sb.length() and sb.deletCharAt in T1 even
> if sb is a StringBuffer.

     If T2 makes such a call while T1 is in the synchronized block,
T2 will stall until T1's synchronized block finishes (or perhaps
longer).  No race, no trouble.

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

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Safety Of Non-Synchronized Collections Steve <tinker123@gmail.com> - 2013-01-01 15:46 -0500
  Re: Safety Of Non-Synchronized Collections Knute Johnson <nospam@knutejohnson.com> - 2013-01-01 14:18 -0800
    Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-01 15:43 -0800
  Re: Safety Of Non-Synchronized Collections markspace <markspace@nospam.nospam> - 2013-01-01 16:17 -0800
    Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-02 11:12 -0800
      Re: Safety Of Non-Synchronized Collections markspace <markspace@nospam.nospam> - 2013-01-02 12:56 -0800
        Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-02 17:46 -0800
  Re: Safety Of Non-Synchronized Collections Roedy Green <see_website@mindprod.com.invalid> - 2013-01-03 05:59 -0800
    Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-03 15:30 -0800
      Re: Safety Of Non-Synchronized Collections Roedy Green <see_website@mindprod.com.invalid> - 2013-01-09 12:17 -0800
        Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-09 12:56 -0800
          Re: Safety Of Non-Synchronized Collections Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-01-09 13:10 -0800
            Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-09 14:51 -0800
              Re: Safety Of Non-Synchronized Collections Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-01-09 15:22 -0800
                Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-09 16:26 -0800
                Re: Safety Of Non-Synchronized Collections Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-01-09 16:42 -0800
                Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-09 16:54 -0800
                Re: Safety Of Non-Synchronized Collections Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2013-01-10 10:30 -0800
                Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-10 10:58 -0800
                Re: Safety Of Non-Synchronized Collections markspace <markspace@nospam.nospam> - 2013-01-09 16:50 -0800
                Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-09 16:59 -0800
                Re: Safety Of Non-Synchronized Collections markspace <markspace@nospam.nospam> - 2013-01-10 08:18 -0800
                Re: Safety Of Non-Synchronized Collections Lew <lewbloch@gmail.com> - 2013-01-10 10:49 -0800
                Re: Safety Of Non-Synchronized Collections Jukka Lahtinen <jtfjdehf@hotmail.com.invalid> - 2013-01-10 13:37 +0200
                Re: Safety Of Non-Synchronized Collections Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-10 09:27 -0500
                Re: Safety Of Non-Synchronized Collections Jukka Lahtinen <jtfjdehf@hotmail.com.invalid> - 2013-01-10 17:14 +0200
                Re: Safety Of Non-Synchronized Collections Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-10 11:44 -0500
                Re: Safety Of Non-Synchronized Collections Jukka Lahtinen <jtfjdehf@hotmail.com.invalid> - 2013-01-10 23:41 +0200
                Re: Safety Of Non-Synchronized Collections Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-10 17:14 -0500
                Re: Safety Of Non-Synchronized Collections Jukka Lahtinen <jtfjdehf@hotmail.com.invalid> - 2013-01-11 09:10 +0200
                Re: Safety Of Non-Synchronized Collections Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-11 08:25 -0500

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