[concurrency-interest] synchronized on construction
Pete.Soper at Sun.COM
Wed May 24 09:43:55 EDT 2006
Thomas Hawtin wrote:
> Jeremy Manson wrote:
>>Jason Mehrens wrote:
>>>Browsing over the java.util.Vector class I noticed that in the constructors,
>>>the initial writes to the internal data members (non-final and non-volatile)
>>>are not performed under a synchronized block. The same is true for when a
> Under the new JMM would a synchronised block in the constructor
> guarantee to fix the problem? Does the racy write of the Vector
> reference, outside of the synchronised block, necessarily come after the
> block itself. Could it technically be moved up before the lock is
> acquired? Or am I misinterpreting something?
>>>Vector is deserialized. Where does happens-before edge occur so that other
>>>threads don't see uninitialized values of the internals? The only happens
>>>before-edges I can think of are during handoff between threads or on the
>>>call to Thread.start(). Is how this class conforms to the JMM or is
>>>something else going on?
>>Well, bear in mind that only one thread has access to instances of these
>>classes during construction. JMM problems only come up here if you pass
>>a reference to the object to another thread via a data race. These
>>classes are not designed to work correctly if you do that - only the
>>immutable ones, like String, really are.
>>If you are using this class, it is therefore imperative to provide
>>proper synchronization during a handoff.
> Bug 6379897 deals with the thread safety of Random's seed when
> constructed lazily in Collections. It's interested to see that the fix
> for that attempts to make the use of seed thread-safe, rather than
> construct the Random safely in Collections. Presumably the details of
> that fix (setting of the seed value) require that my first paragraph is
There is a special guarantee extended to final fields related to
construction that the experts can explain better than me. And as far as
I can tell there is no possibility of "this" being published before the
constructor completes. So this (current Mustang source for) Random looks
looks OK and I think the attempt was successful.
> Tom Hawtin
> Concurrency-interest mailing list
> Concurrency-interest at altair.cs.oswego.edu
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