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From Rich Dougherty <r...@rd.gen.nz>
Subject Re: [collections] Questions....
Date Fri, 22 Aug 2003 00:39:29 GMT
On Wed, Aug 20, 2003 at 10:25:39AM -0400, Henri Yandell wrote:
> > > FastXxx classes say that they are not cross-platform, but in no way
> > > discuss which platforms they are targetted at. This makes these classes
> > > useless except to the authors.
> > IIRC, nobody can ever answer this question. It may be a theoretical risk, or
> > a real one. Who knows?
> 
> Dunno. Hopefully someone on the project has a clue?

The Fast* classes provide a thread-safe wrapper around collection classes.
Their read operations are faster than the thos of the sychronizing wrappers
in the standard Java libraries. However, the write operations are slower.

This is because all state is stored in a single field. Read
operations can access this state without synchronization. Only write operations
are synchronized. The write operations update the state by copying it,
updating the copy, then replacing the original state with the copy. The copy
operation is atomic, so the classes are thread safe.

However on some platforms they may not behave as expected. As the JVM spec
is written, read operations in different threads may not notice when the
state is changed. This is because different threads are permitted to give
different values for non-volatile fields in unsynchronized code.

The read operations aren't synchronized and the field insn't volatile so the
read operations may not operate on the latest state.

I don't think this happens much in practice because most JVM implementations
always show the same value, even in different threads. However, the
possibility exists that the Fast* classes won't work as expected in some
JVMs.

I'm not very will connected to the net right now, so I haven't been able to
verify this, but I think that may be what whoever wrote that note was
talking about.

Rich
--
Rich Dougherty
http://www.rd.gen.nz/

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