Re: Calendar.getInstance() thread safe?

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Sun, 07 Mar 2010 14:13:07 -0500
On 07-03-2010 12:52, markspace wrote:

Arne Vajh?j wrote:

On 07-03-2010 12:07, markspace wrote:

Peter Duniho wrote:

I doubt the Calendar class is thread-safe, except possibly static
methods like getInstance(). Typically, classes are NOT thread-safe
unless documented otherwise.

By a single thread, sure, they must be.

What is thread safe by single thread?

I'll give you a counter example: Most Swing methods are NOT thread safe
for a single thread to call, since they interact with the EDT and will
not synchronize properly between the caller and the EDT.

my thread + EDT = single thread


Let me try to explain my idea a bit more:

Given some method a(), if the results of a() are visible to the calling
thread (all object properly constructed, any threads started by the
method synchronize properly with objects they interact with, etc.) then
a() is safe for a single thread to call.

If the code in a() is thread safe then a() is thread safe. That
is hardly surprising.

Brian Goetz talks about this in Java Concurrency in Practice, that
single-thread safety is the default. Anything different must be documented.

It is reasonable to expect that the library does not start any threads
working on the objects unless the documentation state so.

But I still don't see much point in considering a single thread
scenario for thread safe.


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