Performance characteristics of mutable static primitives? - JVM Languages

Message from discussion Performance characteristics of mutable static primitives?

John Rose

More options Apr 3, 7:01 pm

From: John Rose <John.R...@Sun.COM>

Date: Thu, 03 Apr 2008 16:01:54 -0700

Local: Thurs, Apr 3 2008 7:01 pm

Subject: Re: [jvm-l] Re: Performance characteristics of mutable static primitives?

  Reply to author
  |
  Forward
  | Print
  | View thread
  | Show original
  | Report this message
  | Find messages by this author
  

On Apr 3, 2008, at 12:31 PM, John Cowan wrote:

> That reminds me of an old question of mine. In the case where a
> thread has been created from an object whose class is a subclass of
> Thread, is Thread.currentThread() guaranteed to give you back the same
> object?

Yes. You can then cast it to the subclass and go on from there.

> If so, that provides a different approach
> to thread-local state, where the state is held in the instance
> variables of MyThread objects, and the JVM is used to thread them
> through :-) the code until the point where they're needed.

That's how Java thread locals are implemented in the JVM.

You can only use this approach if (a) you control the creation of the
thread and can specify your own subclass for it, or (b) you can edit
the rt.jar code for java.lang.Thread. Plan (c) is to make a
ThreadLocal.

You might be interested to know that Hotspot intrinsifies
Thread.currentThread to a couple of instructions; it's cheap. We did
that to make things like thread locals efficient.

Best,
-- John

Reply to author Forward