Re: JavaMemoryModel: Unsynchronized read as an optimization

From: Bill Pugh (
Date: Fri Dec 15 2000 - 13:59:42 EST

At 1:50 PM -0500 12/15/00, Robert Munyer wrote:
>I've been discussing double-check with the author of a high-performance Java
>web server I'm evaluating. He believes unsynchronized read is safe in the
>majority of JVM installations (even if you only count servers)

Doesn't matter. Real Java programmers don't write code that runs
correctly only on a majority of platforms.

>and he would
>like to have an optimization setting to take advantage of this feature.

Use volatile. On platforms with strong memory models, the performance
hit from using volatile will be insignificant.

>After some thought, I think the idiom below will serve this purpose. I want
>to ask two questions of the experts on this list: (1) Is this code correct?
>And if the answer to #1 is yes: (2) Can you give me an idea of which JVMs
>(current and imminent) will be unable to take advantage of the optimization?

The code belong is wrong. It will not work correctly on SMP's with
weak memory models, such as the COMPAQ Alpha.

> Thanks,
> Robert Munyer <>
>class Foo {
> private Helper helper = null;
> private boolean helperExists;
> public Helper getHelper() {
> if (!helperExists)
> synchronized(this) {
> if (helper == null)
> helper = new Helper();
> else
> helperExists = GlobalSettings.ALLOW_UNSYNCHRONIZED_READ;
> }
> return helper;
> }
> // other functions and members...
>JavaMemoryModel mailing list -

JavaMemoryModel mailing list -

This archive was generated by hypermail 2b29 : Thu Oct 13 2005 - 07:00:29 EDT