Magazine Article

Letters to the Editor

2001

Overview

I am a Java developer, but I never realized the subtleties that optimizations could cause. I enjoy a good test of my abilities and took the article as a challenge to find a solution. I am by no means an expert in the workings of microprocessors and optimizing compilers/ JITs. However, I have come up with a technique that I would like you to comment on. It involves creating a second instance variable to hold the reference to the singleton. The call to getHelper() still checks to see if the helper instance variable reference is null. If it is, instead of instantiating a new Helper object, it calls a second synchronized method, getSingleton(), that has its own null check. That method then instantiates a helper() method and sets the singleton reference to it; the singleton is then returned. Before A exits the synch block, data written to A's memory may not be moved to main memory, or it could be moved in an order different than the order in which it was written to A's memory. Also, some values may be written out slowly to main memory. But let's say that the reference to the new Resource is written out to main memory before the constructor finishes, which is certainly possible. If B goes to main memory to examine the reference, it may see the valid value of resource, but not the fields of resource (because it's still being constructed or those fields have not yet been written to main memory). That's bad; it sees a partially constructed Resource. Those are good questions. Clearly, there are differences between XPath and XPointer, as you mentioned. XPointer is built on top of XPath, but allows you to select areas of an XML document that a pure XPath could not address.

• Publisher: Foundry
• Subject: Goetz, Brian / Vervaet, Erwin