Thursday, November 29, 2007


There are two kinds of people: those who quote the Red Dwarf going-to-bed-with-Wilma-Flintstone line in their signatures, and those who have gotten over it.

Yes, it's funny. But not that funny.

Monday, November 05, 2007

Concurrency issues in Restlet

Issue #368 in the Restlet issue tracker was originally focused on the Guard class. Since then, I've taken a look at the rest of the org.restlet package and found that the fields of most classes are accessed without appropriate synchronization. [Update 2008-Feb-6: All of these have been fixed as of Restlet 1.1 M1.]

Most of these problems would disappear if the fields could be final, but that would mean taking away setter-injectability.

There is hope, however: Methods that are used only for setter injection (and not, for example, used to reconfigure an instance dynamically), should be documented as setter-injection-only, meaning they must not be called after the instance has been (safely) published. Fields set by such methods get their values before publication, and those values never change; such fields are effectively immutable. I don't know of any standard documentation conventions for this, but the important thing is to decide for each non-final, non-volatile field whether it is truly mutable or only setter-injected before publication.

I have a long-standing gripe about the Spring documentation that it doesn't state, at least not in terms that I recognize, under what conditions beans are safely published. The vanilla use cases (standard ApplicationContext implementations) are almost certainly fine, but if you have any doubts about whether your dependency injection framework can guarantee safe publication, then you should either guard your setter-injected fields with a lock or make them volatile. I have a personal preference for using volatile in this case, but most of my Java Concurrency in Practice co-authors have the opposite preference.

Even if you're fairly confident about your DI framework's guarantees, it is never wrong to guard field access or make a field volatile, only potentially wasteful. For a framework like Restlet that should be usable in any DI context, I think it would be prudent to assume the worst of that context.

But is setter injection really necessary? Most of the injected fields are of distinct types, so constructor injection in Spring would be straightforward. Getting rid of setter injection entirely would allow most, if not all, fields to be final, which would greatly reduce the risk of concurrency problems in Restlet.