RVM - Beat Me To It & I AM SO THANKFUL!
Changing JDK version's in Java has always been a peace of cake. Mac OS X actually made it very smooth from an OS implementation perspective. Simple classpath, path changes in a script or (DOS) batch file and your in the different environments very quickly.
Ruby has never really been that far off the beaten path and never been far off to implement the same type of scripts. The issue is the method of how GEMS "system/local" cache are handled is the issue and what the best way is to handle isolation from one version of Ruby and its gems from another. This can be implemented in a few very smart scripts. But, we haven't done it, have we? Well Wayne did and its called "RVM"!
RVM is the answer and it is not just a few scripts, is substantial effort and a professionally thorough effort. Wayne has done a great job!
Now, why is RVM a great tool? Because it eases the burden of a developer from switching from version to version and learning the edge releases of the language and frameworks. Here's a real world use case for beginners.
In learning Ruby picking up The MUST HAVE books are "The Pick Axe" and "Agile Web Development With Rails A Pragmatic Guide Pragmatic". Sometimes "newbies" have a challenge of which version to use. The case of having newest edition of the "The Pick Axe" book describes version 1.9 and "Agile Web Development With Rails A Pragmatic Guide Pragmatic" is on its 3rd edition working into Rails 2.3.x. Depending on what you've installed and what books you have you may be out of synch with 1.8.7 or 1.9.x in Ruby and Rails 2.0 to 2.3 and coming soon version 3.0.1 RVM is your friend! Setup the right version for your the books, class or development with a few key strokes.
Depending on what versions you install in terms of Ruby & the Rails framework or a project that you may inherit, start or re-factor your versions may be out of synch. Some cases you may be forced into Ruby version 1.8.6 with a Rails version 2.1.1 or some mix that may not fit your current rig!
What to do? Last night Adam Keyes gave a great presentation on "RVM" and basically that's what you do. You use "RVM".
Basically, multiple version of Rubies and Gems. It's a fantastic tool and if your're not using this type of scripting tool to control your Ruby environment, you should. Take a look it will save yo a lot of time and open you up to the edge releases with out risking your deve environment.
1 Note, RVM manages the RUBY VM and Gems loaded for the specified Ruby environment. the reference to Rails (which is a gem) is only for a discussion to explain the management of Ruby VMs and Gems and using Rails as an example of a Gem.