You can specify which java version you're compiling and execute (unit and integration test included) within maven execution. Granted it's not rvm/rbenv that literally install multiple version of Ruby.

It doesn't. You just install the latest java and it's fully backwards compatible.

That's not really true. We still run some older Apache camel components that require Java 7. And when Java 8 first came out it had bugs that crippled Solr, it took a long time before that was stable enough to use everywhere.

I'm not sure that's really true, but it regardless misses the point. No matter what language you're using, you'll eventually need to be able to deal with multiple versions of it.

Even if you're running an operation small enough that you can upgrade every single runtime in use all in one step, you'll still need to have two versions around while you test. But many, many places have to deal with multiple language versions simultaneously. E.g., if you maintain an open source library and have any regard for your users, you have to test how it behaves in multiple language versions.

Well Ruby is too. If something worked on 1.9 it works on 2.3.

Yeah? Try rails 3.2.17 which work perfectly on ruby 1.9 and see how well it works on 2.2.

(java is not fully backwards compatible either, but it may be a bit more tolerant in some cases)

I've done the work before to make rails 3.2 work on 2.2. I recall the problem is due to a breaking Bcrypt name change and Yaml serialization, and it's a one line patch. The maintainers are aware of the issue but held back so as to not break every 3.2 installation on a minor upgrade (would have to be called Rails 3.3 for semver?). Rails 4.0 isnt that hard of an upgrade path if you include the attraccessible gem.

It took quite a while before rails released a version of 3.2 that worked.

As for how fun upgrading to rails4 is, you obviously don't have a project with 200 database tables that has models using the attr_protected for the five columns (appwide) that actually need it. Forced whitelisting is not fun, regardless of if it is on model or controller level.

Rails isn't ruby.

Did I claim that? However, it is written in ruby and as such it is a valid counterexample to the claim in the comment I commented on.

It's Ruby on Rails

No, Java is not fully backwards compatible. It may be more backwards compatible compared to ruby, but claiming that it is fully backwards compatible is simple false.