Q:

Clojure dependency on Java 1.5 only?

I hope this is not off-topic for SO (I hesitated between SO and programmers.stackexchange) but as far as I can tell this is a "practical, answerable problem that is unique to the programming profession" so it's complying with the FAQ.
Which version of the JVM do you need in order to run which version of Clojure (Clojure on the JVM, this question is not about ClojureScript)?
The page here: http://clojure.org/getting_started states that:

Clojure requires only Java 1.5 or greater

But is this always going to be the case?
And what about the Clojure ecosystem, like Leiningen?
Basically I'd like to know if I can count on Clojure to be able to develop a desktop app that should run on systems, including OS X systems, that are never going to get Java 6 nor more recent versions of Java (for example on some OS X versions Apple stated that no JVM 6 would ever see the light).

A:

I am not on the Clojure/core team, so I don't have inside information, but here is how I would approach this situation.
Take the latest version of Clojure (1.5 as of this writing) and test it against Java 1.5 for the things you need to do and any Clojure libraries you need to use and stick with that.  If Clojure 1.5 is Java 1.5 compatable, it will always be so, since that release is immutable.
I would not make the assumption that all Clojure versions after 1.5 will be Java 1.5 compatible and you definitely can't assume that Clojure libraries will be.  For example, I just released a Clojure library that requires Java 1.7 (since it uses a java.util.concurrent class introduced in Java 1.7).
Since Leiningen gives you maven-like dependency resolution if you test all your libraries and your chosen version of Clojure against Java 1.5 and they work, then you can stick with that set of versioned dependencies for as long as you want to keep the app running.  Your only risk at that point is that some bug fix releases of a non core library might not be Java 1.5 compatible any longer.  This risk is proportional to how many non-core Clojure libraries you need to use. 

A:

If you are selective about what libraries you use, then targeting Java 1.5 is certainly feasible.

Clojure is very conservative about it's Java version requirements - hence the dependency on version 1.5 only.
Many libraries depend only on Clojure itself, so will run quite happily on the 1.5 JVM
Some libraries require >1.5, these tend to be more advanced libraries that have requirements for interop with specific Java features (e.g. newer concurrency code).

Note: I personally write all my apps/libraries to target Java 1.6, since I think that is a reasonable minimum and the vast majority of Java-based systems are at 1.6 or above. I'm willing to live with the potential loss of a few users who are stuck on 1.5. Also, even if a future version of Clojure does abandon 1.5, I expect it will continue support for Java 1.6 for a long time.

