Ehcache supports 1.3, 1.4, 1.5 and 1.6 at runtime. Ehcache final releases are compiled with -target 1.3. This produces Java class data, version 47.0.
When compiling from source, the build process requires at least JDK 1.4, because 1.4 features are compile in but switched out at runtime if the JDK is 1.3. JDK1.3 is supported by catching NoSuchMethodError and providing an alternate implementation. No JDK1.4 or 1.5 language features are used.
Ehcache is known not to work with JDK1.1 and is not tested on JDK1.2.
Because of an RMI bug, in JDKs before JDK1.5 ehcache is limited to one CacheManager operating in distributed mode per virtual machine. (The bug limits the number of RMI registries to one per virtual machine). Because this is the expected deployment configuration, however, ther should be no practical effect.
On JDK1.5 and higher it is possible to have multiple CacheManagers per VM each participating in the same or different clusters. Indeed the replication tests do this with 5 CacheManagers on the same VM all run from JUnit.
For JDK1.4, JDK1.5 and JDK 1.6, ehcache requires commons-logging and commons-collections 2.1.1 from Apache's Jakarta project.
For JDK 1.3, ehcache also requires Apache xerces (xml-apis.jar and xercesImpl.jar), version 2.5.
These dependencies are very common, so they are probably already met in your project.