Java 8 will not work with CF9. Support for this has only been introduced into CF10 and 11 this year. As CF 9 is EoL, support was not put in.
Thanks for your quick response and confirming. I wanted to make sure I wasn't missing a simple setting or file copy that would make it run.
@haxtbh I don't think your statement is correct -- it is possible to run Java 8 on CF9, however it is not supported. The support issue is somewhat moot now anyways because CF9 has reached End Of Life so there is no official support.
If you do want to run Java 8 on CF9 make sure you apply all the hotfixes first, especially the one where they added support for Java 7.
Here's an example showing someone getting Java8 running on CF9: http://www.trunkful.com/index.cfm/2014/12/8/Preventing-SSLv3-Fallback-in-ColdFusion
For most Java apps updating the JVM to a newer version does not cause major issues, with CF there are some parts that are not pure java, so you might see issues with those parts, or if a library was using an undocumented class in a JVM that was removed, etc.
Granted since CF9 is not supported by adobe the best route is to upgrade to CF10 or CF11.
If you are using Solr collections on CF9, Java 7 u 31 and later (including Java 8) will break Solr collections on CF9. Not sure about CF10. If you download CF11 that already has Java 8, Solr collections seem to work fine.
If you are not using Solr collections and have no intention of doing so, this is not going to be an issue. Just a heads-up, in case Solr collections are a consideration.