What version of JaspeReports has support for Java 8?

As far as i know, JasperReport does not support Java 8 yet, just like you said. In this post a member of JasperSoft staff clearly states that they will introduce it on later versions of their library, so i would trust him: JasperReport - Java 8 compatibility.

We are not aware of any issue with running JasperReports Library on Java 8.

At some point we needed to upgrade the JDT compiler to support Java 8, but after doing so, there were no other issues we know about.