How to debug something that works under Java 7 but not under Java 8

I have a project with a unit test that works under Java 7, but not under Java 8. Is there a good way to investigate such things? (I'm sure that the test is correct; this suggests that there's a subtle bug in the implementation.)

Really I suppose what I would like is a quick way to identify where the code paths diverge. But this is hard, because there might be all sorts of differences in the code paths at a very low level through the JDK, and I don't want to get bogged down in irrelevant differences that are down to tiny optimisations.

So the nice thing would maybe be to ask at what top level trace the paths diverge; and then, starting from just before that point, to ask at what second level trace the paths diverge; and so on.

But I've no idea whether there's a way to do this. I fear I could waste a lot of time if I don't have a systematic approach.

The code, by the way, is the Apache Phoenix repository, where under Java 8, I get the following failure:

Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 0.006 sec <<< FAILURE! - in org.apache.phoenix.schema.PMetaDataImplTest
testEviction(org.apache.phoenix.schema.PMetaDataImplTest)  Time elapsed: 0.006 sec  <<< FAILURE!
java.lang.AssertionError: expected:<3> but was:<2>
    at org.junit.Assert.fail(Assert.java:88)
    at org.junit.Assert.failNotEquals(Assert.java:834)
    at org.junit.Assert.assertEquals(Assert.java:645)
    at org.junit.Assert.assertEquals(Assert.java:631)
    at org.apache.phoenix.schema.PMetaDataImplTest.testEviction(PMetaDataImplTest.java:98)