doc: description for jni installation
@@ -37,6 +37,10 @@ Install openjdk-9-jdk openjdk-9-jdk-headless openjdk-9-jre from backports.
When manually executing testcases the following LD_LIBRARY_PATH might be needed:
`/usr/lib/jvm/java-9-openjdk-amd64/lib:/usr/lib/jvm/java-9-openjdk-amd64/lib/server`
+Make sure that 9 is also default java, otherwise CMake will not be able to locate it:
+`/usr/lib/jvm/default-java -> java-9-openjdk-amd64`
+
+
openjdk-8 is known not to work (jvm crashes without usable backtrace).
