You are definitely using Oracle JDK 8 so I don't understand why you
end up with this kind of error.
I just upgraded to XWiki 8.0 with pretty much the same setup and all seems OK:
* Ubuntu Server 14.04
* Oracle JDK 1.8.0_91 installed trough
http://www.webupd8.org/2012/09/install-oracle-java-8-in-ubuntu-via-ppa.html
* I commented the JAVA_HOME property in /etc/default/tomcat7 and rely
on oracle-java8-set-default package (provided by webupd8 ppa) to set
the proper default version
On Wed, May 25, 2016 at 12:48 PM, Stéphane LASSIRE
<slassire(a)cesap.asso.fr> wrote:
  Hello Thomas,
 The answer :
 root@CESAP-WIKI01:/var/log/tomcat7# ps aux | grep tomcat
 tomcat7   6355  7.3  6.7 2182572 275136 ?      Sl   12:34   0:59
 /usr/lib/jvm/java-8-oracle/bin/java
-Djava.util.logging.config.file=/var/lib/tomcat7/conf/logging.properties
  -Djava.awt.headless=true -Xmx512m -XX:+UseConcMarkSweepGC
-Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager
  -Djava.endorsed.dirs=/usr/share/tomcat7/endorsed -classpath
 /usr/share/tomcat7/bin/bootstrap.jar:/usr/share/tomcat7/bin/tomcat-juli.jar
 -Dcatalina.base=/var/lib/tomcat7 -Dcatalina.home=/usr/share/tomcat7
-Djava.io.tmpdir=/tmp/tomcat7-tomcat7-tmp
 org.apache.catalina.startup.Bootstrap start
 thanks for the help
 Cordialement
 Stéphane Lassire
 -----Message d'origine-----
 De : users [mailto:users-bounces@xwiki.org] De la part de Thomas Mortagne
 Envoyé : mercredi 25 mai 2016 12:46
 À : XWiki Users
 Objet : Re: [xwiki-users] Error upgrading Debian
 "Invalid byte tag in constant pool: 15" still looks like a Java version
 issue.
 Could you check what version of Java started with "ps aux | grep tomcat" ?
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Thomas Mortagne