Java version used to compile Tomcat7

I was trying to deploy a war that is working on Tomcat7 + Java 7 on Ubuntu 14.04 to a Tomcat7 on Ubuntu 16.04

The Webapp needs Java 7 to works, so I used a PPA to install it.

However I still get a compatibility errors that prevent the webapp from starting.

Below is the error:

==========================================
org.springframework.beans.factory.access.BootstrapException: Error executing bootstraps; nested exception is java.lang.NoSuchMethodError: java.util.concurrent.ConcurrentHashMap.keySet()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;
        at org.codehaus.groovy.grails.web.context.GrailsContextLoader.initWebApplicationContext(GrailsContextLoader.java:124)
        at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:111)
        at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:5068)
        at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5584)
        at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:147)
        at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:899)
        at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:875)
        at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:652)
        at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1259)
        at org.apache.catalina.startup.HostConfig$DeployDirectory.run(HostConfig.java:1998)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
        at java.util.concurrent.FutureTask.run(FutureTask.java:262)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NoSuchMethodError: java.util.concurrent.ConcurrentHashMap.keySet()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;
        ... 5 more
==========================

Googling around, I find that the error is related to some Java version compatibility, and some folks mentioned that if Tomcat7 was compiled using Java8 that will cause that problem too even if the Java version used is Java7

How can I check if tomcat7 was compiled using Java8 or not?

Thanks in advance!