JAVA_HOME is not defined correctly on Ubuntu?

Add both JAVA_HOME & PATH to your ~/.profile

export JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64
export PATH=$JAVA_HOME/bin:$PATH

And, add following to your /etc/profile.d/java.sh

JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64
export JAVA_HOME
PATH=${JAVA_HOME}/bin:${PATH}
export PATH
JRE_HOME=/usr/lib/jvm/jre
export JRE_HOME
JAVA_OPTS="-XX:+AggressiveOpts -Xms256m -Xmx512m -XX:MaxPermSize=256m -XX:+DisableExplicitGC"
export JAVA_OPTS

For more info, Refer Documentation

Hope it helps.

According to Your editor.

sudo vim /etc/profile

add these 2 lines at the end of the file

export JAVA_HOME="/usr/lib/jvm/java-8-oracle"  
export PATH=JAVA_HOME/bin:$PATH

Then

source /etc/profile

Check

mvn -version

Instead of

export JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64

I did

EXPORT JAVA_HOME='/usr/lib/jvm/java-1.7.0-openjdk-amd64'

Works.

from Ubantu terminal execute:

export JAVA_HOME=/usr/lib/jvm/java-7-openjdk-amd64/

to test maven run:

mvn --version

it will give output:

Maven home: /usr/share/maven
Java version: 1.7.x.xxx, vendor: Oracle Corporation, runtime: /usr/lib/jvm/java-7-openjdk-amd64/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "4.15.0-47-generic", arch: "amd64", family: "unix"