Set javac version 1.8 on Ubuntu 16.04 (#1483)

* set up javac version

* fix update-java-alternatives

* add version check

* change java check

* fix checks and position of update-java-alternatives

* resolve comments

Co-authored-by: Dmitry Shibanov <v-dmshib@microsoft.com>

images/linux/scripts/installers/java-tools.sh

@@ -10,6 +10,15 @@ source $HELPER_SCRIPTS/os.sh
 
 set -e
 
+function javaTool {
+    if [[ "$2" =~ ([1]{0,1}.)?$DEFAULT_JDK_VERSION.* ]]; then
+        echo "$1 $2 is equal to default one $DEFAULT_JDK_VERSION"
+    else
+        echo "$1 $2 is not equal to default one $DEFAULT_JDK_VERSION"
+        exit 1
+    fi
+}
+
 # Install GPG Key for Adopt Open JDK. See https://adoptopenjdk.net/installation.html
 wget -qO - "https://adoptopenjdk.jfrog.io/adoptopenjdk/api/gpg/key/public" | apt-key add -
 add-apt-repository --yes https://adoptopenjdk.jfrog.io/adoptopenjdk/deb/
@@ -38,6 +47,11 @@ apt-get -y install adoptopenjdk-8-hotspot=\*
 apt-get -y install adoptopenjdk-11-hotspot=\*
 
 # Set Default Java version.
+if isUbuntu16; then
+    # issue: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=825987
+    # stackoverflow: https://askubuntu.com/questions/1187136/update-java-alternatives-only-java-but-not-javac-is-changed
+    sed -i 's/(hl|jre|jdk|plugin|DUMMY) /(hl|jre|jdk|jdkhl|plugin|DUMMY) /g' /usr/sbin/update-java-alternatives
+fi
 update-java-alternatives -s /usr/lib/jvm/adoptopenjdk-${DEFAULT_JDK_VERSION}-hotspot-amd64
 
 echo "JAVA_HOME_8_X64=/usr/lib/jvm/adoptopenjdk-8-hotspot-amd64" | tee -a /etc/environment
@@ -85,6 +99,11 @@ for cmd in gradle java javac mvn ant; do
     fi
 done
 
+javaVersion=$(java -version |& head -n 1 | cut -d\" -f 2)
+javaTool "Java" $javaVersion
+javacVersion=$(javac -version |& cut -d" " -f2)
+javaTool "Javac" $javacVersion
+
 # Document what was added to the image
 echo "Lastly, documenting what we added to the metadata file"
 if isUbuntu16 || isUbuntu18 ; then