JAVA_HOME env is unset when starting any process as a service

Although JAVA_HOME is set as an environmental var, when starting any process as a service, almost all environmental variables are dropped and the application is required to define JAVA_HOME.

I found that CDH uses the script /usr/lib/bigtop-utils/bigtop-detect-javahome to determine which version of java to use. While this works if a user has multiple version of Java installed, bigtop-detect-javahome will prefer java 6 over java 7 and java 7 over java 8. The only way for a user to configure which java to use is to hardcode JAVA_HOME into /usr/lib/bigtop-utils/bigtop-detect-javahome. This is not elegant.

I propose that bigtop-detect-javahome read from the file /etc/default/java_home to determine the $JAVA_HOME. This is an elegant solution that allows users to define JAVA_HOME and have it used in all CDH supported software.

The only line of code needed to be added to bigtop-detect-javahome is:

[ -f /etc/default/java_home ] && . /etc/default/java_home

The user can than choose to define JAVA_HOME in /etc/default/java_home or they can allow bigtop-detect-javahome to detect it for them.