A command to print JAVA_VERSION used by Hadoop/HDFS

The java version used by hadoop is controlled by JAVA_HOME variable defined in hadoop_env.sh. We log this information when HDFS starts in the log file.

STARTUP_MSG:   java = 1.8.0_112

However, it is quite possible that a user might have many versions of java installed on his/her machine. Generally, users tend to check for the java version via

java -version

This just means we are printing out the java version in the current shell path.

This jira proposes adding a new simple command or an extension to existing hadoop version command where the current java version used by hadoop is also printed out.

This avoids customer confusion when they are looking at if the java stack is properly configured. For example, checking if JCE is installed correctly.

This is a very minor change that can be done by modifying hdfs.cmd or hdfs shell script in the /bin directory. Thanks to sujit for bringing this to my attention.