Is there an easy way to insert data/time in front of a log file with just shell scripting?

Don't reinvent the wheel. Write the messages to syslog using logger(1), which is supported on just about every Unix flavor. Syslog will take care of the timestamps for you. Your log data will be stored to a system log like /var/log/messages (This is configurable). You don't need to worry about cleaning up the logfiles later on (especially if this job runs forever), because the system logs are automatically rotated by logrotate/newsyslog.

top | grep java | logger -t java_cpuhog

Or, if you really want to write it to your own file. But then you need to clean up the file afterwards:

top | grep java | logger -t java_cpuhog -f /var/log/java-top.log

... | awk -f timetag.awk > ...

timetag.awk:

{
  print "[" strftime() "] " $0
}

In shell: top | grep java | while read LINE ; do date "+%F %T ${LINE}" ; done

You could create something like stamp containing

#!/usr/bin/perl -p
s/^/localtime()." "/e;

Then run

top -b | grep java | stamp > out.log

Don't forget to use top -b if you are not using it interactively. Of course, top already puts a timestamp each time it runs. If you don't necessarily need the date on each line, just use top on its own since it puts a timestamp on each iteration. I sometimes use this to get 10 second samples for a full day to review later:

top -b -d 10 -c -n 8640