If you start your own Java function or a Lifecylce Object Action, you can also print messages to your job log. Simply add following code in your function:

JobContext.log("Start to read stream");

If you run your function not as Job via Scheduler, your message will be shown via System.out in your console output.

An example job:

will show following log message:

Successful 
Start to read stream
Read stream after 10000millis