Where is default Logstash Java classpath?

[thangld (Thang Le)]

Hi,
I'm running logstash in Docker Container.
When I config input for logstash like this:

jdbc {
        jdbc_connection_string => "jdbc:mysql://localhost:3306/mysql?user=root"
        jdbc_driver_class => "com.mysql.jdbc.Driver"
        jdbc_user => "root"
        statement => "SELECT * from songs where artist = 'Bee'"
        add_field => {
        input_plugin => "jdbc"
        log_parser => "none"
        log_source_name => "sdfx"
        }
    }

It gives out this error:

  Error: com.mysql.jdbc.Driver not loaded. :jdbc_driver_library is not set, are you sure you included
                    the proper driver client libraries in your classpath?

Then, I add this config line:
jdbc_driver_library => "/etc/logstash/mysql-connector-java-5.1.36.jar"
And it works.

My question is how can I config jdbc right without adding that line?
If not provided, Plugin will look for the driver class in the Logstash Java classpath.
Where is Logstash Java classpath? So I can mount mysql-connector-java-5.1.36.jar file to that directory.

Thanks