Listener.log shows same DB connections every minute - what is this?

Dear all,

I encountered a very mysterious fact:

When grep'ing in the listener.log file for all entries of some particular database (it could any of the databases I administer) I found following entries occurring (at least) every minute:

08-MAR-2018 15:10:41 * (CONNECT_DATA=(SID=<db_sid>)(CID=(PROGRAM=)(HOST=__jdbc__)(USER=oracle))) * (ADDRESS=(PROTOCOL=tcp)(HOST=<ip_address>)(PORT=<some_port>)) * establish * <db_sid> * 0

08-MAR-2018 15:11:41 * [...........same as above.........]

08-MAR-2018 15:12:41 * [...........same as above.........]

[...]

I already know that the OS user "oracle" is correctly establishing (hence the "0" at the end of the line) some sort of JDBC connection (derived by "__jdbc__") to itself, as the <ip_adress> is the one of the server the databases and listener are running on.