How to create a database connection pool manager without using third party libraries?

Question

Java

Hello.

The following task arose: it is necessary to write a connection manager that will contain two pools of connections to databases, one of them is the main one, and the second is a backup. Those. so that when communication with the main pool fails, it automatically switches to the backup one, and in case of restoration of communication with the main base, switch back to it. The task also states that the connection manager should not be any Open Source library.

I would appreciate any information on this topic.