What is a database connection pool?

Database connection pool, for short, DBCP database connection pool

Existence Meaning:
Database connection is very expensive system resources, an application usually need to deal with the database, but also need to connect to the database frequently, which leads to wasting a lot of system resources;

The principle of connection pooling is: I have created several database connections in advance, when my system needs to operate the database from the connection pool directly to take the connection, and the connection is marked as busy, after the use of the pool, and marked as free;
When connections in the connection pool are in use, if you are connecting at this point, the connection pool will be placed in the pool where the connection is created, and the number of these connections is controlled by you in the configuration file.

What is a database connection pool?