HikariCP Connection Pool immediately creates 100 connections

Question

I have this code that uses HikariCP Connection Pool:

config.setMaximumPoolSize(100);
config.setDataSourceClassName("com.mysql.jdbc.jdbc2.optional.MysqlDataSource");
config.addDataSourceProperty("serverName", hostname);
config.addDataSourceProperty("port", portnumber);
config.addDataSourceProperty("databaseName", dbname);
config.addDataSourceProperty("user", username);
config.addDataSourceProperty("password", password);
config.setConnectionTimeout(30000);
config.setInitializationFailFast(false);
pooledDataSource = new HikariDataSource(config);

I monitor connections in mysql by issuing command "Show Processlist" and I see that 100 connections is created after line:

pooledDataSource = new HikariDataSource(config);

...is run. I'm sure this is not meant to happen, right? It should create connections later when I do pooledDataSource.getConnection().

What am I doing wrong? Why is it creating 100 connections immediately??

Answer 1

By default HikariCP runs as a fixed-sized pool. You need to set minimumIdle. That's it.

From the documentation for minimumIdle:

This property controls the minimum number of idle connections that HikariCP tries to maintain in the pool. If the idle connections dip below this value, HikariCP will make a best effort to add additional connections quickly and efficiently. However, for maximum performance and responsiveness to spike demands, we recommend not setting this value and instead allowing HikariCP to act as a fixed size connection pool. Default: same as maximumPoolSize