I've modified spring.datasource.hikari.maximumPoolSize values (with differents values 1, 2, 3 and 4) and when the application execute a findAll or a query without the clause WHERE the execution is blockade.

In theory there are 2 free connections but the error returned is:

org.springframework.transaction.CannotCreateTransactionException: Could not open JPA EntityManager for transaction; nested exception is org.hibernate.exception.JDBCConnectionException: Unable to acquire JDBC Connection

Hi Warlin! thank you for your answer, I tried modifying the connection pool size to 1, 2 and 3 and the problem continued. 

Sure! You have it available here:

https://github.com/es-comunidad-intersystems/webinar-multimodelo/tree/master/jdbc-jpa-rest

It's a MAVEN project so you only need MAVEN installed in your PC and modify the actions.properties to define the parameters to access to the database.