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I am able to achieve a connection pool to my database using SpringTransactionManager(dataSource) . But as my dao layer use transaction { } blocks, I get the IllegalStateException: Already value for key bound to thread [main]

I reproduced that issue in tests in going to look why it happens. 
ATM I can only recommend to do not mix SpringTransactionManager with transaction {} block from ThreadLocalTransactionManager file.

<@U8RNPUYAX>, fixed in master. Will be available on next release

<@U0EC2UPDF> I couldn't accomplish the task of configuring exposed with hikariCP on a spring-boot project using spring's application.properties file for specifying the jdbc params. (without using the SpringTransactionManager)
I was able to get the DataSource instance but the subsequent call to Database.connect(ds) failed with error: 
java.lang.IllegalArgumentException: one of either dataSource or dataSourceClassName or jdbcUrl must be specified

And hence I had to import the spring-transaction dependency for the Database.connect to happen properly

Can you provide me with a sample of same ?

I have no experience with spring boot + hikariCP, but you can take <https://github.com/sdeleuze/geospatial-messenger> as base example and extend <https://github.com/sdeleuze/geospatial-messenger/blob/master/src/main/resources/application.yml> by adding HikariCP support (<https://www.mkyong.com/spring-boot/spring-boot-jdbc-mysql-hikaricp-example/> )