Spring boot application with JpaRespository and c3p0 leaking connections

Question

I found a deadlock in my system, but my attempts at getting rid of it failed.

There's this endpoint that is firing 3 or more requests to the DB, so when there's enough requests fo my endpoint fired simultaneously, the threads are locking each other, and the service is dead.

I tried to solve this by getting my endpoint to reuse its connection by annotating the first method call at controller-level with @Transactional.

This doesn't work though, the first request is still trying to get its own connection from the pool.

I used a TransactionManagerConfig.java for the configuration

@Configuration

public class TransactionManagerConfig {

@Bean(name ="oracleDataSourceTransactionManager", "transactionManager")
@Profile("!test")

@Primary

public DataSourceTransactionManager dataSourceTransactionManager(DataSource dataSource){
return new DataSourceTransactionManager(dataSource);}

The method talking to the Database is called from a RestController, so I tried to use a @Transactional annotation at the controller level:

@RestController
@RequestMapping(value ={"/customer", "/agent", produces = {"application/json"})

@Transactional
public class UserController extends BaseController {

further down the calling stack, I'm getting a Customer number from the DB:

public interface UserRegistrationRepository extends JpaRepository<UserRegistration, Long>{

@Query(value = "SELECT CUSTOMER_NO FROM CUSTOMERS WHERE PK_USER =?1", nativeQuery = true)
Long getCustomerNumber(Long userId);
...

For some reason this is trying to get a new Transaction, causing a deadlock, if my connection pool runs out of connections.