I have a setup where Spring-Boot, meaning Spring-Data in the end, manages all my database connections with the default settings. No problems there. For integration-tests I overrode the DataSource, like this:

@Configuration
public class DataSourceConfiguration {
  @Bean
  public DataSource dataSource() { return new ExtendedHikariDataSource(); }

  // If you override dataSource, Spring needs the 3 other Beans to be provided too:
  @Bean
  public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
    Map<String, String> jpaProperties = new HashMap<>();
    jpaProperties.put("hibernate.dialect", "org.hibernate.dialect.PostgreSQLDialect");
    jpaProperties.put(TransactionSettings.AUTO_CLOSE_SESSION, "false");
    jpaProperties.put(MappingSettings.PHYSICAL_NAMING_STRATEGY ...);
    ...
    return new EntityManagerFactoryBuilder(new HibernateJpaVendorAdapter(), 
                               jpaProperties, null)
        .dataSource(dataSource())
        .packages("org.mypackage")
        .build();
  }

  @Bean
  public JdbcTemplate jdbcTemplate() { 
    return new JdbcTemplate(dataSource());
  }

  @Bean
  public PlatformTransactionManager transactionManager() {
    return new JpaTransactionManager(entityManagerFactory().getObject());
  }
}

The problem is now, that Spring closes the database connection after almost all operations, even a simple save. Like this:

...
// Standard Spring "CrudRepository".
@Autowired private MyCrudRepository myCrudRepository;

@Test
void testSave() {
  myCrudRepository.save(myObject);
  // Connection gets closed. Instantly, no timeouts or anything.
}

Now I constantly run into exceptions being thrown, because the connection got closed. What setting do I have to set, to tell Spring / Hibernate / JPA / etc. that I want the “normal” behavior?