I have a Spring Boot app that works smoothly when it connects to an Oracle instance in the classic way:
jdbc:oracle:thin:@<server name>:<port num>/<service name>
However, when I turn to a connection via tns through a tnsnames.ora stored in my local drive "C:\ORACLE\Client11g\network\admin" something doesn't work and I get this exception:
Caused by: org.hibernate.service.spi.ServiceException: Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:267)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:231)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:210)
    at org.hibernate.engine.jdbc.internal.JdbcServicesImpl.configure(JdbcServicesImpl.java:51)
    at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.configureService(StandardServiceRegistryImpl.java:94)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.initializeService(AbstractServiceRegistryImpl.java:240)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.getService(AbstractServiceRegistryImpl.java:210)
    at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.handleTypes(MetadataBuildingProcess.java:352)
    at org.hibernate.boot.model.process.spi.MetadataBuildingProcess.complete(MetadataBuildingProcess.java:111)
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.metadata(EntityManagerFactoryBuilderImpl.java:858)
    at org.hibernate.jpa.boot.internal.EntityManagerFactoryBuilderImpl.build(EntityManagerFactoryBuilderImpl.java:885)
    at org.springframework.orm.jpa.vendor.SpringHibernateJpaPersistenceProvider.createContainerEntityManagerFactory(SpringHibernateJpaPersistenceProvider.java:60)
    at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:353)
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.buildNativeEntityManagerFactory(AbstractEntityManagerFactoryBean.java:373)
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:362)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1642)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1579)
    ... 36 more
Caused by: org.hibernate.HibernateException: Access to DialectResolutionInfo cannot be null when 'hibernate.dialect' not set
    at org.hibernate.engine.jdbc.dialect.internal.DialectFactoryImpl.determineDialect(DialectFactoryImpl.java:100)
    at org.hibernate.engine.jdbc.dialect.internal.DialectFactoryImpl.buildDialect(DialectFactoryImpl.java:54)
    at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:137)
    at org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator.initiateService(JdbcEnvironmentInitiator.java:35)
    at org.hibernate.boot.registry.internal.StandardServiceRegistryImpl.initiateService(StandardServiceRegistryImpl.java:88)
    at org.hibernate.service.internal.AbstractServiceRegistryImpl.createService(AbstractServiceRegistryImpl.java:257)
    ... 52 more
10:37:32,201 ERROR [org.jboss.msc.service.fail] (ServerService Thread Pool -- 381) MSC000001: Failed to start service jboss.undertow.deployment.default-server.default-host./my_service: org.jboss.msc.service.StartException in service jboss.undertow.deployment.default-server.default-host./my_service: java.lang.RuntimeException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [app/persistence/OracleConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.service.spi.ServiceException: Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
    at org.wildfly.extension.undertow.deployment.UndertowDeploymentService$1.run(UndertowDeploymentService.java:85)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)
    at org.jboss.threads.JBossThread.run(JBossThread.java:320)
Caused by: java.lang.RuntimeException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [app/persistence/OracleConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.service.spi.ServiceException: Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
    at io.undertow.servlet.core.DeploymentManagerImpl.deploy(DeploymentManagerImpl.java:236)
    at org.wildfly.extension.undertow.deployment.UndertowDeploymentService.startContext(UndertowDeploymentService.java:100)
    at org.wildfly.extension.undertow.deployment.UndertowDeploymentService$1.run(UndertowDeploymentService.java:82)
    ... 6 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [app/persistence/OracleConfiguration.class]: Invocation of init method failed; nested exception is org.hibernate.service.spi.ServiceException: Unable to create requested service [org.hibernate.engine.jdbc.env.spi.JdbcEnvironment]
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1583)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:553)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1081)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:856)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
    at org.springframework.boot.context.embedded.EmbeddedWebApplicationContext.refresh(EmbeddedWebApplicationContext.java:122)
    at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:761)
    at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:371)
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:315)
    at org.springframework.boot.web.support.SpringBootServletInitializer.run(SpringBootServletInitializer.java:151)
    at org.springframework.boot.web.support.SpringBootServletInitializer.createRootApplicationContext(SpringBootServletInitializer.java:131)
    at org.springframework.boot.web.support.SpringBootServletInitializer.onStartup(SpringBootServletInitializer.java:86)
    at org.springframework.web.SpringServletContainerInitializer.onStartup(SpringServletContainerInitializer.java:169)
    at io.undertow.servlet.core.DeploymentManagerImpl$1.call(DeploymentManagerImpl.java:186)
    at io.undertow.servlet.core.DeploymentManagerImpl$1.call(DeploymentManagerImpl.java:171)
    at io.undertow.servlet.core.ServletRequestContextThreadSetupAction$1.call(ServletRequestContextThreadSetupAction.java:42)
    at io.undertow.servlet.core.ContextClassLoaderSetupAction$1.call(ContextClassLoaderSetupAction.java:43)
    at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
    at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
    at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
    at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
    at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
    at io.undertow.servlet.api.LegacyThreadSetupActionWrapper$1.call(LegacyThreadSetupActionWrapper.java:44)
    at io.undertow.servlet.core.DeploymentManagerImpl.deploy(DeploymentManagerImpl.java:234)
    ... 8 more
The following is my application.properties + application-oracleprod.properties files. Keep in mind that this approach works when I use a SID connection as previously said.
application.properties:
spring.profiles.active=oracleprod
spring.jpa.database-platform=org.hibernate.dialect.Oracle10gDialect
spring.jpa.hibernate.ddl-auto=validate
hibernate.show_sql=true
hibernate.hbm2ddl.auto=update
hibernate.generate_statistics=true
application-oracleprod.properties (here the magic should happen due to this property "oracle.net.tns_admin" which sets the "tnsnames.ora" folder plus the link "@MY_SERVICE" which is obviously present in the "tnsnames.ora" file, but I really don't see what Oracle does under the hood):
spring.datasource.driver-class-name=oracle.jdbc.OracleDriver
#/tnsnames.ora
oracle.net.tns_admin=C:/ORACLE/Client11g/network/admin
oracle.username=my_user
oracle.password=xyz
oracle.url=jdbc:oracle:thin:@MY_SERVICE
The Configuration class is clean and straightforward:
@Configuration
@ConfigurationProperties(prefix="oracle" )
public class OracleConfiguration {
    @NotNull
    private String username;
    @NotNull
    private String password;
    @NotNull
    private String url;
    /*@Value("${oracle.net.tns_admin}")
    private String tns;*/
    public void setUsername(String username) {
        this.username = username;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    public void setUrl(String url) {
        this.url = url;
    }
    /*public void setTns(String tns) {
        this.tns = tns;
    }*/
    @Bean
    DataSource dataSource() throws SQLException {
        OracleDataSource dataSource = new OracleDataSource(); // we use oracle data source API but it could be mysql
        dataSource.setURL(this.url);
        dataSource.setUser(this.username);
        dataSource.setPassword(this.password);
        dataSource.setImplicitCachingEnabled(true);
        dataSource.setFastConnectionFailoverEnabled(true);
        return dataSource;
    }
    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
        LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
        try {
            em.setDataSource(dataSource());
        } catch (SQLException e) {
            e.printStackTrace();
        }
        em.setPackagesToScan(new String[] { "app.persistence.entity" }); // put the name of entity classes package 
        JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        em.setJpaVendorAdapter(vendorAdapter);
        //em.setJpaProperties(additionalProperties()); // this properties are set by "application-oracle.properties" file
        return em;
    }
    @Bean
    public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
        JpaTransactionManager transactionManager = new JpaTransactionManager();
        transactionManager.setEntityManagerFactory(emf);
        return transactionManager;
    }
To summarize, I have followed the approach suggested in the Oracle documentation in combination with Spring Boot configuration files/class with no luck.
I have also tried different path formats:
C:/ORACLE/Client11g/network/admin/tnsnames.ora
C:\ORACLE\Client11g\network\admin
C:\\ORACLE\\Client11g\\network\\admin
I think I'm doing well because with a SID connection everything is perfect but probably something hidden is missing.
UPDATE
It turned out the db url is not correct, at least for a TNS connection. The thing is we have something like multiple hosts and not only a single one:
    MY_SERVICE =
  (DESCRIPTION_LIST =
    (LOAD_BALANCE = off)
    (FAILOVER = on)
    (DESCRIPTION =
      (CONNECT_TIMEOUT = 5)
      (TRANSPORT_CONNECT_TIMEOUT = 3)
      (RETRY_COUNT = 3)
      (ADDRESS_LIST =
        (LOAD_BALANCE = on)
        (ADDRESS = (PROTOCOL = TCP)(HOST = <ip_1> )(PORT = 1621))
        (ADDRESS = (PROTOCOL = TCP)(HOST = <ip_2> )(PORT = 1621))
      )
      (CONNECT_DATA =
        (SERVICE_NAME = <my_service_name>)
      )
    )
    (DESCRIPTION =
      (CONNECT_TIMEOUT = 5)
      (TRANSPORT_CONNECT_TIMEOUT = 3)
      (RETRY_COUNT = 3)
      (ADDRESS_LIST =
        (LOAD_BALANCE = on)
        (ADDRESS = (PROTOCOL = TCP)(HOST = <ip_3> )(PORT = 1621))
        (ADDRESS = (PROTOCOL = TCP)(HOST = <ip_4> )(PORT = 1621))
      )
      (CONNECT_DATA =
        (SERVICE_NAME = <my_service_name> )
      )
    )
  )
And I got:
 javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory
    Caused by: java.lang.RuntimeException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaAutoConfiguration.class]: Invocation of init method failed; nested exception is javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory
    Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaAutoConfiguration.class]: Invocation of init method failed; nested exception is javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory
    Caused by: javax.persistence.PersistenceException: [PersistenceUnit: default] Unable to build Hibernate SessionFactory
    Caused by: org.hibernate.exception.JDBCConnectionException: Unable to open JDBC Connection for DDL execution
    Caused by: java.sql.SQLRecoverableException: Errore di I/O: Unknown host specified 
    Caused by: oracle.net.ns.NetException: Unknown host specified "
Again, I'm a little bit confused how to tell Spring how to read this TNS configuration.
Any idea?