Connection conn = null; 
   try
   conn = ((Session) entityManager.getDelegate()).   connection();

"ERROR [STDERR] java.sql.SQLException: Connection handle has been closed and is unusable"

@Resource
DataSource dataSource; //sun.jdbc.odbc.ee.DataSource;

....

 Connection conn = null; 
 CallableStatement cs = null;
 try { 
    conn =   dataSource.getConnection();
...

<?xml version="1.0" encoding="UTF-8"?>
<!-- Persistence deployment descriptor for dev profile -->
<persistence xmlns="http://java.sun.com/xml/ns/persistence" 
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" 
             version="1.0">
             
   <persistence-unit name="medoK" transaction-type="JTA">
      <provider>org.hibernate.ejb.HibernatePersistence</provider>
      <jta-data-source>java:/medoKDatasource</jta-data-source>
      <properties>
         <property name="hibernate.dialect" value="org.hibernate.dialect.OracleDialect"/>
<!--strochka nizhe byla zakommentirovana!!   -->
    <!--        <property name="hibernate.hbm2ddl.auto" value="validate"/>-->
         <property name="hibernate.show_sql" value="true"/>
         <property name="hibernate.format_sql" value="true"/>
         <property name="hibernate.default_schema" value="MEDO"/>
<!--strochka nizhe byla zakommentirovana!!   -->         
<!--            <property name="jboss.entity.manager.factory.jndi.name" value="java:/medoKEntityManagerFactory"/> --> 
         <property name="hibernate.transaction.manager_lookup_class" value="org.hibernate.transaction.JBossTransactionManagerLookup"/>  
      </properties>
   </persistence-unit>
    
</persistence>

import javax.sql.DataSource;

conn = defaultDS.getConnection();

import javax.sql.DataSource;

conn = defaultDS.getConnection();

Context ctx = new InitialContext();
DataSource ds = 
            (DataSource)ctx.lookup(
               "java:/medoKDatasource");
Connection conn = ds.getConnection();

    import javax.naming.InitialContext;
    import javax.naming.NamingException;
    import javax.sql.DataSource;
    ...
    
     public Connection GetDbConnection() {
       DataSource ds;
	  try {
 		try {
		  InitialContext initialContext = new InitialContext();   
		  InitialContext ctx = initialContext;
		  ds = (DataSource)ctx.lookup( "java:/medoKDatasource" );  
		} catch ( NamingException ne) {
		  System.out.println( "in CATCH -> NamingException!" );
		  throw new RuntimeException(ne);
		}
		return ds.getConnection();  
	  } catch (SQLException e) {
		System.out.println( "in CATCH -> GetDbConnection()!" );
		throw new RuntimeException(e);
	  }
    }

conn.close();