...
Map<String, Object> properties = new HashMap<String, Object>();

// Enable DDL Generation
properties.put(PersistenceUnitProperties.DDL_GENERATION, PersistenceUnitProperties.NONE);
properties.put(PersistenceUnitProperties.CREATE_JDBC_DDL_FILE, "EclipseLink_DDL.sql");
properties.put(PersistenceUnitProperties.DDL_GENERATION_MODE, PersistenceUnitProperties.DDL_SQL_SCRIPT_GENERATION);

//eManager is the injected EntityManager with @PersistenceContext
ServerSession session = eManager.unwrap(ServerSession.class);  
SchemaManager schemaManager = new SchemaManager(session);  
    			
schemaManager.setCreateSQLFiles(true);
StringWriter w = new StringWriter();

schemaManager.outputDDLToWriter(w);
schemaManager.createDefaultTables(true);
String s =w.toString();
...