oracle - Java persistence - getSingleResult() did not retrieve any entities -

i testing jpa project follows:

try {             emfactory = persistence.createentitymanagerfactory(persistence_unit_name);             entitymanager em = emfactory.createentitymanager();             query qry = em.createquery("select t testcon t");             system.out.println(qry);             testcon tcon = (testcon) qry.getsingleresult(); //qry.getsingleresult();             rslt = tcon.getb();             if (rslt == null || rslt.equals(""))                 rslt = "could not result";             system.out.println(rslt); //new showmsg(rslt);         } catch (exception ex) {             system.out.println(ex); //new showmsg("exception occured");         } 

the output is:

javax.persistence.noresultexception: getsingleresult() did not retrieve entities. 

the qry string is:

ejbqueryimpl(readallquery(referenceclass=testcon sql="select a, b c##test.testcon")) 

when test directly in sqlplus:

select a, b c##test.testcon 

it gives me correct result. can not find out program fails.

create transaction before executing query.

try {     emfactory = persistence.createentitymanagerfactory(persistence_unit_name);     entitymanager em = emfactory.createentitymanager();      em.gettransaction().begin(); //added      query qry = em.createquery("select t testcon t");     system.out.println(qry);      //need use getresultlist() if table has > 1 row     list<testcon> tcon = (list<testcon>) qry.getresultlist();      for(testcon rslt:tcon){           rslt = tcon.getb();          if (rslt == null || rslt.equals("")){              rslt = "could not result";          }          system.out.println(rslt); //new showmsg(rslt);     }      entitymanager.gettransaction().commit(); //added     entitymanager.close(); //added } catch (exception ex) {  system.out.println(ex); //new showmsg("exception occured"); }