org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session

Exception: 

com.mysql.jdbc.exceptions.MySQLSyntaxErrorException: Access denied for user ''@'localhost' to database 'school' at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:936) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2985) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:885) at com.mysql.jdbc.MysqlIO.secureAuth411(MysqlIO.java:3421) at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1247) at com.mysql.jdbc.Connection.createNewIO(Connection.java:2775) at com.mysql.jdbc.Connection.(Connection.java:1555) at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285) at java.sql.DriverManager.getConnection(DriverManager.java:582) at java.sql.DriverManager.getConnection(DriverManager.java:154)...

Reason  : 

1. you are passing transient object instead of persistent object for delete

Possible solution : 

1. Pass persistent object instead of transient object.

For example :

you would be trying something like this.Here studentToDelete is transient.

    public void deleteStudent(Student studentToDelete)
    {
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();
        // Here studentToDelete is transient object
        session.delete(studentToDelete);
        session.getTransaction().commit();
        session.flush();
    }

We will remove persisted object and pass to delete function like below one.

    public void deleteStudentA(Student studentToDelete)
    {
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();
        // Getting persisted object of Student.
        Student persistentStudent = (Student) session.get(Student.class, studentToDelete.getId());
        session.delete(persistentStudent);
        session.getTransaction().commit();
        session.flush();
    }