Hibernate, exception when doing update

I have an exception when calling an update () method of the UserService class, with the aim of updating a user's data stored in the db.

UserService class:

import java.util.List;

import db.hibernate.dao.UserDao;
import models.User;

public class UserService {
    private static UserDao userDao;

    public UserService() {
        userDao = new UserDao();
    }

    public void persist(User entity) {
        userDao.openCurrentSessionwithTransaction();
        userDao.persist(entity);
        userDao.closeCurrentSessionwithTransaction();
    }

    public void update(User entity) {
        userDao.openCurrentSessionwithTransaction();
        userDao.update(entity);
        userDao.closeCurrentSessionwithTransaction();
    }

/*otros metodos*/

The class is called from the PaneUser view controller when you click on the "Delete" button:

PaneUserController class:

public class PaneUserController {

    private UserService userService = new UserService();

    List<User> allUsers = userService.findAll();

    public PaneUserController(PaneUser paneUser) {

        //codigo

        paneUser.addCrudListener(new CrudListener());
        initUserView();

    }

    public void initUserView() {

        theUserView.setFieldData(allUsers.get(0));
    }

    class CrudListener implements ActionListener {

        @Override
        public void actionPerformed(ActionEvent e) {

    //crea un user con los datos de input
            User user = theUserView.getFieldData();

            switch (e.getActionCommand()) {
            case "Search":
                // opens the searchUI

                break;
            case "New...":

                CardLayout cardLayout = (CardLayout) pUser.getLayout();
                cardLayout.show(pUser, "Pane attributeSet");

                break;
            case "Update":

                userService.update(user); // lanza el error en cuestion
                break;
            case "Delete":

                userService.delete(user.getUserID());
                break;

            }

        }
    }

UserDao Class:

import models.User;

public class UserDao implements UserDaoInterface<User, String> {

    private Session currentSession;

    private Transaction currentTransaction;

    public UserDao() {
    }

    public Session openCurrentSession() {
        currentSession = getSessionFactory().openSession();
        return currentSession;
    }

    public Session openCurrentSessionwithTransaction() {
        currentSession = getSessionFactory().openSession();
        currentTransaction = currentSession.beginTransaction();
        return currentSession;
    }

    public void closeCurrentSession() {
        currentSession.close();
    }

    public void closeCurrentSessionwithTransaction() {
        currentTransaction.commit();
        currentSession.close();
    }

    private static SessionFactory getSessionFactory() {
        Configuration configuration = new Configuration()
                .addAnnotatedClass(models.User.class)
                .addAnnotatedClass(models.Client.class).configure();

        StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder()
                .applySettings(configuration.getProperties());
        SessionFactory sessionFactory = configuration.buildSessionFactory(builder.build());
        return sessionFactory;
    }

    public Session getCurrentSession() {
        return currentSession;
    }

    public void setCurrentSession(Session currentSession) {
        this.currentSession = currentSession;
    }

    public Transaction getCurrentTransaction() {
        return currentTransaction;
    }

    public void setCurrentTransaction(Transaction currentTransaction) {
        this.currentTransaction = currentTransaction;
    }

    public void persist(User entity) {
        getCurrentSession().save(entity);
    }

    public void update(User entity) {
        getCurrentSession().update(entity);
    }

    public User findById(String id) {
        User book = (User) getCurrentSession().get(User.class, id);
        return book;
    }

    public void delete(User entity) {
        getCurrentSession().delete(entity);
    }

    @SuppressWarnings("unchecked")
    public List<User> findAll() {
        List<User> users = (List<User>) getCurrentSession().createQuery("from User order by userid").list();
        return users;
    }

    public void deleteAll() {
        List<User> entityList = findAll();
        for (User entity : entityList) {
            delete(entity);
        }
    }
}

The StackTrace:

ERROR: HHH000346: Error during managed flush [Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) : [models.User#1]]
Exception in thread "AWT-EventQueue-0" javax.persistence.OptimisticLockException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) : [models.User#1]
    at org.hibernate.internal.ExceptionConverterImpl.wrapStaleStateException(ExceptionConverterImpl.java:226)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:93)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:181)
    at org.hibernate.internal.ExceptionConverterImpl.convert(ExceptionConverterImpl.java:188)
    at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1460)
    at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:511)
    at org.hibernate.internal.SessionImpl.flushBeforeTransactionCompletion(SessionImpl.java:3283)
    at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:2479)
    at org.hibernate.engine.jdbc.internal.JdbcCoordinatorImpl.beforeTransactionCompletion(JdbcCoordinatorImpl.java:473)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.beforeCompletionCallback(JdbcResourceLocalTransactionCoordinatorImpl.java:178)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl.access$300(JdbcResourceLocalTransactionCoordinatorImpl.java:39)
    at org.hibernate.resource.transaction.backend.jdbc.internal.JdbcResourceLocalTransactionCoordinatorImpl$TransactionDriverControlImpl.commit(JdbcResourceLocalTransactionCoordinatorImpl.java:271)
    at org.hibernate.engine.transaction.internal.TransactionImpl.commit(TransactionImpl.java:98)
    at db.hibernate.dao.UserDao.closeCurrentSessionwithTransaction(UserDao.java:38)
    at db.hibernate.service.UserService.update(UserService.java:24)
    at controllers.users.PaneUserController$CrudListener.actionPerformed(PaneUserController.java:63)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)
Caused by: org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect) : [models.User#1]
    at org.hibernate.persister.entity.AbstractEntityPersister.check(AbstractEntityPersister.java:2522)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3355)
    at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:3229)
    at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3630)
    at org.hibernate.action.internal.EntityUpdateAction.execute(EntityUpdateAction.java:146)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:604)
    at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:478)
    at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:356)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:39)
    at org.hibernate.internal.SessionImpl.doFlush(SessionImpl.java:1454)
    ... 47 more

Sorry if I wrote a lot of code, but I see it as necessary, what could be the cause of this error?