/*init rpc service */
        final MainRpcServiceAsync rpcService = GWT.create(MainRpcService.class);
        button.addClickHandler(new ClickHandler() {
            @Override
            public void onClick(ClickEvent event) {
                rpcService.saveWeatherEntity(new WeatherEntity(), new AsyncCallback<WeatherEntity>() {
                    @Override
                    public void onFailure(Throwable caught) {
                        Window.alert("Failure: " + caught.getMessage());
                    }

                    @Override
                    public void onSuccess(WeatherEntity result) {
                        result.setCity("Сарапулка");
                        result.setDate("11/11/2002");
                        result.setPrcp(1110);
                        result.setTempLo(12);
                        result.setTempHi(13);
                    }
                });

            }
        });

 @Override
    public WeatherEntity saveWeatherEntity(WeatherEntity weatherEntity) {

        System.out.println("Записываю....." + weatherEntity.getCity());

        session.beginTransaction();
        session.save(weatherEntity);
        session.close();
        return null;

@Override
    public WeatherEntity saveWeatherEntity(WeatherEntity weatherEntity) {

        System.out.println("Записываю....." + weatherEntity.getCity());

        session.beginTransaction();
        session.save(weatherEntity);
        session.getTransaction().commit();
        session.close();
        return null;

    }

@Override
    public WeatherEntity saveWeatherEntity(WeatherEntity weatherEntity) {

        WeatherEntity we = new WeatherEntity();

        System.out.println("Записываю.....");
        
        session.beginTransaction();
        we.setCity("Бухарест");
        session.save(we);
        session.getTransaction().commit();
        session.close();
        return null;

    }

  @Override
                    public void onSuccess(WeatherEntity result) {
                        if (result == null) Window.alert("result = null");
                        else { Window.alert("result <> null"); }
                    }

package ru.javastudy.gwtPersistence.client;

public class Main implements EntryPoint {
    public void onModuleLoad() {

        final TextBox tbCity = new TextBox();
        final Button button = new Button("Send");

        /*init rpc service */
        final MainRpcServiceAsync rpcService = GWT.create(MainRpcService.class);

        /* --------------------------------------------------------------------------------------------------------- */
        rpcService.getAllWeatherEntity(new AsyncCallback<List<WeatherEntity>>() {
            @Override
            public void onFailure(Throwable caught) {
                Window.alert("Fail getAllWeatherEntity"); }
            @Override
            public void onSuccess(List<WeatherEntity> result){
                final WeatherEntity weatherEntity = result.get(3);
                tbCity.setText(weatherEntity.getCity());
            }
        });


        /* --------------------------------------------------------------------------------------------------------- */
        button.addClickHandler(new ClickHandler() {
            @Override
            public void onClick(ClickEvent event) {

                rpcService.saveWeatherEntity(new WeatherEntity(), new AsyncCallback<WeatherEntity>() {
                    @Override
                    public void onFailure(Throwable caught) {
                        Window.alert("Failure: " + caught.getMessage());
                    }
                    @Override
                    public void onSuccess(WeatherEntity result) {
                        //if (result == null) Window.alert("result = null");
                        //else { Window.alert("result <> null"); }
                    }
                });
            }
        });

        /*add view to Main.html without data*/
        RootPanel.get("slot1").add(tbCity);
        RootPanel.get("slot2").add(button);

    }
}

package ru.javastudy.gwtPersistence.client;

import ... ;

@RemoteServiceRelativePath("gwtPersistService")
public interface MainRpcService extends RemoteService {
    List<WeatherEntity> getAllWeatherEntity();
    WeatherEntity saveWeatherEntity(WeatherEntity weatherEntity);
}

package ru.javastudy.gwtPersistence.client;

import ...;

public interface MainRpcServiceAsync {
    void getAllWeatherEntity(AsyncCallback<List<WeatherEntity>> async);
    void saveWeatherEntity(WeatherEntity weatherEntity, AsyncCallback<WeatherEntity> async);
}

package ru.javastudy.gwtPersistence.server;

import...

public class MainRpcServiceImpl extends RemoteServiceServlet implements MainRpcService {

    private SessionFactory sessionFactory;
    private Session session;
    private List weatherEntityList = new ArrayList<>();

    public MainRpcServiceImpl() {
        sessionFactory = HibernateUtil.getSessionFactory();
        session = sessionFactory.openSession();
    }

    @Override
    public List<WeatherEntity> getAllWeatherEntity() {
        Query query =  session.createQuery("from WeatherEntity");
        weatherEntityList = query.list();
        return weatherEntityList;
    }

    @Override
    public WeatherEntity saveWeatherEntity(WeatherEntity weatherEntity) {
        session.beginTransaction();
        session.save(weatherEntity); //Здесь weatherEntity уже null. Как будто бы он и не передавался вовсе ((
        session.getTransaction().commit();
        session.close();
        return null;
    }
}

package ru.javastudy.gwtPersistence.server;

...

public class HibernateUtil {

    private static SessionFactory sessionFactory = buildSessionFactory();

    protected static SessionFactory buildSessionFactory() {
        // A SessionFactory is set up once for an application!
        final StandardServiceRegistry registry = new StandardServiceRegistryBuilder()
                .configure() // configures settings from hibernate.cfg.xml
                .build();
        try {
            sessionFactory = new MetadataSources( registry ).buildMetadata().buildSessionFactory();
        }
        catch (Exception e) {
            // The registry would be destroyed by the SessionFactory, but we had trouble building the SessionFactory
            // so destroy it manually.
            StandardServiceRegistryBuilder.destroy( registry );
        }
        return sessionFactory;
    }


    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

    public static void shutdown() {
        // Close caches and connection pools
        getSessionFactory().close();
    }
}

package ru.javastudy.gwtPersistence.shared;
...

@Entity
@Table(name = "weather", schema = "public", catalog = "myDB")
public class WeatherEntity implements Serializable {
    private int id;
    private String city;

    @Id @GeneratedValue
    @Column(name = "id", nullable = false, insertable = true, updatable = true)
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
    @Basic
    @Column(name = "city", nullable = true, length = 80)
    public String getCity() {
        return city;
    }

    public void setCity(String city) {
        this.city = city;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        WeatherEntity that = (WeatherEntity) o;
        if (city != null ? !city.equals(that.city) : that.city != null) return false;
        return true;
    }

    @Override
    public int hashCode() {
        int result = city != null ? city.hashCode() : 0;
        return result;
    }
}

        final WeatherEntity weatherEntity = new WeatherEntity();

        /*init rpc service */
        final MainRpcServiceAsync rpcService = GWT.create(MainRpcService.class);

        button.addClickHandler(new ClickHandler() {
             @Override
             public void onClick(ClickEvent event) {

                 //Запись в экземпляр класса производится ДО метода обработки событий записи. Всё.
                 weatherEntity.setCity("Астана");
                 weatherEntity.setTempLo(1);
                 weatherEntity.setTempHi(15);
                 weatherEntity.setPrcp(3333);
                 weatherEntity.setDate("11/05/2006");

                 rpcService.saveWeatherEntity(weatherEntity, new AsyncCallback<WeatherEntity>() {
                     @Override
                     public void onFailure(Throwable caught) {
                         Window.alert("Failure: " + caught.getMessage());
                     }
                     @Override
                     public void onSuccess(WeatherEntity result) {
                         //Здесь ничего не надо. Событие записи УЖЕ СЛУЧИЛОСЬ!
                     }
                 });
             }
        });