我在一个小项目上工作,我有2个表,用户和应用程序。用户可以拥有多个应用程序,并且多个用户可能会使用某个应用程序,因此它们之间存在多对多的关系。每个表都有一些字段(id,名称,密码,技术等),我还在User和Application类中使用@ManyToMany注释声明了2个arraylists。问题是,在我的业务层中,我编写了一个方法,该方法应该向用户添加应用程序,当我尝试执行user.getListOfApplications()时,添加(app)它会给我异常...
public class ManagerHibernate {private SessionFactory sessionFactory;
public void setup()
{
sessionFactory = new Configuration().configure().buildSessionFactory();
}
public void exit()
{
sessionFactory.close();
}
public void create(Object obj)
{
Session session = sessionFactory.openSession();
session.beginTransaction();
session.save(obj);
session.getTransaction().commit();
session.close();
}
public Object read(Class<?> c, int idObj)
{
Session session = sessionFactory.openSession();
session.beginTransaction();
Object obj = session.get(c, idObj);
System.out.println(obj);
session.getTransaction().commit();
session.close();
return obj;
}
public void update(Object obj)
{
Session session = sessionFactory.openSession();
session.beginTransaction();
session.update(obj);
session.getTransaction().commit();
session.close();
}
public void delete(Object obj)
{
Session session = sessionFactory.openSession();
session.beginTransaction();
session.delete(obj);
session.getTransaction().commit();
session.close();
}
public <T> List<T> loadAllData(Class<T> type)
{
Session session = sessionFactory.openSession();
session.beginTransaction();
CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<T> criteria = builder.createQuery(type);
criteria.from(type);
List<T> data = session.createQuery(criteria).getResultList();
session.getTransaction().commit();
session.close();
return data;
}
}
public Boolean addNewApplicationToUser(String userUserName, String applicationName)
{
int okUser = 0;
int okApp = 0;
listOfApplications = managerHibernate.loadAllData(Application.class);
listOfUsers = managerHibernate.loadAllData(User.class);
User user = null;
Application app = null;
for(Application index: listOfApplications)
{
if(index.getApplicationName().equals(applicationName))
{
okApp = 1;
app = index;
}
}
for(User index: listOfUsers)
{
if(index.getUserUserName().equals(userUserName))
{
okUser = 1;
user = index;
}
}
if(okUser == 0 || okApp == 0)
return false;
else
{
user.getListOfApplications().add(app);
//app.getUserList().add(user);
return true;
}
}
addNewApplicationToUser方法是在另一个名为ControllerHibernate的类中编写的。只有else分支很重要,其余的是检查参数是否确实存在于数据库中
使用以下方法managerHibernate.loadAllData加载数据时,问题就开始了
public <T> List<T> loadAllData(Class<T> type)
{
// New session was opened here
Session session = sessionFactory.openSession();
session.beginTransaction();
CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<T> criteria = builder.createQuery(type);
criteria.from(type);
List<T> data = session.createQuery(criteria).getResultList();
session.getTransaction().commit();
session.close();
//session is close here
return data;
}
因此,当您加载数据时,hibernate框架将只加载用户对象。由于您已选择在模型类中使用延迟加载,因此仅当您尝试访问列表时才会加载应用程序值。由于您已经关闭了会话,因此框架无法再获取应用程序列表,从而导致延迟加载异常。
listOfApplications = managerHibernate.loadAllData(Application.class);
//loading user data and close the session associated with it
listOfUsers = managerHibernate.loadAllData(User.class);
User user = null;
Application app = null;
for(Application index: listOfApplications)
{
if(index.getApplicationName().equals(applicationName))
{
okApp = 1;
app = index;
}
}
for(User index: listOfUsers)
{
if(index.getUserUserName().equals(userUserName))
{
okUser = 1;
user = index;
}
}
if(okUser == 0 || okApp == 0)
return false;
else
{
// when you run this line the hibernate framework will try to retrieve the application data.Since you have the closed session lazy load exception occurs
user.getListOfApplications().add(app);
return true;
}
如何克服这个问题
1)尝试保持会话打开,以便框架可以获取应用程序数据
2)在模型pojo类中将延迟加载更改为急切加载(因为您使用多对多关系不建议使用这种方式)
由于没有用于在用户中获取惰性listofApplication的事务,因此您需要先获取它。为此,您可以更改loadAllData,如下所示:
public interface CriteriaSpec
{
public void joinFetch(CriteriaBuilder builder, CriteriaQuery criteria, Root root);
}
public <T> List<T> loadAllData(Class<T> type, Optional<CriteriaSpec> spec)
{
Session session = sessionFactory.openSession();
session.beginTransaction();
CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<T> criteria = builder.createQuery(type);
Root root = criteria.from(type);
if(spec.isPresent())
spec.joinFetch(builder, criteria, root);
List<T> data = session.createQuery(criteria).getResultList();
session.getTransaction().commit();
session.close();
return data;
}
然后使用它:
managerHibernate.loadAllData(Application.class, Optional.empty());
listOfUsers = managerHibernate.loadAllData(User.class, (rootEntity, query,
criteriaBuilder) -> {
rootEntity.fetch("listOfApplications", JoinType.Left_OUTER_JOIN);
});