Question

For an answer scroll down to the end of this…

The basic problem is the same as asked multiple time. I have a simple program with two POJOs Event and User – where a user can have multiple events.

@Entity
@Table
public class Event {
 private Long id;
 private String name;
 private User user;

 @Column
 @Id
 @GeneratedValue
 public Long getId() {return id;}
 public void setId(Long id) { this.id = id; }

 @Column
 public String getName() {return name;}
 public void setName(String name) {this.name = name;}

 @ManyToOne
 @JoinColumn(name="user_id")
 public User getUser() {return user;}
 public void setUser(User user) {this.user = user;}

}

The User:

@Entity
@Table
public class User {
 private Long id;
 private String name;
 private List<Event> events;

 @Column
 @Id
 @GeneratedValue
 public Long getId() { return id; }
 public void setId(Long id) { this.id = id; }

 @Column
 public String getName() { return name; }
 public void setName(String name) { this.name = name; }

 @OneToMany(mappedBy="user", fetch=FetchType.LAZY)
 public List<Event> getEvents() { return events; }
 public void setEvents(List<Event> events) { this.events = events; }

}

Note: This is a sample project. I really want to use Lazy fetching here.

Now we need to configure spring and hibernate and have a simple basic-db.xml for loading:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans 
           http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
           http://www.springframework.org/schema/aop 
           http://www.springframework.org/schema/aop/spring-aop-3.0.xsd">


 <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource"
  destroy-method="close"  scope="thread">
  <property name="driverClassName" value="com.mysql.jdbc.Driver" />
  <property name="url" value="jdbc:mysql://192.168.1.34:3306/hibernateTest" />
  <property name="username" value="root" />
  <property name="password" value="" />
  <aop:scoped-proxy/>
 </bean>

 <bean class="org.springframework.beans.factory.config.CustomScopeConfigurer">
  <property name="scopes">
   <map>
    <entry key="thread">
     <bean class="org.springframework.context.support.SimpleThreadScope" />
    </entry>
   </map>
  </property>
 </bean>

 <bean id="mySessionFactory"
  class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean" scope="thread">
  <property name="dataSource" ref="myDataSource" />
  <property name="annotatedClasses">
   <list>
    <value>data.model.User</value>
    <value>data.model.Event</value>
   </list>
  </property>
  <property name="hibernateProperties">
   <props>
    <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
    <prop key="hibernate.show_sql">true</prop>
    <prop key="hibernate.hbm2ddl.auto">create</prop>
   </props>
  </property>
  <aop:scoped-proxy/>

 </bean>

 <bean id="myUserDAO" class="data.dao.impl.UserDaoImpl">
  <property name="sessionFactory" ref="mySessionFactory" />
 </bean>

 <bean id="myEventDAO" class="data.dao.impl.EventDaoImpl">
  <property name="sessionFactory" ref="mySessionFactory" />
 </bean>

</beans>

Note: I played around with the CustomScopeConfigurer and SimpleThreadScope, but that didnt change anything.

I have a simple dao-impl (only pasting the userDao – the EventDao is pretty much the same – except with out the “listWith” function:

public class UserDaoImpl implements UserDao{

 private HibernateTemplate hibernateTemplate;

 public void  setSessionFactory(SessionFactory sessionFactory) {
  this.hibernateTemplate = new HibernateTemplate(sessionFactory);

 }

 @SuppressWarnings("unchecked")
 @Override
 public List listUser() {
  return hibernateTemplate.find("from User");
 }

 @Override
 public void saveUser(User user) {
  hibernateTemplate.saveOrUpdate(user);

 }

 @Override
 public List listUserWithEvent() {

  List users = hibernateTemplate.find("from User");
  for (User user : users) {
   System.out.println("LIST : " + user.getName() + ":");
   user.getEvents().size();
  }
  return users;
 }

}

I am getting the org.hibernate.LazyInitializationException – failed to lazily initialize a collection of role: data.model.User.events, no session or session was closed at the line with user.getEvents().size();

And last but not least here is the Test class I use:

public class HibernateTest {

 public static void main(String[] args) {

  ClassPathXmlApplicationContext ac = new ClassPathXmlApplicationContext("basic-db.xml");


  UserDao udao = (UserDao) ac.getBean("myUserDAO");
  EventDao edao = (EventDao) ac.getBean("myEventDAO");


  System.out.println("New user...");
  User user = new User();
  user.setName("test");

  Event event1 = new Event();
  event1.setName("Birthday1");
  event1.setUser(user);

  Event event2 = new Event();
  event2.setName("Birthday2");
  event2.setUser(user);

  udao.saveUser(user);
  edao.saveEvent(event1);
  edao.saveEvent(event2);

  List users = udao.listUserWithEvent();
  System.out.println("Events for users");
  for (User u : users) {

   System.out.println(u.getId() + ":" + u.getName() + " --");
   for (Event e : u.getEvents())
   {
    System.out.println("\t" + e.getId() + ":" + e.getName());
   }
  }

  ((ConfigurableApplicationContext)ac).close();
 }

}

and here is the Exception:

1621 [main] ERROR org.hibernate.LazyInitializationException - failed to lazily initialize a collection of role: data.model.User.events, no session or session was closed
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: data.model.User.events, no session or session was closed
 at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:380)
 at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:372)
 at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:119)
 at org.hibernate.collection.PersistentBag.size(PersistentBag.java:248)
 at data.dao.impl.UserDaoImpl.listUserWithEvent(UserDaoImpl.java:38)
 at HibernateTest.main(HibernateTest.java:44)
Exception in thread "main" org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: data.model.User.events, no session or session was closed
 at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:380)
 at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:372)
 at org.hibernate.collection.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:119)
 at org.hibernate.collection.PersistentBag.size(PersistentBag.java:248)
 at data.dao.impl.UserDaoImpl.listUserWithEvent(UserDaoImpl.java:38)
 at HibernateTest.main(HibernateTest.java:44)

Things tried but did not work:

• assign a threadScope and using beanfactory (I used “request” or “thread” – no difference noticed):

  // scope stuff
  Scope threadScope = new SimpleThreadScope();
  ConfigurableListableBeanFactory beanFactory = ac.getBeanFactory();
  beanFactory.registerScope("request", threadScope);
  ac.refresh();
...

• Setting up a transaction by getting the session object from the deo:

...
  Transaction tx = ((UserDaoImpl)udao).getSession().beginTransaction();
  tx.begin();
  users = udao.listUserWithEvent();
...

• getting a transaction within the listUserWithEvent()

 public List listUserWithEvent() {
  SessionFactory sf = hibernateTemplate.getSessionFactory();
  Session s = sf.openSession();
  Transaction tx = s.beginTransaction();
  tx.begin();

  List users = hibernateTemplate.find("from User");
  for (User user : users) {
   System.out.println("LIST : " + user.getName() + ":");
   user.getEvents().size();
  }
  tx.commit();
  return users;
 }

I am really out of ideas by now. Also, using the listUser or listEvent just work fine.

Step forward:

Thanks to Thierry I got one step further (I think). I created the MyTransaction class and do my whole work in there, getting everything from spring. The new main looks like this:

 public static void main(String[] args) {

  ClassPathXmlApplicationContext ac = new ClassPathXmlApplicationContext("basic-db.xml");

  // getting dao
  UserDao udao = (UserDao) ac.getBean("myUserDAO");
  EventDao edao = (EventDao) ac.getBean("myEventDAO");

  // gettting transaction template
  TransactionTemplate transactionTemplate = (TransactionTemplate) ac.getBean("transactionTemplate");

  MyTransaction mt = new MyTransaction(udao, edao);
  transactionTemplate.execute(mt);

  ((ConfigurableApplicationContext)ac).close();
 }

Unfortunately now there is a null-pointer Exception @: user.getEvents().size(); (in the daoImpl).

I know that it should not be null (neither from the output in the console nor from the db layout).

Here is the console output for more information (I did a check for user.getEvent() == null and printed “EVENT is NULL”):

New user...
Hibernate: insert into User (name) values (?)
Hibernate: insert into User (name) values (?)
Hibernate: insert into Event (name, user_id) values (?, ?)
Hibernate: insert into Event (name, user_id) values (?, ?)
Hibernate: insert into Event (name, user_id) values (?, ?)
List users:
Hibernate: select user0_.id as id0_, user0_.name as name0_ from User user0_
1:User1
2:User2
List events:
Hibernate: select event0_.id as id1_, event0_.name as name1_, event0_.user_id as user3_1_ from Event event0_
1:Birthday1 for 1:User1
2:Birthday2 for 1:User1
3:Wedding for 2:User2
Hibernate: select user0_.id as id0_, user0_.name as name0_ from User user0_
Events for users
1:User1 --
EVENT is NULL
2:User2 --
EVENT is NULL

You can get the sample project from http://www.gargan.org/code/hibernate-test1.tgz (it’s an eclipse/maven project)

The solution (for console applications)

There are actually two solutions for this problem – depending on your environment:

For a console application you need a transaction template which captures the actutal db logic and takes care of the transaction:

public class UserGetTransaction implements TransactionCallback{

 public List users;

 protected ApplicationContext context;

 public UserGetTransaction (ApplicationContext context) {
  this.context = context;
 }

 @Override
 public Boolean doInTransaction(TransactionStatus arg0) {
  UserDao udao = (UserDao) ac.getBean("myUserDAO");
  users = udao.listUserWithEvent();
  return null;
 }

}

You can use this by calling:

 TransactionTemplate transactionTemplate = (TransactionTemplate) context.getBean("transactionTemplate");
 UserGetTransaction mt = new UserGetTransaction(context);
 transactionTemplate.execute(mt);

In order for this to work you need to define the template class for spring (ie. in your basic-db.xml):

<bean id="transactionTemplate" class="org.springframework.transaction.support.TransactionTemplate">
    <property name="transactionManager" ref="transactionManager"/>
</bean>

Another (possible) solution

thanks andi

    PlatformTransactionManager transactionManager = (PlatformTransactionManager) applicationContext.getBean("transactionManager");
    DefaultTransactionAttribute transactionAttribute = new DefaultTransactionAttribute(TransactionDefinition.PROPAGATION_REQUIRED);

transactionAttribute.setIsolationLevel(TransactionDefinition.ISOLATION_SERIALIZABLE);
    TransactionStatus status = transactionManager.getTransaction(transactionAttribute);
    boolean success = false;
    try {
      new UserDataAccessCode().execute();
      success = true;
    } finally {
      if (success) {
        transactionManager.commit(status);
      } else {
        transactionManager.rollback(status);
      }
    }

The solution (for servlets)

Servlets are not that big of a problem. When you have a servlet you can simply start and bind a transaction at the beginning of your function and unbind it again at the end:

public void doGet(...) {
  SessionFactory sessionFactory = (SessionFactory) context.getBean("sessionFactory");
  Session session = SessionFactoryUtils.getSession(sessionFactory, true);
  TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(session));

// Your code....

  TransactionSynchronizationManager.unbindResource(sessionFactory);
}

Answer 1

I think you should not use the hibernate session transactional methods, but let spring do that.

Add this to your spring conf:

<bean id="txManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
    <property name="sessionFactory" ref="mySessionFactory" />
</bean>

<bean id="transactionTemplate" class="org.springframework.transaction.support.TransactionTemplate">
    <property name="transactionManager" ref="txManager"/>
</bean>

and then I would modify your test method to use the spring transaction template:

public static void main(String[] args) {
    // init here (getting dao and transaction template)

    transactionTemplate.execute(new TransactionCallback() {
        @Override
        public Object doInTransaction(TransactionStatus status) {
          // do your hibernate stuff in here : call save, list method, etc
        }
    }
}

as a side note, @OneToMany associations are lazy by default, so you don’t need to annotate it lazy. (@*ToMany are LAZY by default, @*ToOne are EAGER by default)

EDIT: here is now what is happening from hibernate point of view:

• open session (with transaction start)
• save a user and keep it in the session (see the session cache as an entity hashmap where the key is the entity id)
• save an event and keep it in the session
• save another event and keep it in the session

• … same with all the save operations …

• then load all users (the “from Users” query)

• at that point hibernate see that it has already the object in its session, so discard the one it got from the request and return the one from the session.
• your user in the session does not have its event collection initialized, so you get null.
• …

Here are some points to enhance your code:

• in your model, when collection ordering is not needed, use Set, not List for your collections (private Set events, not private List events)
• in your model, type your collections, otherwise hibernate won’t which entity to fetch (private Set<Event> events)
• when you set one side of a bidirectional relation, and you wish to use the mappedBy side of the relation in the same transaction, set both sides. Hibernate will not do it for you before the next tx (when the session is a fresh view from the db state).

So to address the point above, either do the save in one transaction, and the loading in another one :

public static void main(String[] args) {
    // init here (getting dao and transaction template)
    transactionTemplate.execute(new TransactionCallback() {
        @Override
        public Object doInTransaction(TransactionStatus status) {
          // save here
        }
    }

    transactionTemplate.execute(new TransactionCallback() {
        @Override
        public Object doInTransaction(TransactionStatus status) {
          // list here
        }
    }
}

or set both sides:

...
event1.setUser(user);
...
event2.setUser(user);
...
user.setEvents(Arrays.asList(event1,event2));
...

(Also do not forget to address the code enhancement points above, Set not List, collection typing)