Develop Reference

How do I achieve a hibernate batch insert with Spring transactional (#Transactional)?

I am using Spring 5.x and plain Hibernate in backend. I want to transfer data from one DataBase to another, 1000 records in one go as a batch. I have same tables in the both databases. We have configured 2 transaction mangers with 2 data source details.
I have configured batch-size="1000" in the hbm file as below.
<hibernate-mapping>
<class table="User" name="com.my.User" batch-size="1000">
<id name="id" .../>
<property name="name" ...>
...
I have the 2 service classes annotated with #Transactional and implemented as below.
#Transactional(value="oldDBTransactionManager" propagation = Propagation.SUPPORTS, readOnly = true)
public class GetUserServiceImpl extends HibernateDaoImpl implements GetUserService {
...
public List<User> getAllUsers(int firstResult, int maxResults) {
DetachedCriteria criteria = DetachedCriteria.forClass(User.class);
criteria.addOrder(Order.asc("id"));
List results = getHibernateTemplate().findByCriteria(criteria, firstResult, maxResults);
if (!results.isEmpty()) {
for (User user: results ) {
saveUserService.saveUser(user);
}
}
return results;
...
}
#Transactional(value="newDBTransactionManager" propagation = Propagation.SUPPORTS, readOnly = true)
public class SaveUserServiceImpl extends HibernateDaoImpl implements SaveUserService{
...
#Transactional(value="newDBTransactionManager", propagation = Propagation.REQUIRED, readOnly = false)
#Qualifier("newDBTransactionManager")
public void saveUser(User user) {
getHibernateTemplate().save(user);
}
...
I am calling the above getAllUsers service method from outside of the Service layer(outside Transaction) and it will internally calls saveUser service method (which usesanother transaction manger) as below:
public class UserProcessActivity{
private int maxUsersPerIteration = 50;
...
private void processUsers(){
List<User> usersList;
long counter = 0;
int firstResult = 0;
usersList = getUserService.getAllUsers(firstResult, maxUsersPerIteration);
while (!usersList.isEmpty()) {
// some logic...
firstResult += usersList .size();
usersList = getUserService.getAllUsers(firstResult, maxUserPerIteration);
}
}
...
My question here is whether the above code handles batch insert or not?
If yes, what is the number of records insert as a batch 1000(batch-size declared in hbm) or 50 (calling service method with maxUserPerIteration assigned with 50).
If it does not perform insert operation as batch, how to achieve the batch insert with Spring #Trasactional and Hibernate?
How to enable logging for this batch insert operation queries to understand actual batch operation information?
Thanks in advance.

javax.persistence.TransactionRequiredException Error: jpa 2 + hibernate - Native Query

I'm getting the above error when I'm trying to update sphinx real time indexes. Since sphinx uses a database very close to mysql, I'm trying to use entity manager to update indexes. But sphinxql isn't a full mysql database so if I use entitymanager.merge method the sql generated isn't understood by sphinxql. Therefore, I decided to use createNativeQuery to insert new indices or delete indices from the Real Time Index. However, I'm getting the exception in the title when I invoke .executeUpdate method of createNativeQuery.
This is my Dao:
public interface RTIndexGeoDao {
public boolean deleteIndex(long id);
public boolean insertIndex(long id, long agentActivityId, long agentId, double latitude, double longitude);
}
This is my Implementation of Dao:
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import org.springframework.stereotype.Repository;
#Repository(value = "rtIndexGeoDao")
public class JPARTIndexDao implements RTIndexGeoDao {
private EntityManager em;
#PersistenceContext(unitName = "rtIndexPU")
public void setRtIndexEntityManager(EntityManager em)
{
this.em = em;
}
public boolean deleteIndex(long id) {
//This is to update our geodistance index
try
{
this.em.createNativeQuery("delete from rt_geo where agent_agent_id = ?1").setParameter(1, id).executeUpdate();
return true;
}
catch (Exception ex)
{
return false;
}
}
public boolean insertIndex(long id, long agentActivityId, long agentId,
double latitude, double longitude) {
try
{
this.em.createNativeQuery("insert into rt_geo(id, agent_activity_id, agent_agent_id, latitude, longitude) values(?1, ?2, ?3, ?4, ?5)")
.setParameter(1, id)
.setParameter(2, agentActivityId)
.setParameter(3, agentId)
.setParameter(4, latitude)
.setParameter(5, longitude).executeUpdate();
return true;
}
catch (Exception ex)
{
return false;
}
}
}
This is my RTIndexManager:
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
import org.springframework.transaction.annotation.Transactional;
import org.springtest.mavenspringapp.repository.RTIndexGeoDao;
#Component
#Transactional
public class SphinxRTIndexManagerImpl implements SphinxRTIndexManager {
/**
*
*/
private static final long serialVersionUID = 1L;
#Autowired
private RTIndexGeoDao rtIndexGeoDao;
public boolean deleteIndex(long id) {
return rtIndexGeoDao.deleteIndex(id);
}
public boolean insertIndex(long id, long agentActivityId, long agentId,
double latitude, double longitude) {
return rtIndexGeoDao.insertIndex(id, agentActivityId, agentId, latitude, longitude);
}
}
This is my persistence.xml
<persistence-unit name="rtIndexPU" transaction-type="RESOURCE_LOCAL">
</persistence-unit>
This is the controller:
#Autowired
private SphinxRTIndexManager rtIndexManager;
private AgentActivity createActivity(Agent agent, double latitude, double longitude, String agentActivityDescription)
{
//Steps to update indexes
boolean deleteEntry = rtIndexManager.deleteIndex(agent.getAgentId());
if (!deleteEntry) return null;
UPDATE: I'm posting my applicationContext.xml
<!-- enabling annotation driven configuration /-->
<context:annotation-config/>
<bean id="rtIndexEntityManagerFactory"
class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"
p:dataSource-ref="rtIndexDataSource"
p:jpaVendorAdapter-ref="jpaAdapter">
<property name="loadTimeWeaver">
<bean class="org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver"/>
</property>
<property name="persistenceUnitName" value="rtIndexPU"></property>
</bean>
<bean id="rtIndexTransactionManager" class="org.springframework.orm.jpa.JpaTransactionManager"
p:entityManagerFactory-ref="rtIndexEntityManagerFactory"/>
<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager"
p:entityManagerFactory-ref="entityManagerFactory"/>
I have another persistence unit and every manager, dao, etc works. But when I use createNativeQuery.executeUpdate on entitymanager whose persistenceContext is rtIndexPU, this exception occurs:
javax.persistence.TransactionRequiredException: Executing an update/delete query
I've searched at every question available on the internet, but cannot have this issue solved. Thanks in advance.

Since you have two transaction managers in your code, the problem is most likely that your SphinxRTIndexManagerImpl is not using the correct one (or any).
You should change the code to:
#Component
#Transactional("rtIndexTransactionManager")
public class SphinxRTIndexManagerImpl implements SphinxRTIndexManager {
}
Also you have to configure the second transaction manager (rtIndexTransactionManager) for working with annotations, by doing:
<tx:annotation-driven transaction-manager="rtIndexTransactionManager"/>

Trying to remove and insert data using Spring's JdbcTemplate

I'm having problems trying to use batchUpdate, from Spring's JdbcTemplate.
The problem is that i want to execute two SQL operations: a DELETE method (to clear my table) and then an INSERT method. It works fine the first time i make the call (from jsp). But from the second attempt on, when i try to do the call, the DELETE procedure isn't called or executed, just the INSERT procedure, causing an unique constraint exception.
First i tried this:
public class MyTableDAOStoredProcedure extends JdbcDaoSupport implements MyTableDAO {
...
....
public void insert(final List<MyObject> myObjectList) {
...
String deleteSql = "DELETE FROM ......";
String insertSql = "INSERT INTO ......";
// Delete Procedure
jdbcTemplate.execute(deleteSql);
//Insert Procedure
jdbcTemplate.batchUpdate(insertSql, new BatchPreparedStatementSetter() {
#Override
public int getBatchSize() {
return myObjectList.size();
}
#Override
public void setValues(PreparedStatement ps, int i) throws SQLException {
MyObject object = myObjectList.get(i);
ps.setString(1, myObject.getA());
ps.setInt(2, myObject.getB());
}
});
}
}
Then i tried this:
public class MyTableDAOStoredProcedure extends JdbcDaoSupport implements MyTableDAO {
...
...
String deleteSql = "DELETE FROM MY OBJECT";
String insertsql = "INSERT INTO MY_OBJECT values(1,2)";
jdbcTemplate.batchUpdate(new String[] { deleteSql, insertSql});
}
I think it might be some Spring Transaction problem. Here it is the config of my DAO procedure on applicationContext.xml, it's quite simple:
<bean id="txManager"
class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
<property name="dataSource" ref="dataSource" />
</bean>
.
.
.
<bean id="myTableDAOStoredProcedure" class="....dao.spring.MyTableDAOStoredProcedure">
<property name="dataSource">
<ref bean="dataSource" />
</property>
</bean>
Any ideas or suggestions?

How to configure a Spring beans with properties that are stored in a database table

In my project we'd like to externalize the properties of our Spring managed beans, that is very easy to do with standard Java .properties files, however we want to be able to read those properties from a DB table that behaves like a Map (key is the property name, value is the value assigned to that property).
I found this post that suggest the usage of Commons Configuration but I don't know if there's a better way to do the same with Spring 3.x. Maybe implementing my own PropertyResource or something.
Any clues?

I'd use a FactoryBean of type <Properties> that I'd implement using JdbcTemplate. You can then use the generated Properties object with the <context:property-placeholder> mechanism.
Sample code:
public class JdbcPropertiesFactoryBean
extends AbstractFactoryBean<Properties>{
#Required
public void setJdbcTemplate(final JdbcTemplate jdbcTemplate){
this.jdbcTemplate = jdbcTemplate;
}
private JdbcTemplate jdbcTemplate;
#Required
public void setTableName(final String tableName){
this.tableName = tableName;
}
private String tableName;
#Required
public void setKeyColumn(final String keyColumn){
this.keyColumn = keyColumn;
}
private String keyColumn;
#Required
public void setValueColumn(final String valueColumn){
this.valueColumn = valueColumn;
}
private String valueColumn;
#Override
public Class<?> getObjectType(){
return Properties.class;
}
#Override
protected Properties createInstance() throws Exception{
final Properties props = new Properties();
jdbcTemplate.query("Select " + keyColumn + ", " + valueColumn
+ " from " + tableName, new RowCallbackHandler(){
#Override
public void processRow(final ResultSet rs) throws SQLException{
props.put(rs.getString(1), rs.getString(2));
}
});
return props;
}
}
XML Configuration:
<bean id="props" class="foo.bar.JdbcPropertiesFactoryBean">
<property name="jdbcTemplate">
<bean class="org.springframework.jdbc.core.JdbcTemplate">
<!-- reference to a defined data source -->
<constructor-arg ref="dataSource" />
</bean>
</property>
<property name="tableName" value="TBL_PROPERTIES" />
<property name="keyColumn" value="COL_KEY" />
<property name="valueColumn" value="COL_VAL" />
</bean>
<context:property-placeholder properties-ref="props" />

In addition to Sean's suggestion, you can extend PropertyPlaceholderConfigurer. Look at the two current implementations - PreferencesX and ServletContextX, and roll out your own, jdbc-based.

There are ways to create "PropertyPlaceholderConfigurer" Programmatically , please see below.
Write a DAO which reads Properties and create a PropertyPlaceholderConfigurer as shown below.
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml"));
PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer();
cfg.setProperties(yourProperties);
cfg.postProcessBeanFactory(factory);

Hibernate/Spring: failed to lazily initialize - no session or session was closed

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);
}

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)

In case of Web application, it is also possible to declare a special Filter in web.xml, that will do session-per-request:
<filter>
<filter-name>openSessionInViewFilter</filter-name>
<filter-class>org.springframework.orm.hibernate3.support.OpenSessionInViewFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>openSessionInViewFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
After that you can lazyload your data anytime during the request.

I got here looking for a hint regarding a similar problem. I tried the solution mentioned by Thierry and it didnt work. After that I tried these lines and it worked:
SessionFactory sessionFactory = (SessionFactory) context.getBean("sessionFactory");
Session session = SessionFactoryUtils.getSession(sessionFactory, true);
TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(session));
Indeed what I'm doing is a batch process that must leverage Spring existings managers/services. After loading the context and doing some invocations I founded the famous issue "failed to lazily initialize a collection". Those 3 lines solved it for me.

The issue is that your dao is using one hibernate session but the lazy load of the user.getName (I assume that is where it throws) is happening outside that session -- either not in a session at all or in another session. Typically we open up a hibernate session before we make DAO calls and don't close it until we are done with all lazy loads. Web requests are usually wrapped in a big session so these problems do not happen.
Typically we have wrapped our dao and lazy calls in a SessionWrapper. Something like the following:
public class SessionWrapper {
private SessionFactory sessionFactory;
public void setSessionFactory(SessionFactory sessionFactory) {
this.hibernateTemplate = new HibernateTemplate(sessionFactory);
}
public <T> T runLogic(Callable<T> logic) throws Exception {
Session session = null;
// if the session factory is already registered, don't do it again
if (TransactionSynchronizationManager.getResource(sessionFactory) == null) {
session = SessionFactoryUtils.getSession(sessionFactory, true);
TransactionSynchronizationManager.bindResource(sessionFactory, new SessionHolder(session));
}
try {
return logic.call();
} finally {
// if we didn't create the session don't unregister/release it
if (session != null) {
TransactionSynchronizationManager.unbindResource(sessionFactory);
SessionFactoryUtils.releaseSession(session, sessionFactory);
}
}
}
}
Obviously the SessionFactory the same SessionFactory that was injected into your dao.
In your case, you should wrap the entire listUserWithEvent body in this logic. Something like:
public List listUserWithEvent() {
return sessionWrapper.runLogic(new Callable<List>() {
public List call() {
List users = hibernateTemplate.find("from User");
for (User user : users) {
System.out.println("LIST : " + user.getName() + ":");
user.getEvents().size();
}
}
});
}
You will need to inject the SessionWrapper instance into your daos.

Interesting!
I had the same problem in a #Controller's #RequestMapping handler method.
The simple solution was to add a #Transactional annotation to the handler method so that the session is kept open for the whole duration of the method body execution

Easiest solution to implement:
Within the scope of the session[inside the API annotated with #Transactional], do the following:
if A had a List<B> which is lazily loaded, simply call an API which makes sure the List is loaded
What's that API ?
size(); API of the List class.
So all that's needed is:
Logger.log(a.getBList.size());
This simple call of logging the size makes sure it gets the whole list before calculating the size of the list. Now you will not get the exception !

What worked for us in JBoss was the solution #2 taken from this site at Java Code Geeks.
Web.xml:
<filter>
<filter-name>ConnectionFilter</filter-name>
<filter-class>web.ConnectionFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>ConnectionFilter</filter-name>
<url-pattern>/faces/*</url-pattern>
</filter-mapping>
ConnectionFilter:
import java.io.IOException;
import javax.annotation.Resource;
import javax.servlet.*;
import javax.transaction.UserTransaction;
public class ConnectionFilter implements Filter {
#Override
public void destroy() { }
#Resource
private UserTransaction utx;
#Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
try {
utx.begin();
chain.doFilter(request, response);
utx.commit();
} catch (Exception e) { }
}
#Override
public void init(FilterConfig arg0) throws ServletException { }
}
Maybe it would work with Spring too.