Spring, Hibernate - many-to-many - LazyInitializationException

I have 2 models.

User:

    @Entity
public class User implements Serializable {
    Long id;
    String name;
    List<Car> cars;
    public User() {

    }
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<Car> getCars() {
        return cars;
    }
    public void setCars(List<Car> cars) {
        this.cars = cars;
    }
}

Car:

@Entity
public class Car implements Serializable {
Long id;
String mark;
public Car() {

}
public Long getId() {
    return id;
}
public void setId(Long id) {
    this.id = id;
}
public String getMark() {
    return mark;
}
public void setMark(String mark) {
    this.mark = mark;
}
}

Mapping fies:

User:

    <hibernate-mapping>
<class name="com.bontade.phone_book.mvc.spring.models.User" table="USERS">
    <id name="id" type="java.lang.Long">
    <column name="ID" />
    <generator class="identity" />
    </id>        
    <property name="name" not-null="true" length="100" type="java.lang.String">
    <column name="NAME" />
    </property>       
    <list name="cars" table="USER_CAR" cascade="all">
    <key>
        <column name="USER_ID" />
    </key>
    <list-index></list-index>
    <many-to-many column="CAR_ID" class="com.bontade.phone_book.mvc.spring.models.Car" />
    </list>        
</class>
</hibernate-mapping>

Car:

    <hibernate-mapping>
<class name="com.bontade.phone_book.mvc.spring.models.Car" table="CARS">
    <id name="id" type="java.lang.Long">
    <column name="ID" />
    <generator class="identity" />
    </id>
    <property name="mark"  not-null="true" length="20" type="java.lang.String">
    <column name="MARK" />
    </property>     
</class>
</hibernate-mapping>

HomePageController:

public class HomePageController extends AbstractController {
private UserDAO userDAO;
private CarDAO carDAO;

@Override
protected ModelAndView handleRequestInternal(HttpServletRequest request,
        HttpServletResponse response) throws Exception {        
    Car markCar = new Car();
    markCar.setId(null);
    markCar.setMark("111");
    carDAO.saveCar(markCar);

    User mark = new User();
    mark.setId(null);
    mark.setName("mark");
    List a = new ArrayList<Car>();
    a.add(markCar);
    mark.setCa开发者_运维百科rs(a);
    userDAO.saveUser(mark);
    List<User> users = userDAO.getAll();
    System.out.println("==" + users.size() + "===");
    System.out.println(users.get(0).getCars().get(0).getMark());

    ...
}
...
}

But when I executes line :

System.out.println(users.get(0).getCars().get(0).getMark());

I'm getting error with following stack trace:

        Hibernate: select user0_.ID as ID0_, user0_.NAME as NAME0_ from USERS user0_
==1==
2011-02-23 17:35:10 org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [dispatcher] in context with path [/PhoneBook] threw exception [Request processing failed; nested exception is org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.bontade.phone_book.mvc.spring.models.User.cars, no session or session was closed] with root cause
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.bontade.phone_book.mvc.spring.models.User.cars, 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.readElementByIndex(AbstractPersistentCollection.java:173)
    at org.hibernate.collection.PersistentList.get(PersistentList.java:293)
    at com.bontade.phone_book.mvc.spring.controllers.HomePageController.handleRequestInternal(HomePageController.java:59)
    at org.springframework.web.servlet.mvc.AbstractController.handleRequest(AbstractController.java:153)
    at org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter.handle(SimpleControllerHandlerAdapter.java:48)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:875)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:807)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:571)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:501)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:306)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:96)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:244)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:108)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:558)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:379)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:243)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:259)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:281)
    at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
    at java.lang.Thread.run(Thread.java:662)

Have I wrong constructed mapping files, especially many-to-many relation?

By default Hibernate will lazy load collections. In other words, it won't go to the database to retrieve the list of cars until it absolutely needs to. What that means is the returned object from your dao layer won't have the cars list initialized until you try to access it. When you do try to access it, you're no longer within the session, and so you get the exception.

You can explicitly disable lazy fetching on that list property by setting lazy="false" in the hibernate mapping, which will make sure the entire property is populated before returning from your dao layer.