Hello,

My problem would seem like an FAQ, but I've spent two days going through the forum for a solution and haven't found one.

I use Hibernate 2.1 and Oracle 9i, and a simple command line program.

I have the following mapping, with the class HeaderImpl associated with 3 collections of child entities: child1List,
child2List, child3List.

For performance reasons, I would like to load the child2 and child3 lists lazily.

But what I observe is that even though I indicate lazy="true" in all the collection declarations, when I call
header.getAttr1(), hibernate loads the object graphs of all the child entities (down to children of the child2 and child3).

<hibernate-mapping package="testApp">
<class name="Root" dynamic-insert="true" dynamic-update="true" table="ROOT">

<id name="ID">
<column name="ID" sql-type="NUMBER(15)"/>
<generator class="assigned"/>
</id>

<!-- versioning attribute for defining the version of an instance -->
<timestamp name="lastModifiedDate" column="TIME_STAMP"/>

<property name="createDate" column="S_ROT_CREATE_DATE"/>

</class>

<joined-subclass name="Header" dynamic-insert="true" dynamic-update="true" extends="Root" lazy="true" table="Header">

<key column="OBJ_ID"/>

<property name="attr1" column="ATTR_1" length="30"/>
<property name="attr2" column="ATTR_2" length="30"/>
<property name="attr3" column="ATTR_3" length="255"/>

<property name="status">
<column name="STATUS" sql-type="NUMBER(10)"/>
</property>

<many-to-one name="owner" class="User" column="OWNER_USER" cascade="none"/>
<many-to-one name="createdBy" class="User" column="CREATED_BY" cascade="none"/>

<bag name="child1List" cascade="all">
<key column="HEADER_ID"/>
<one-to-many class="Child1"/>
</bag>

<bag name="child2List" lazy="true" cascade="all">
<key column="HEADER_ID"/>
<one-to-many class="Child2"/>
</bag>

<bag name="child3List" lazy="true" cascade="all">
<key column="HEADER_ID"/>
<one-to-many class="Child3"/>
</bag>

</joined-subclass>

<class name="Child1" dynamic-insert="true" dynamic-update="true" lazy="true" table="CHILD1">
<property name="child_attr1" column="ATTR_1" length="30"/>
<property name="child_attr2" column="ATTR_2" length="30"/>
<property name="child_attr3" column="ATTR_3" length="255"/>
<property name="child_attr4" column="ATTR_3" length="60"/>
</class>

<class name="Child2" dynamic-update="true" lazy="true" table="CHILD2">
...
collections of Child3
...
</class>

<class name="Child3" dynamic-update="true" lazy="true" table="CHILD3">
...
collections of Child3
...
</class>

<class name="User" mutable="false" lazy="true" table="USER">
...
</class>

</hibernate-mapping>


Application code:

Header header = (Header)session.load(Header.class, id); // does not fire a query: a proxy is returned

String attr1 = header.getAttr1(); // this call causes Hibernate to issue large number of queries
// to load almost the entire object graph, down to the leaf level
// of child2 and child3.
// However, many-to-one associations of Header are not loaded.

Hibernate log (segment):

09:47:39,728 DEBUG SessionImpl:2198 - resolving associations for [testApp.Header#226112]
09:47:39,728 DEBUG SessionImpl:1982 - loading [testApp.User#173340]
09:47:39,728 DEBUG SessionImpl:1982 - loading [testApp.User#173340]
09:47:39,728 DEBUG SessionImpl:3929 - creating collection wrapper:[testApp.Header.child1List#226112]
09:47:39,743 DEBUG SessionImpl:3929 - creating collection wrapper:[testApp.Header.child2List#226112]
09:47:39,743 DEBUG SessionImpl:3929 - creating collection wrapper:[testApp.Header.child3List#226112]
09:47:39,759 DEBUG SessionImpl:3256 - initializing collection [testApp.Header.child1List#226112]
09:47:39,759 DEBUG SessionImpl:3257 - checking second-level cache
09:47:39,759 DEBUG SessionImpl:3263 - collection not cached
09:47:39,759 DEBUG BatcherImpl:196 - about to open: 0 open PreparedStatements, 0 open ResultSets
09:47:39,759 DEBUG SQL:237 - select child1....

09:47:39,775 DEBUG BatcherImpl:241 - preparing statement
...

Can someone point out what is wrong here?? Child1 should not be initialized at all.

Another thing. All my entity classes are generated using JAXB, which uses a wrapper List implementation, com.sun.xml.bind.util.ListImpl. Can this cause the strange behaviour??

Thank you

http://forum.hibernate.org/viewtopic.php?t=933117.
you need to specify lazy and proxy attributes in class and joined-sublcass elements as well ....
check out the hibernate.dtd:

http://hibernate.sourceforge.net/hibern ... ng-2.0.dtd

palash

Thanks for your response, Palash. It turns out my problem was not with my mapping, but because of the JAXB generated classes I was using.

JAXB uses it's own proxy implementations of lists. When Hibernate checks the object identity of such a proxied field, it appears that a new collection was assigned to this field. This results in Hibernate loading the entire collection, even though the mapping indicates lazy="true" for such fields.

HyperJAXB, which aims to bridge the gaps between JAXB and Hibernate provides the solution to this by generating an alternate field for all collections which are not proxies. Mapping these alternate fields instead of the JAXB generated ones, fixes the lazy loading issue.

This took some time for me to figure out (and a new release of HyperJAXB), but it's good to have a resolution.