休眠延迟加载不适用于多对一映射

Question

I have problem of performance in my many-to-one mapping. When I debug the SQL query in log file the principal query it's ok, but after i have other query representing many-to-one object mapping.

Entity.hbm.xml :

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >

<hibernate-mapping>
    <class name="com.omb.database.mapping.MyEntity" table="MY_ENTITY">
        <id name="id" type="java.lang.Integer">
            <column name="ENTITY_ID"/>
            <generator class="sequence">
                <param name="sequence">SEQ_MY_ENTITY</param>
            </generator>
        </id>

        <property name="prop1" type="string" column="PROP1" />
        <many-to-one name="object1" column="OBJECT1_ID" class="com.omb.database.mapping.Object1" />
        <many-to-one name="object2" column="OBJECT2_ID" class="com.omb.database.mapping.Object2" /> 

    </class>
</hibernate-mapping>

Object1.hbm.xml :

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >

<hibernate-mapping default-lazy="true">

    <class name="com.omb.database.mapping.Object1" table="TABLE_OBJECT_1">
        <id name="id" type="java.lang.Integer" column="OBJECT1_ID" />
        <property name="label" type="string" column="LABEL_OBJECT_1" length="15" />
    </class>

</hibernate-mapping>

Object2.hbm.xml :

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >

<hibernate-mapping default-lazy="true">

    <class name="com.omb.database.mapping.Object2" table="TABLE_OBJECT_2">
        <id name="id" type="java.lang.Integer" column="OBJECT2_ID" />
        <property name="label" type="string" column="LABEL_OBJECT_2" length="15" />
    </class>

</hibernate-mapping>

Query HBM :

public List<Entity> findByObject1Id(Integer object1Id) throws DataAccesException {

        List<Entity> results = null;
        try {
            Query query = this.getSession().createQuery(
                    "from Entity ent where ent.object1.id = :object1Id");
            query.setParameter("object1Id", object1Id);
            results = query.list();
        } catch (HibernateException hbe) {
            throw new DataAccesException(hbe);
        }

        return results;
    }

in pom.xml

<!-- Hibernate 3 -->
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate</artifactId>
            <version>3.2.6.ga</version>
            <exclusions>
                <exclusion>
                    <groupId>javax.transaction</groupId>
                    <artifactId>jta</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>net.sf.ehcache</groupId>
                    <artifactId>ehcache</artifactId>
                </exclusion>
                <exclusion>
                    <groupId>asm</groupId>
                    <artifactId>asm-attrs</artifactId>
                </exclusion>
            </exclusions>
        </dependency>

Answer 1

Your mapping seems to be ok. As documented here 5.1.1. Entity

The <class> attribute lazy is by default true

• lazy (optional): lazy fetching can be disabled by setting lazy="false".

The same for <many-to-one> : 5.1.7.1. Using a foreign key or an association table lazy attribute:

• lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched.

So, where is the issue?

I would say in your debug window . Because you do have a reference to your list, and you are watching the result - in the moment it is executed - the reference is also loaded. Lazily - but loaded. That's in fact what we want. Proxy - when firstly tuched - is forcing the load.

Try to remove it from watch. Or close the session and then put it in the watch... You should see, taht the query used above - is not loading references... only when really accessed... even via debug window

Answer 2

您是否尝试过像这样的FetchMode.SELECT？

<many-to-one name="object1" column="OBJECT1_ID" class="com.omb.database.mapping.Object1" fetch="select" />