java – Hibernate急切加载(获取所有属性不起作用)

基本上我想要加载属性.我有以下HQL查询：

SELECT u.id AS id, u.name AS text, u AS obj FROM User AS u fetch all properties

我希望这只执行一个查询.相反,我得到了N 1个查询.

代码如下：

Query q = mySession.createQuery(
    "SELECT u.id AS id, u.name AS text, u AS obj FROM User AS u fetch all properties")
    .setResultTransformer(Transformers.ALIAS_TO_ENTITY_MAP);

for (Iterator i = q.iterate(); i.hasNext();) {
    Object line = i.next();
    System.out.println(line);
}

我得到的输出(hibernate.show_sql设置为true)是：

Hibernate: select user0_.id as col_0_0_, user0_.name as col_1_0_, user0_.id as col_2_0_ from user user0_
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_, user0_.location as location0_0_ from user user0_ where user0_.id=?
{id=1, obj=User@b6548 [id='1' name='John' ], text=John}
Hibernate: select user0_.id as id0_0_, user0_.name as name0_0_, user0_.location as location0_0_ from user user0_ where user0_.id=?
{id=2, obj=User@4865ce [id='2' name='Arnold' ], text=Arnold}

Ps：没有变形金刚的情况也是如此.

编辑：

具有实体映射的文件：

<hibernate-mapping>

    <class name="User" table="user">
        <id name="id" column="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <property name="location"/>
        <map name="customPrices" table="custprice">
            <key column="user"/>
            <map-key-many-to-many column="product" class="Product"/>
            <element column="price" type="double"/>
        </map>
    </class>

    <class name="Product" table="product">
        <id name="id" column="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <property name="listprice"/>
    </class>

</hibernate-mapping>

我尝试将lazy =“false”添加到类和各个属性中.没有不同.

我的配置文件：

<hibernate-configuration>
<session-factory>
    <property name="connection.url">jdbc:mysql://192.168.0.203/hibtest</property>
    <property name="connection.username">hibtest</property>
    <property name="connection.password">hibb345</property>
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="transaction.factory_class">org.hibernate.transaction.JDBCTransactionFactory</property>
    <property name="current_session_context_class">thread</property>
    <property name="hibernate.show_sql">true</property>

    <mapping resource="combined.hbm.xml" />
</session-factory> 
</hibernate-configuration>

EDIT2：

即使以下代码也会导致N 1查询.虽然我只获取ID字段,但根据文档不应该导致加载对象.

for (Iterator i = q.iterate(); i.hasNext();) {
    Object line = i.next();
    User u = (User)((Map)line).get("obj");
    System.out.println(u.getId());
}

解决方法:

问题出在.iterate()上.根据Hibernate API docs：

Entities returned as results are initialized on demand. The first SQL query returns identifiers only.

当我们期望生成的对象已经被缓存时,这是一个特殊的函数.访问时它们将被(懒惰)加载.

因此,对于一般用途,要在查询结果上获取迭代器,您应该使用.list().iterate().

感谢Eran Harel的帮助.

标签：java,hibernate,hql,lazy-loading,eager-loading
来源： https://codeday.me/bug/20190610/1211098.html