关于Hibernate discriminator标签的问题?报错

定义"一棵对象继承树对应一个表"的映射表的时候，对于discriminator标签在xml文件中的位置有什么特殊的要求么？ 如果这样子定义的话会报错：

<class name="com.line.model.Animal" table="animal">
		<id name="id" column="id">
			<generator class="native"/>
		</id>
		<property name="sex"/>
		<property name="year"/>
		<discriminator type="string" column="type"/>
		<subclass name="com.line.model.Pig" discriminator-value="P">
			<property name="weight"/>
		</subclass>
		<subclass name="com.line.model.Bird" discriminator-value="B">
			<property name="height"/>
		</subclass>
	</class>

17:04:35,598 ERROR XMLHelper:61 - Error parsing XML: XML InputStream(19) The content of element type "class" must match "(meta*,subselect?,cache?,synchronize*,comment?,tuplizer*,(id|composite-id),discriminator?,natural-id?,(version|timestamp)?,(property|many-to-one|one-to-one|component|dynamiccomponent|properties|any|map|set|list|bag|idbag|array|primitive-array)*,((join*,subclass*)|joined-subclass*|union-subclass*),loader?,sql-insert?,sql-update?,sql-delete?,filter*,resultset*,(query|sql-query)*)".

<class name="com.line.model.Animal" table="animal">
		<id name="id" column="id">
			<generator class="native"/>
		</id>
		<!-- 这个标签一定要主键定义之后，其他属性定义之前，不明所以 -->
		<discriminator type="string" column="type"/>
		<property name="sex"/>
		<property name="year"/>
		<subclass name="com.line.model.Pig" discriminator-value="P">
			<property name="weight"/>
		</subclass>
		<subclass name="com.line.model.Bird" discriminator-value="B">
			<property name="height"/>
		</subclass>
	</class>

   求知道原因的牛人给小弟解答一下，谢谢！