異常：could not initialize proxy - the owning Session was closed

解決方法：設定lazy為false

<class name="Grade" table="t_grade">

        <id name="id">

            <generator class="increment" />

        </id>

        <many-to-one name="student" class="Student" column="student_id" lazy="false"/>

        <many-to-one name="course" class="Course" column="course_id" lazy="false"/>

        <property name="state"/>

        <property name="mark"/>

    </class>