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

解决方法：设置lazy为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>