public class Student extends Model<Student> {
    public static final Student dao = new Student();
    public Classes getClasses() {
        return Classes.dao.findById(get("class_id"));
    }
}

班级类:

public class Classes extends Model<Classes> {
    public static final Classes dao = new Classes();
}

DemoConfig:

	@Override
	public void configPlugin(Plugins me) {
		//	数据库的ORM映射的配置。
		C3p0Plugin cp = new C3p0Plugin("jdbc:mysql://localhost:3306/wt",  
                "root", "111111");
        me.add(cp); 
        ActiveRecordPlugin arp = new ActiveRecordPlugin(cp);  
        me.add(arp);  
        arp.addMapping("t_user", User.class);
        arp.addMapping("t_class", "class_id", Classes.class);
        arp.addMapping("t_student", "id", Student.class);
	}

jsp:

  <c:forEach items="${studentList}" var="student">
            <tr>
                <td>
                    ${student.name}
                </td>
                <td>
                    ${student.age}
                </td>
                <td>
                    ${student.sex}
                </td>
                <td>
                    ${student.getClasses().class_name}
                </td>
                <td>
                    <a href="/student/delete/${student.studentid}">删除</a>
                    <a href="/student/get/${student.studentid}">修改</a>
                </td>
            </tr>
		</c:forEach>

${student.getClasses().class_name}这一句报空指针异常,望高手解惑 ,哪里没弄对 .