jpa中@JoinColumn使用出错

jpa中@JoinColumn使用出错。

Employee类代码：

 package com.yunkouan.jpatest;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;

@Entity
public class Employee {

    @Id
    private int id;

    private String name;

    @ManyToOne
    @JoinColumn(name="dept_id",referencedColumnName="id")
    private Department dept;
}

Department代码：

 package com.yunkouan.jpatest;

import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class Department {

    @Id private int id;
    private String der_name;

}

dept_id这个在哪里定义的。