Java开发中同一个对象中多次以同一个对象的主键作为属性的@JoinColumn注解

Java开发过程中，常常会遇到一个对象中多次用到另一个对象的主键作为属性的情况。比如记录一个人的部门变化情况，需要同时记录变更前后的部门编号。之前有误区，以为在这种情况下的@JoinColumn属性必须是实体类的主键，而实际上这个注解的属性值是当前实体类的外键。举例如下：

首选创建Department实体类

@Entity
@Table(name="department")
public class Department implements Serializable{

    private static final long serialVersionUID = 1L;
    
    private int departmentId;

    private String departmentName;
    
    @Id
    @GenerateValue
    @Column(name = "departmentId",nullable = false)
    public int getDepartmentId(){
            return departmentId;    
    }

    public void setDepartmentId(int departmentId) {
	    this.departmentId = departmentId;
    }

    @Column(name = "departmentName",nullable = false)
    public int getDepartmentName(){
            return departmentName;    
    }

    public void setDepartmentName(String departmentName) {
	    this.departmentName = departmentName;
    }

}

    

　　和Employee实体类

@Entity
@Table(name="employee")
public class Employee implements Serializable{

    private static final long serialVersionUID = 1L;
    
    private int employeetId;

    private String employeeName;
    
    @Id
    @GenerateValue
    @Column(name = "employeeId",nullable = false)
    public int getEmployeeId(){
            return employeeId;    
    }

    public void setEmployeeId(int employeeId) {
	    this.employeeId = employeeId;
    }

    @Column(name = "employeeName",nullable = false)
    public int getEmployeeName(){
            return employeeName;    
    }

    public void setEmployeeName(String employeeName) {
	    this.employeeName = employeeName;
    }

}

　　接下来，创建用户的部门调动情况，实现如下：

@Entity
@Table(name="empDeptChangeInfo")
public class EmpDeptChangeInfo implements Serializable{

    private static final long serialVersionUID = 1L;

    private int itemId;
　　
    private Employee employee;

    private Department dept1;

    private Department dept2;

    @Id
    @GenerateValue
    @Column(name = "itemId",nullable = false)
    public int getItemId(){
         return itemId;    
    }

    public void setItemId(int itemId) {
	 this.itemId = itemId;
    }

    @ManyToOne(cascade = CascadeType.REFRESH, optional = true)
    @JoinColumn(name = "employeeId")
    public int getEmployeeId(){
    　　　return employeeId;　
    }
    
    public void setEmployee(Employee employee){
         this.employee = employee;
    }

    @ManyToOne(cascade = CascadeType.REFRESH, optional = true)
    @JoinColumn(name = "deptId1")
    public getDept1(){
         return dept1;
    }
    
    public void setDept1(Department dept1){
         this.dept1 = dept1;
    }

    @ManyToOne(cascade = CascadeType.REFRESH, optional = true)
    @JoinColumn(name = "deptId2")
    public getDept2(){
         return dept2;
    }
    
    public void setDept2(Department dept2){
         this.dept2 = dept2;
    }
}

　　值得注意的是，@JoinColumn后面的属性，并非所关联的表中定义的属性字段名或数据库名，如Dept1和Dept2在数据库表empDeptChangeInfo中对应的字段为deptId1和的平台deptId2，而并不是department表的字段departmentId。