多对一关系表 java类描述

少的一方把它查询出来，多的一方看需求把它查出来

涉及java对象涉及到多个对象相互引用，要尽量避免使用一对多，或多对多关系，而应使用多对一描述对象之间的关系(或使用延迟加载的方式)。

下个例子employee是多，而department是一 的关系

sql语句

create table department_table(
    id int primary key,
    name varchar(100)
);
create table employee_table(
    id int primary key,
    name varchar(100),
    salary float(8,2),
    dept_id int,
    constraint dept_id_fk foreign key(dept_id) references department(id)
);

domain

employee.java

package cn.itcast.domain;

import java.io.Serializable;
import java.util.List;
/*
create table department(
    id int primary key,
    name varchar(100)
);
create table employee(
    id int primary key,
    name varchar(100),
    salary float(8,2),
    dept_id int,
    constraint dept_id_fk foreign key(dept_id) references department(id)
);
 */
public class Employee implements Serializable {
    Integer id;
    String name;
    float salary;
    int dept_id;
    Department dep;
    public Employee() {
        super();
        // TODO Auto-generated constructor stub
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getSalary() {
        return salary;
    }
    public void setSalary(float salary) {
        this.salary = salary;
    }
    public int getDept_id() {
        return dept_id;
    }
    public void setDept_id(int dept_id) {
        this.dept_id = dept_id;
    }
    @Override
    public String toString() {
        return "Employee [id=" + id + ", name=" + name + ", salary=" + salary
                + ", dept_id=" + dept_id + ", dep=" + dep + "]";
    }
    public Department getDep() {
        return dep;
    }
    public void setDep(Department dep) {
        this.dep = dep;
    }
    
}

departmenet.java

package cn.itcast.domain;

import java.io.Serializable;
import java.util.List;

public class Department implements Serializable {

    private Integer id;
    private String name;
    List<Employee> emps;
    public List<Employee> getEmps() {
        return emps;
    }
    public void setEmps(List<Employee> emps) {
        this.emps = emps;
    }
    public Department() {
        super();
        // TODO Auto-generated constructor stub
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public String toString() {
        return "Department [id=" + id + ", name=" + name + "]";
    }
    
}

EmployeeDaoImpl.java

package cn.itcast.dao.impl;

import java.sql.SQLException;
import java.util.List;

import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanHandler;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import org.apache.commons.dbutils.handlers.ScalarHandler;

import cn.itcast.domain.Department;
import cn.itcast.domain.Employee;
import cn.itcast.utils.JDBCUtil;

public class EmployeeDaoImpl {
    QueryRunner qr=new QueryRunner(JDBCUtil.getDataSource());
    public void addEmployee(Employee e) throws SQLException
    {
        Department d=e.getDep();            //因为员工是外键依赖部门，所以先考察部门在不在然后考虑员工表
        if(d!=null)
        {
            Object num=qr.query("select 1 from department_table where id= ?", new ScalarHandler(1),d.getId());
            if(null==num)
            {
                qr.update("insert into department_table (id,name) values(?,?)",d.getId(),d.getName());
            }
        }
        String sql="insert into employee_table (id,name,salary,dept_id) values(?,?,?,?)";
        qr.update(sql,e.getId(),e.getName(),e.getSalary(),e.getDept_id());
    }
    
    public Department findDepartment(String id) throws SQLException
    {
        Department d=qr.query("select * from department_table where id =?", new BeanHandler<Department>(Department.class),id);
        if(d!=null)
        {
            List<Employee> emps=qr.query("select * from employee_table where dept_id =?", new BeanListHandler<Employee>(Employee.class),d.getId());
            d.setEmps(emps);
        }
        return d;
    }
}

测试

    EmployeeDaoImpl dao=new EmployeeDaoImpl();
    @Test
    public void addEmployee() throws SQLException
    {
        Department d=new Department();
        d.setId(1);
        d.setName("开发部");

        Employee e=new Employee();
        e.setId(1);
        e.setName("chenlongfei");
        e.setSalary(10000);
        e.setDept_id(1);
        e.setDep(d);
        
        dao.addEmployee(e);
        
    }
    @Test
    public void findDepartment() throws SQLException
    {
        Department d=dao.findDepartment("1");
        System.out.println(d.getName());
        for(Employee e:d.getEmps())
        {
            System.out.println(e.getName());
        }
    }