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());
         }
     }