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

1 create table department_table( 2     id int primary key, 3     name varchar(100) 4 ); 5 create table employee_table( 6     id int primary key, 7     name varchar(100), 8     salary float(8,2), 9     dept_id int,10     constraint dept_id_fk foreign key(dept_id) references department(id)11 );

domain

employee.java

1 package cn.itcast.domain; 2  3 import java.io.Serializable; 4 import java.util.List; 5 /* 6 create table department( 7     id int primary key, 8     name varchar(100) 9 );10 create table employee(11     id int primary key,12     name varchar(100),13     salary float(8,2),14     dept_id int,15     constraint dept_id_fk foreign key(dept_id) references department(id)16 );17  */18 public class Employee implements Serializable {19     Integer id;20     String name;21     float salary;22     int dept_id;23     Department dep;24     public Employee() {25         super();26         // TODO Auto-generated constructor stub27     }28     public Integer getId() {29         return id;30     }31     public void setId(Integer id) {32         this.id = id;33     }34     public String getName() {35         return name;36     }37     public void setName(String name) {38         this.name = name;39     }40     public float getSalary() {41         return salary;42     }43     public void setSalary(float salary) {44         this.salary = salary;45     }46     public int getDept_id() {47         return dept_id;48     }49     public void setDept_id(int dept_id) {50         this.dept_id = dept_id;51     }52     @Override53     public String toString() {54         return "Employee [id=" + id + ", name=" + name + ", salary=" + salary55                 + ", dept_id=" + dept_id + ", dep=" + dep + "]";56     }57     public Department getDep() {58         return dep;59     }60     public void setDep(Department dep) {61         this.dep = dep;62     }63     64 }

departmenet.java

1 package cn.itcast.domain; 2  3 import java.io.Serializable; 4 import java.util.List; 5  6 public class Department implements Serializable { 7  8     private Integer id; 9     private String name;10     List
      
        emps;11     public List
       
         getEmps() {12         return emps;13     }14     public void setEmps(List
        
          emps) {15         this.emps = emps;16     }17     public Department() {18         super();19         // TODO Auto-generated constructor stub20     }21     public Integer getId() {22         return id;23     }24     public void setId(Integer id) {25         this.id = id;26     }27     public String getName() {28         return name;29     }30     public void setName(String name) {31         this.name = name;32     }33     @Override34     public String toString() {35         return "Department [id=" + id + ", name=" + name + "]";36     }37     38 }
        
       
      

EmployeeDaoImpl.java

1 package cn.itcast.dao.impl; 2  3 import java.sql.SQLException; 4 import java.util.List; 5  6 import org.apache.commons.dbutils.QueryRunner; 7 import org.apache.commons.dbutils.handlers.BeanHandler; 8 import org.apache.commons.dbutils.handlers.BeanListHandler; 9 import org.apache.commons.dbutils.handlers.ScalarHandler;10 11 import cn.itcast.domain.Department;12 import cn.itcast.domain.Employee;13 import cn.itcast.utils.JDBCUtil;14 15 public class EmployeeDaoImpl {16     QueryRunner qr=new QueryRunner(JDBCUtil.getDataSource());17     public void addEmployee(Employee e) throws SQLException18     {19         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20         if(d!=null)21         {22             Object num=qr.query("select 1 from department_table where id= ?", new ScalarHandler(1),d.getId());23             if(null==num)24             {25                 qr.update("insert into department_table (id,name) values(?,?)",d.getId(),d.getName());26             }27         }28         String sql="insert into employee_table (id,name,salary,dept_id) values(?,?,?,?)";29         qr.update(sql,e.getId(),e.getName(),e.getSalary(),e.getDept_id());30     }31     32     public Department findDepartment(String id) throws SQLException33     {34         Department d=qr.query("select * from department_table where id =?", new BeanHandler
      
       (Department.class),id);35         if(d!=null)36         {37             List
       
         emps=qr.query("select * from employee_table where dept_id =?", new BeanListHandler
        
         (Employee.class),d.getId());38             d.setEmps(emps);39         }40         return d;41     }42 }
        
       
      

测试

1     EmployeeDaoImpl dao=new EmployeeDaoImpl(); 2     @Test 3     public void addEmployee() throws SQLException 4     { 5         Department d=new Department(); 6         d.setId(1); 7         d.setName("开发部"); 8  9         Employee e=new Employee();10         e.setId(1);11         e.setName("chenlongfei");12         e.setSalary(10000);13         e.setDept_id(1);14         e.setDep(d);15         16         dao.addEmployee(e);17         18     }19     @Test20     public void findDepartment() throws SQLException21     {22         Department d=dao.findDepartment("1");23         System.out.println(d.getName());24         for(Employee e:d.getEmps())25         {26             System.out.println(e.getName());27         }28     }29