三层结构下的多表查询，实体类写法

一对一关系

　　查询所有学生信息，年级信息以年级名称显示

　　一、DAO层搭建：

　　　　1.实体类：

　　　　　　Student类：

package com.myschool.entity;

import java.io.Serializable;
import java.util.Date;

public class Student implements Serializable {
	
	private static final long serialVersionUID = 6439763802252472361L;
	
	//定义实体属性
	private int studentNo;
	private String longinPwd;
	private String studentName;
	private int sex;
	private grade grade;//把年级对象作为属性
	private String phone;
	private String address;
	private Date bornDate;
	private String email;
	private String identityCard;
	
        //封装
	public int getStudentNo() {
		return studentNo; 
	}
	public void setStudentNo(int studentNo) {
		this.studentNo = studentNo;
	}
	public String getLonginPwd() {
		return longinPwd;
	}
	public void setLonginPwd(String longinPwd) {
		this.longinPwd = longinPwd;
	}
	public String getStudentName() {
		return studentName;
	}
	public void setStudentName(String studentName) {
		this.studentName = studentName;
	}
	public int getSex() {
		return sex;
	}
	public void setSex(int sex) {
		this.sex = sex;
	}
	public grade getGrade() {
		return grade;
	}
	public void setGrade(grade grades) {
		this.grade = grades;
	}
	public String getPhone() {
		return phone;
	}
	public void setPhone(String phone) {
		this.phone = phone;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	public Date getBornDate() {
		return bornDate;
	}
	public void setBornDate(Date bornDate) {
		this.bornDate = bornDate;
	}
	public String getEmail() {
		return email;
	}
	public void setEmail(String email) {
		this.email = email;
	}
	public String getIdentityCard() {
		return identityCard;
	}
	public void setIdentityCard(String identityCard) {
		this.identityCard = identityCard;
	}
	
       //有参构造
	public Student(int studentNo, String longinPwd, String studentName,
			int sex, grade gradeID, String phone, String address, Date bornDate,
			String email, String identityCard) {
		this.studentNo = studentNo;
		this.longinPwd = longinPwd;
		this.studentName = studentName;
		this.sex = sex;
		this.gradeID = gradeID;
		this.phone = phone;
		this.address = address;
		this.bornDate = bornDate;
		this.email = email;
		this.identityCard = identityCard;
	}

	//无参构造
	public Student() {
		
	}

　　　　　Grade类：

package com.myschool.entity;

public class grade {
    private int gradeid;
    private String gradeName;
    public int getGradeid() {
        return gradeid;
    }
    public void setGradeid(int gradeid) {
        this.gradeid = gradeid;
    }
    public String getGradeName() {
        return gradeName;
    }
    public void setGradeName(String gradeName) {
        this.gradeName = gradeName;
    }
    public grade(int gradeid, String gradeName) {
        super();
        this.gradeid = gradeid;
        this.gradeName = gradeName;
    }
    public grade() {
        super();
        
    }
    
}

　　　　因为一个学生对应一个年级，但是学生表中不能涉及年级名称，所以直接将年级对象封装到学生属性中

　　　　2.Daobase：数据库连接访问类

package com.myschool.dao;

import java.beans.Statement;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

public class baseDao {

    // 创建连接参数
    private final static String DRIVER = "com.mysql.jdbc.Driver";
    private final static String URL = "jdbc:mysql:///myschool";
    private final static String USER_NAME = "root";
    private final static String PASSWORD = "123";

    Connection con = null;
    PreparedStatement prestatement = null;
    ResultSet rs = null;

    // 获取连接
    private Connection getConnection() {
        try {
            Class.forName(DRIVER);
            if (con == null) {
                con = DriverManager.getConnection(URL, USER_NAME, PASSWORD);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        return con;
    }

    // 增删改
    public int executeUpdate(String sql, Object... obj) throws Exception {
        // 获取连接
        getConnection();

        // 获取prepareStatement对象
        prestatement = con.prepareStatement(sql);

        // 循环添加参数
        for (int i = 1; i <= obj.length; i++) {
            prestatement.setObject(i, obj[i - 1]);
        }
        // 执行SQL语句
        int count = prestatement.executeUpdate();
        return count;

    }

    // 查
    public ResultSet executeQuery(String sql, Object... obj) throws Exception {
        // 获取连接
        getConnection();

        // 获取prepareStatement对象
        prestatement = con.prepareStatement(sql);

        // 循环添加参数
        for (int i = 1; i <= obj.length; i++) {
            prestatement.setObject(i, obj[i - 1]);
        }
        // 执行SQL语句
        rs = prestatement.executeQuery();

        return rs;

    }
    
    //回收资源
    public void closeResouse() throws Exception
    {
        if (rs!=null) {
            rs.close();
        }
        if (prestatement!=null) {
            prestatement.close();
        }
        if (con!=null) {
            con.close();
        }
        
    }
 
}

　　　　3.DAO接口：

package com.myschool.dao;

import java.util.List;

import com.myschool.entity.Student;

public interface IStudentDao {

    /*
     * 查询所有学生记录,年级名称
     */
    public List<Student> Search() throws Exception;

    
}

　　　　　4.DAO接口实现类：

package com.mychool.dao.impl;

import java.sql.ResultSet;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;

import com.myschool.dao.IStudentDao;
import com.myschool.dao.baseDao;
import com.myschool.entity.Student;
import com.myschool.entity.grade;

public class IStudentDaoImpl extends baseDao implements IStudentDao {

    
    @Override
    public List<Student> Search() throws Exception {
        //创建student对象泛型集合
        List<Student> stus=new ArrayList<Student>();
        
        String sql="Select studentName,GradeName from Student,grade where Grade.gradeid=Student.gradeid ";

        //定义resultSet对象接收basedao的查询方法查出来的数据
        ResultSet rSet=executeQuery(sql);
        
        if (rSet!=null) {
            while (rSet.next()) {
                Student stu=new Student();
                grade grade=new grade();//创建年级对象
//                stu.setAddress(rSet.getString("address"));
//                stu.setEmail(rSet.getString("email"));
//                stu.setIdentityCard(rSet.getString("identityCard"));
//                stu.setLonginPwd(rSet.getString("longinPwd"));
//                stu.setPhone(rSet.getString("phone"));
//                stu.setSex(rSet.getInt("sex"));
                grade.setGradeName(rSet.getString("gradeName"));//给年级对象赋值
                stu.setStudentName(rSet.getString("studentName"));//给学生对象赋值
                stu.setGradeID(grade);
//                stu.setStudentNo(rSet.getInt("StudentNo"));
//                SimpleDateFormat sdFormat=new SimpleDateFormat("yyyy-MM-dd");
//                stu.setBornDate(sdFormat.parse(rSet.getString("bornDate")));
                stus.add(stu);//将学生对象添加到对象集合中
            }
            
        }
　　　　//回收释放资源
        closeResouse();
        return stus;
    }

    
}
为了简单明洁，部分属性不进行值查询

　　二、service层：

　　这里没有啥逻辑，我就直接将dao方法引用一遍，不做详解

　　　　1.service接口：

package com.myschool.service;

import java.util.List;

import com.myschool.entity.Student;

public interface IService {
    
    public List<Student> Search() throws Exception;
}

　　　　2.service接口实现类：

package com.mychool.service.Impl;

import java.util.List;

import com.mychool.dao.impl.IStudentDaoImpl;
import com.myschool.dao.IStudentDao;
import com.myschool.entity.Student;
import com.myschool.service.IService;

public class IServiceImpl implements IService{

    IStudentDao isd=new IStudentDaoImpl();
    @Override
    public List<Student> Search() throws Exception {
        return isd.Search();
    }
    
}

　　三、UI层：

　　　　

package com.myschool.ui;

import java.util.List;

import com.mychool.service.Impl.IServiceImpl;
import com.myschool.entity.Student;
import com.myschool.service.IService;

public class test {    
    public static void main(String[] args) throws Exception {
　　　　　//创建service接口实现类的对象
        IService isv=new  IServiceImpl();
        List<Student> lsList=isv.Search();

        for (Student student : lsList) {
            System.out.println(student.getStudentName()+"	"+student.getGradeID().getGradeName());
        }
    }
}

一对多关系

　　这里最主要的区别就是封装和dao实现类的写法：

　　因为一个年级对应多个学生，所以在grade类中封装一个student对象的泛型集合。

　　　　DAO实现类写法：