java三层架构，有图有案例有代码，一看就懂！！！

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三层架构

三层结构解释：

视图层：主要是用于与用户进行交互，比如接收用户输入的内容将返回结果向用户展示等。

业务逻辑层：实现每个功能的特定的逻辑方法。

数据访问层：主要是与数据库进行连接，然后对数据库进行增删改查工作。

结构一：

包的层级结构：

三层结构目录架构：

命名规则为公司域名反写 eg:com.li.xxx

bean：存放JavaBean，一张表对应一个类，一个字段对应一个属性

dao：存放连接数据库及对数据库进行增删改查的操作的接口

dao.impl：存放dao接口的实现类

service：存放逻辑层的接口

service.impl：存放逻辑层的接口的实现类

view：用户交互层的接口

view.impl：用户交互层的接口的实现类

包内的实现类：

在编写代码之前先要创建一个数据库：

CREATE TABLE `student` (
  `Sid` int NOT NULL AUTO_INCREMENT,
  `Sname` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
  `birthday` datetime DEFAULT NULL,
  `ssex` varchar(10) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci NOT NULL DEFAULT '男',
  `classid` int DEFAULT NULL,
  PRIMARY KEY (`Sid`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=23 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci ROW_FORMAT=DYNAMIC;

以学生类为例下面是实现代码：

Student的JavaBean：

public class Student {
    //JavaBean实体类
    private int sid;
    private String sname;
    private Date birthday;
    private String ssex;
    private int classid;
    
    public Student(){
    }
    public Student(int sid,String sname,Date birthday,String ssex,int classid){
        this.sid = sid;
        this.sname = sname;
        this.birthday = birthday;
        this.ssex = ssex;
        this.classid = classid;
    }

    public int getSid() {
        return sid;
    }

    public void setSid(int sid) {
        this.sid = sid;
    }

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public String getSsex() {
        return ssex;
    }

    public void setSsex(String ssex) {
        this.ssex = ssex;
    }

    public int getClassid() {
        return classid;
    }

    public void setClassid(int classid) {
        this.classid = classid;
    }

    @Override
    public String toString() {
        return "Student{" +
                "sid=" + sid +
                ", sname='" + sname + '\'' +
                ", birthday=" + birthday +
                ", ssex='" + ssex + '\'' +
                ", classid=" + classid +
                '}';
    }

Dao层的接口：

稍作赘述：接口的出现实际上是为了制定规则，代码编写三要素：方法名、参数、返回值类型；而在接口中定义这三个东西然后让子类实现，当某一层未开发时也不影响其他层的开发。

public interface IStudentDao {
    //增加
    public int addStudent(Student s);
    //删除
    public int deleteStudent(int sid);
    //修改
    public int updateStudent(Student s);
    //查询
    public Student findStudentBySid(int sid);
    //全查
    public List<Student> findAllStudent();
}

Dao层接口的实现类：

//增加记录
    @Override
    public int addStudent(Student s) {
        Connection conn = null;
        PreparedStatement presta = null;
        int update = 0;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/myschool?charset=utf8mb4&useSSL=false&useTimezone=true&serverTimezone=GMT%2B8&characterEncoding=utf-8&allowPublicKeyRetrieval=true";
            String user = "root";
            String pwd = "123456";
            conn = DriverManager.getConnection(url, user, pwd);
            String sql = "insert into student(sname,birthday,ssex,classid) values (?,?,?,?)";
            presta = conn.prepareStatement(sql);
//            presta.setObject(1,s.getSid());
            presta.setObject(1,s.getSname());
            presta.setObject(2,s.getBirthday());
            presta.setObject(3,s.getSsex());
            presta.setObject(4,s.getClassid());
            update = presta.executeUpdate();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if (presta != null){
                try {
                    presta.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if (conn != null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }
        return update;
    }

    //删除记录
    @Override
    public int deleteStudent(int sid) {
        int update  = 0;
        PreparedStatement presta = null;
        Connection conn = null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/myschool?charset=utf8mb4&useSSL=false&useTimezone=true&serverTimezone=GMT%2B8&characterEncoding=utf-8&allowPublicKeyRetrieval=true";
            String user = "root";
            String pwd = "123456";
            conn = DriverManager.getConnection(url, user, pwd);
            String sql = "delete from student where sid = ?";
            presta = conn.prepareStatement(sql);
            presta.setObject(1,sid);
            update = presta.executeUpdate();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if(presta != null){
                try {
                    presta.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if(conn != null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }
        return update;
    }

    //修改记录
    @Override
    public int updateStudent(Student s) {
        int update  = 0;
        PreparedStatement presta = null;
        Connection conn = null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/myschool?charset=utf8mb4&useSSL=false&useTimezone=true&serverTimezone=GMT%2B8&characterEncoding=utf-8&allowPublicKeyRetrieval=true";
            String user = "root";
            String pwd = "123456";
            conn = DriverManager.getConnection(url, user, pwd);
            String sql = "update student set sname = ?,birthday = ? ,ssex = ? ,classid = ?  where sid = ?";
            presta = conn.prepareStatement(sql);
            presta.setObject(1,s.getSname());
            presta.setObject(2,s.getBirthday());
            presta.setObject(3,s.getSsex());
            presta.setObject(4,s.getClassid());
            presta.setObject(5,s.getSid());
            update = presta.executeUpdate();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if (presta != null){
                try {
                    presta.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if (conn != null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }
        return update;
    }

    //sid查询
    @Override
    public Student findStudentBySid(int sid) {
        Connection conn = null;
                Student student = null;
        PreparedStatement statement = null;
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/myschool?charset=utf8mb4&useSSL=false&useTimezone=true&serverTimezone=GMT%2B8&characterEncoding=utf-8&allowPublicKeyRetrieval=true";
            String user = "root";
            String pwd = "123456";
            conn = DriverManager.getConnection(url, user, pwd);
            String sql = "select * from student where sid = ?";
            statement = conn.prepareStatement(sql);
            statement.setObject(1,sid);
            ResultSet resultSet = statement.executeQuery();
            if(resultSet.next()){
                student = new Student();
                student.setSid(resultSet.getInt("sid"));
                student.setSname(resultSet.getString("sname"));
                student.setBirthday(resultSet.getDate("birthday"));
                student.setSsex(resultSet.getString("ssex"));
                student.setClassid(resultSet.getInt("classid"));
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if (statement != null){
                try {
                    statement.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if(conn != null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }

        return student;
    }

    //全查
    @Override
    public List<Student> findAllStudent() {
        Connection conn = null;
        PreparedStatement statement = null;
        List list = new ArrayList();
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/myschool?charset=utf8mb4&useSSL=false&useTimezone=true&serverTimezone=GMT%2B8&characterEncoding=utf-8&allowPublicKeyRetrieval=true";
            String user = "root";
            String pwd = "123456";
            conn = DriverManager.getConnection(url, user, pwd);
            String sql = "select * from student";
            statement = conn.prepareStatement(sql);
            ResultSet resultSet = statement.executeQuery();
            while (resultSet.next()){
                Student stu = new Student();
                stu.setSid(resultSet.getInt("sid"));
                stu.setSname(resultSet.getString("sname"));
                stu.setBirthday(resultSet.getDate("birthday"));
                stu.setSsex(resultSet.getString("ssex"));
                stu.setClassid(resultSet.getInt("classid"));
                list.add(stu);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            if (statement != null){
                try {
                    statement.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
            if(conn != null){
                try {
                    conn.close();
                } catch (SQLException throwables) {
                    throwables.printStackTrace();
                }
            }
        }

        return list;
    }

service层的接口：

public interface IStudentService {

    //注册（添加字段）
    public boolean register(Student s);

    //删除学生
    public boolean deleteStu(int sid);

    //修改学生信息
    public boolean modifyStu(Student s);

    //Sid查询学生
    public Student info(int sid);

    //所有学生列表
    public List<Student> student();

}

service层的接口的实现类：

//添加学生字段
    @Override
    public boolean register(Student s) {
        boolean isok = false;
        IStudentDao isd = new StudentDaoImpl();
        int ret = isd.addStudent(s);
        if (ret > 0) {
            isok = true;
        }
        return isok;
    }

    //删除学生字段
    @Override
    public boolean deleteStu(int sid) {
        boolean isok = false;
        IStudentDao isd = new StudentDaoImpl();
        int ret = isd.deleteStudent(sid);
        if (ret > 0){
            isok = true;
        }
        return isok;
    }

    //修改学生字段
    @Override
    public boolean modifyStu(Student s) {
        boolean isok = false;
        IStudentDao isd = new StudentDaoImpl();
        int i = isd.updateStudent(s);
        if (i > 0){
            isok = true;
        }
        return isok;
    }

    //Sid查学生字段
    @Override
    public Student info(int sid) {
        IStudentDao isd = new StudentDaoImpl();
        Student studentBySid = isd.findStudentBySid(sid);
        return studentBySid;
    }

    //学生字段全查
    @Override
    public List<Student> student() {
        IStudentDao isd = new StudentDaoImpl();
        List<Student> allStudent = isd.findAllStudent();
        return allStudent;
    }

View的是接口：

public interface IStudentView {
    //学生的注册
    public void showstuadd();

    //学生的修改
    public void showstumodify();

    //学生的全部查询
    public void showstuinfo();

    //学生的Sid查询
    public void showstuinfoSid();

    //学生的删除
    public void showstudelete();

    //学生的菜单
    public void showstumenu();
}

View的是接口的实现类：

//学生的注册（添加字段）
    @Override
    public void showstuadd() {

        SimpleDateFormat format = new SimpleDateFormat();
        Scanner input  = new Scanner(System.in);
        System.out.println("添加学生：");
        System.out.println("请输入学生姓名");
        String sname = input.next();
        System.out.println("请输入学生生日");
        String bir = input.next();
        System.out.println("请输入学生性别");
        String sex = input.next();
        System.out.println("请输入学生班级");
        int classid = input.nextInt();
        Student stu = new Student();
        stu.setSname(sname);
        try {
            stu.setBirthday(format.parse(bir));
        } catch (ParseException e) {
            stu.setBirthday(new Date());
        }
        stu.setSsex(sex);
        stu.setClassid(classid);
        IStudentService iss = new StudentServiceImpI();
        boolean register = iss.register(stu);
        if (register){
            System.out.println("插入成功");
        }else{
            System.out.println("插入失败");
        }

    }

    //学生的修改
    @Override
    public void showstumodify() {
        SimpleDateFormat format = new SimpleDateFormat();
        Scanner input  = new Scanner(System.in);
        System.out.println("修改学生信息：");
        System.out.println("请输入需要修改的学生编号");
        int sid = input.nextInt();
        System.out.println("请输入学生新姓名");
        String sname = input.next();
        System.out.println("请输入学生新生日");
        String bir = input.next();
        System.out.println("请输入学生新性别");
        String sex = input.next();
        System.out.println("请输入学生新班级");
        int classid = input.nextInt();
        Student stu = new Student();
        stu.setSname(sname);
        try {
            stu.setBirthday(format.parse(bir));
        } catch (ParseException e) {
            stu.setBirthday(new Date());
        }
        stu.setSsex(sex);
        stu.setClassid(classid);
        stu.setSid(sid);
        IStudentService iss = new StudentServiceImpI();
        boolean register = iss.modifyStu(stu);
        if (register){
            System.out.println("修改成功");
        }else{
            System.out.println("修改失败");
        }
    }

    //学生的全部查询
    @Override
    public void showstuinfo() {
        IStudentService iss = new StudentServiceImpI();
        List<Student> list = iss.student();
        list.forEach(System.out::println);
    }

    //学生的Sid查询
    @Override
    public void showstuinfoSid() {
        Scanner scan = new Scanner(System.in);
        System.out.println("请输入需要查询的ID：");
        int i = scan.nextInt();
        IStudentService iss = new StudentServiceImpI();
        Student info = iss.info(i);
        System.out.println(info);
    }

    //学生的删除
    @Override
    public void showstudelete() {
        Scanner scan = new Scanner(System.in);
        System.out.println("请输入要删除学生的ID：");
        int anInt = scan.nextInt();
        IStudentService iss = new StudentServiceImpI();
        boolean deleteStu = iss.deleteStu(anInt);
        if (deleteStu){
            System.out.println("删除成功！");
        }else{
            System.out.println("删除失败！");
        }
    }

    //学生的菜单
    @Override
    public void showstumenu() {
        Scanner input = new Scanner(System.in);
        System.out.println("欢迎使用校园系统--学生模块管理");
        int key = -1;
        do {
            System.out.println("输入:\n1.添加学生\n2.根据ID查看信息\n3.查看所有学生\n4.修改学生\n5.开除学生\n0.退出程序");
            System.out.println("请输入：");
            if(input.hasNextInt()) {
                key = input.nextInt();
                switch(key) {
                    case 1:
                    {
                        showstuadd();
                    }
                    break;
                    case 2:
                    {
                        showstuinfoSid();
                    }
                    break;
                    case 3:
                    {
                        showstuinfo();
                    }
                    break;
                    case 4:
                    {
                        showstumodify();
                    }
                    break;
                    case 5:
                    {
                        showstudelete();
                    }
                    break;
                    default:
                        System.out.println("请输入正确的序号");
                        break;
                }
            }else {
                System.out.println("请输入菜单中的序号，否则打死你");
                input.next();
            }

        }while(key != 0);
        System.out.println("感谢使用");
    }

测试类：

public static void main(String[] args) {
       IStudentView isv = new IStudentViewImpI();
       isv.showstumenu();
    }

结构二：

结构一中，在DAO层中频繁的进行数据连接创建访问对象等，结构复杂，代码重复率高，因此结构二中对DAO层进行了抽象。

创建DaoUtil类：将加载驱动，获取连接，流的释放进行抽象。

public class DaoUtil {
    //加载驱动
    //驱动的加载只需加载一次，因此使用静态代码块，当加载此类时，会随着类的加载而加载一次
    static {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
    
    //获取连接
    public static Connection getConn(){
        Connection conn = null;
        try {
            String url = "jdbc:mysql://localhost:3306/myschool?charset=utf8mb4&useSSL=false&useTimezone=true&serverTimezone=GMT%2B8&characterEncoding=utf-8&allowPublicKeyRetrieval=true";
            String user = "root";
            String pwd = "123456";
            conn = DriverManager.getConnection(url, user, pwd);
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }
        return conn;
    }

    //关闭连接
    public static void closeResource(Connection conn, PreparedStatement prep, ResultSet res){
        if (res != null){
            try {
                res.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if (prep != null){
            try {
                prep.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
        if (conn != null){
            try {
                conn.close();
            } catch (SQLException throwables) {
                throwables.printStackTrace();
            }
        }
    }
}

创建BaseDao：用来存放增删改和查的方法；这里需要注意的是，增删改操作时返回值是int的整数值，表示数据库中的受影响的行数。而查询操作返回的是ResultSet类型的结果，因此需要分开进行写。

public class BaseDao {
    protected Connection conn;
    protected PreparedStatement prepstat;
    protected ResultSet rs;

    //增删改
    protected int update(String sql,Object...args){
        int ret = 0;
        try {
            conn = DaoUtil.getConn();
            prepstat = conn.prepareStatement(sql);
            if (args != null) {
                for (int i = 0; i < args.length; i++) {
                    prepstat.setObject(i + 1,args[i]);
                }
            }
            ret = prepstat.executeUpdate();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            DaoUtil.closeResource(conn, prepstat, rs);
        }
        return ret;
    }

    //查询
    protected ResultSet query(String sql,Object...args){
        ResultSet rs = null;
        try {
            conn = DaoUtil.getConn();
            prepstat = conn.prepareStatement(sql);
            if (args != null){
                for (int i = 0; i < args.length; i++) {
                    prepstat.setObject(i + 1,args[i]);
                }
            }
            rs = prepstat.executeQuery();
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }
        return rs;
    }

}

抽象后的实现类如下：

public class StudentDaoImpl extends BaseDao implements IStudentDao {
    //增加记录
    @Override
    public int addStudent(Student s) {
        String sql = "insert into student(sname,birthday,ssex,classid) values (?,?,?,?)";
        return update(sql,s.getSname(),s.getBirthday(),s.getSsex(),s.getClass());
    }

    //删除记录
    @Override
    public int deleteStudent(int sid) {
        String sql = "delete from student where sid = ?";
        return update(sql,sid);
    }

    //修改记录
    @Override
    public int updateStudent(Student s) {
        String sql = "update student set sname = ?,birthday = ? ,ssex = ? ,classid = ?  where sid = ?";
        return update(sql,s.getSname(),s.getBirthday(),s.getSsex(),s.getClassid(),s.getSid());
    }

    //sid查询
    @Override
    public Student findStudentBySid(int sid) {
        Student s = null;
        SimpleDateFormat format = new SimpleDateFormat();
        try {
            String sql = "select * from student where sid = ?";
            rs = query(sql, sid);
            if (rs.next()) {
                s = new Student();
                s.setSid(rs.getInt("sid"));
                s.setSname(rs.getString("sname"));
                try {
                    s.setBirthday(format.parse(rs.getString("birthday")));
                } catch (ParseException e) {
                    e.printStackTrace();
                }
                s.setSsex(rs.getString("ssex"));
                s.setClassid(rs.getInt("classid"));
                s.setSid(rs.getInt("sid"));
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            DaoUtil.closeResource(conn, prepstat, rs);
        }
        return s;
    }
        //全查
    @Override
    public List<Student> findAllStudent() {
        Student s = null;
        List list = new ArrayList();
        SimpleDateFormat format = new SimpleDateFormat();
        try {
            String sql = "select * from student where sid = ?";
            rs = query(sql);
            if (rs.next()) {
                s = new Student();
                s.setSid(rs.getInt("sid"));
                s.setSname(rs.getString("sname"));
                try {
                    s.setBirthday(format.parse(rs.getString("birthday")));
                } catch (ParseException e) {
                    e.printStackTrace();
                }
                s.setSsex(rs.getString("ssex"));
                s.setClassid(rs.getInt("classid"));
                s.setSid(rs.getInt("sid"));
                list.add(s);
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        }finally {
            DaoUtil.closeResource(conn, prepstat, rs);
        }
        return list;
    }
}

不同的分法可能会略微有所区别，划分程度不同，划分的层级可能也不同，但本质思想不变，将代码功能模块化，实现高内聚低耦合，例如Dao用来做数据库连接等操作，就尽量只需调方法传参就能执行即可，不要在执行的过程中还需要其他层数据的介入，主要也是为了开发的高效性，同时便于对代码进行测试。

文章来源:https://blog.csdn.net/LIJINGPO/article/details/132860065
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