设计模式笔记 – 数据访问对象模式

数据访问对象模式（Data Access Object Pattern）或 DAO 模式用于把低级的数据访问 API 或操作从高级的业务服务中分离出来。以下是数据访问对象模式的参与者。

• 数据访问对象接口（Data Access Object Interface） – 该接口定义了在一个模型对象上要执行的标准操作。
• 数据访问对象实体类（Data Access Object concrete class） – 该类实现了上述的接口。该类负责从数据源获取数据，数据源可以是数据库，也可以是 xml，或者是其他的存储机制。
• 模型对象/数值对象（Model Object/Value Object） – 该对象是简单的 POJO，包含了 get/set 方法来存储通过使用 DAO 类检索到的数据。

示例

我们将创建一个作为模型对象或数值对象的 Student 对象。StudentDao 是数据访问对象接口。StudentDaoImpl 是实现了数据访问对象接口的实体类。DaoPatternDemo，我们的演示类使用 StudentDao 来演示数据访问对象模式的用法。

Java 实现

创建数值对象

public class Student {
    private String name;
    private int rollNo;
    Student(String name, int rollNo) {
        this.name = name;
        this.rollNo = rollNo;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getRollNo() {
        return rollNo;
    }
    public void setRollNo(int rollNo) {
        this.rollNo = rollNo;
    }
}

创建数据访问对象接口

import java.util.List;

public interface StudentDao {
    public List<Student> getAllStudents();
    public Student getStudent(int rollNo);
    public void updateStudent(Student student);
    public void deleteStudent(Student student);
}

创建实现了上述接口的实体类

import java.util.ArrayList;
import java.util.List;

public class StudentDaoImpl implements StudentDao {
    List<Student> students;
    public StudentDaoImpl() {
        students = new ArrayList<Student>();
        Student student1 = new Student("Robert", 0);
        Student student2 = new Student("John", 1);
        students.add(student1);
        students.add(student2);
    }
    @Override
    public void deleteStudent(Student student) {
        students.remove(student.getRollNo());
        System.out.println("Studnet: Roll No "+student.getRollNo()+", deleted from database");
    }
    @Override
    public List<Student> getAllStudents() {
        return students;
    }
    @Override
    public Student getStudent(int rollNo) {
        return students.get(rollNo);
    }
    @Override
    public void updateStudent(Student student) {
        students.get(student.getRollNo()).setName(student.getName());
        System.out.println("Student: Roll No "+student.getRollNo()+", updated in the database");
    }
}

使用 StudentDao 来演示数据访问对象模式的用法

public class DaoPatternDemo {
    public static void main(String[] args) {
        StudentDao studentDao = new StudentDaoImpl();
        for (Student student : studentDao.getAllStudents()) {
            System.out.println("Student: [RollNo: "+student.getRollNo()+", Name: "+student.getName()+"]");
        }
        Student student = studentDao.getAllStudents().get(0);
        student.setName("Michael");
        studentDao.updateStudent(student);
        student = studentDao.getStudent(0);
        System.out.println("Student: [RollNo: "+student.getRollNo()+", Name: "+student.getName()+"]");
    }
}

Python 实现

import abc
from typing import Tuple

class Student:
    def __init__(self, name: str, roll_no: int):
        self.name = name
        self.roll_no = roll_no

    def get_name(self) -> str:
        return self.name

    def set_name(self, name: str):
        self.name = name

    def get_roll_no(self) -> int:
        return self.roll_no

    def set_roll_no(self, roll_no: int):
        self.roll_no = roll_no

class StudentDao(abc.ABC):
    @abc.abstractmethod
    def get_all_students(self) -> Tuple[Student, ...]: ...

    @abc.abstractmethod
    def get_student(self, roll_no: int) -> Student: ...

    @abc.abstractmethod
    def update_student(self, student: Student): ...

    @abc.abstractmethod
    def delete_student(self, student: Student): ...

class StudentDaoImpl(StudentDao):
    def __init__(self):
        self.students = []
        student1 = Student("Robert", 0)
        student2 = Student("John", 1)
        self.students.append(student1)
        self.students.append(student2)

    def delete_student(self, student: Student):
        self.students.remove(student)
        print(f"Student: Roll No {student.get_roll_no()}, deleted from database")

    def get_all_students(self):
        return self.students

    def get_student(self, roll_no: int):
        return self.students[roll_no]

    def update_student(self, student: Student):
        self.students[student.get_roll_no()].set_name(student.get_name())
        print(f"Student: Roll No {student.get_roll_no()}, updated in the database")

if __name__ == "__main__":
    student_dao = StudentDaoImpl()
    for student in student_dao.get_all_students():
        print(f"Student: [RollNo: {student.get_roll_no()}, Name: {student.get_name()}]")

    student = student_dao.get_all_students()[0]
    student.set_name("Michael")
    student_dao.update_student(student)

    student = student_dao.get_student(0)
    print(f"Student: [RollNo: {student.get_roll_no()}, Name: {student.get_name()}]")