[Design Pattern] DAO Pattern 简单案例

Data Access Object Pattern, or DAO Pattern, 即 DAO 模式，用于分离底层的数据访问接口，已经上层的业务逻辑。

下面是 Dao 模式的一个简单的案例。

Student 是纯 Java Beans 类。StudentDao 定义 Dao 的访问接口，StudentDaoImpl 实现该接口。 DaoPatternDemo 演示 Dao 模式

代码实现

Student ，是纯数据对象

public class Student implements Cloneable {

    private String name;
    private int rollNo;
    
    public 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 String toString(){
        return "[name : " + name + ", rollNo : " + rollNo + "]";
    }
    
    public Object clone() {
        Student newStud = new Student(this.name, this.rollNo);
        return newStud;
    }
}

StudentDao 定义 Dao 接口

import java.util.List;

public interface StudentDao {
    
    public List<Student> getAllStudents();
    
    public void updateStudent(Student student);
    
    public void deleteStudent(Student student);
    
    public void addStudent(Student student);

}

StudentDaoImpl 是 Dao 的具体类，实现 Dao 接口

import java.util.Hashtable;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

public class StudentDaoImpl implements StudentDao {

    private Map<Integer, Student> no_stud;
        
    public StudentDaoImpl(){
        no_stud = new Hashtable<Integer, Student>();
        Student studa = new Student("aa", 0);
        Student studb = new Student("bb", 1);
        no_stud.put(studa.getRollNo(), studa);
        no_stud.put(studb.getRollNo(), studb);
    }
    
    @Override
    public List<Student> getAllStudents() {
        List<Student> students = new LinkedList<>(no_stud.values());
        return students;
    }

    @Override
    public void updateStudent(Student student) {
        no_stud.get(student.getRollNo()).setName(student.getName());
    }

    @Override
    public void deleteStudent(Student student) {
        no_stud.remove(student.getRollNo());
    }

    @Override
    public void addStudent(Student student) {
        if (no_stud.containsKey(student.getRollNo())){
            System.out.println(" duplicate student No. cannot add ");
        }
        else{
            no_stud.put(student.getRollNo(), student);
        }
    }
    
    public Student getFirstStudentCopy(){
        Student student = (Student)no_stud.get(0).clone();
        return student;
    }
}

演示 Dao 模式。注，代码中 StudentDaoImpl::getFirstStudentCopy ，是为了演示 StudentDaoImpl::updateStudent 而出现的 ，但并未出现在 UML  图中。

public class DaoPatternDemo {

    public static void main(){
        
        StudentDaoImpl studentDaoImpl = new StudentDaoImpl();
        System.out.println(studentDaoImpl.getAllStudents());
        
        Student student = studentDaoImpl.getFirstStudentCopy();
        student.setName("sss");    
        System.out.println(studentDaoImpl.getAllStudents());

        studentDaoImpl.updateStudent(student);        
        System.out.println(studentDaoImpl.getAllStudents());
    }
}

参考资料

Data Access Object Pattern, TutorialsPoint