三十三、数据访问对象模式 ( Data Access Object )

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

数据访问模式涉及到的参与者有:

  • 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

实现

 

1、 创建一个作为模型对象或数值对象的Student对象;
2、 定义StudentDao作为数据访问对象接口;
3、 定义StudentDaoImpl实现了数据访问对象接口的实体类;
4、 定义DaoPatternDemo使用StudentDao来演示数据访问对象模式的用法;

范例

1. 创建数值对象

Student.java

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
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;
   }

2. 创建数据访问对象接口

StudentDao.java

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
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);

3. 创建实现了上述接口的实体类

StudentDaoImpl.java

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
import java.util.ArrayList;
import java.util.List;
public class StudentDaoImpl implements StudentDao {
   //列表是当作一个数据库
   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("Student: Roll No " + student.getRollNo() 
         +", deleted from database");
   }
   //从数据库中检索学生名单
   @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");
   }

4. 使用 StudentDao 来演示数据访问对象模式的用法

CompositeEntityPatternDemo.java

// author: pottercoding.cn 程序员波特,程序员编程资料站(pottercoding.cn)
// Copyright © 2015-2065 pottercoding.cn. All rights reserved.
package com.pottercoding.gof;
public class DaoPatternDemo {
   public static void main(String[] args) {
      StudentDao studentDao = new StudentDaoImpl();
      //输出所有的学生
      for (Student student : studentDao.getAllStudents()) {
         System.out.println("Student: [RollNo : "
            +student.getRollNo()+", Name : "+student.getName()+" ]");
      }
      //更新学生
      Student student =studentDao.getAllStudents().get(0);
      student.setName("Michael");
      studentDao.updateStudent(student);
      //获取学生
      studentDao.getStudent(0);
      System.out.println("Student: [RollNo : "
         +student.getRollNo()+", Name : "+student.getName()+" ]");      
   }

编译运行以上范例,输出结果如下

$ javac -d . src/main/com.pottercoding/gof/DaoPatternDemo.java
$ java  com.pottercoding.gof.DaoPatternDemo
Student: [RollNo : 0, Name : Robert ]
Student: [RollNo : 1, Name : John ]
Student: Roll No 0, updated in the database
Student: [RollNo : 0, Name : Michael ]