數(shù)據(jù)訪問(wèn)對(duì)象模式

數(shù)據(jù)訪問(wèn)對(duì)象模式（Data Access Object Pattern）或 DAO 模式用于把低級(jí)的數(shù)據(jù)訪問(wèn) API 或操作從高級(jí)的業(yè)務(wù)服務(wù)中分離出來(lái)。以下是數(shù)據(jù)訪問(wèn)對(duì)象模式的參與者。

• 數(shù)據(jù)訪問(wèn)對(duì)象接口（Data Access Object Interface） - 該接口定義了在一個(gè)模型對(duì)象上要執(zhí)行的標(biāo)準(zhǔn)操作。
• 數(shù)據(jù)訪問(wèn)對(duì)象實(shí)體類（Data Access Object concrete class） - 該類實(shí)現(xiàn)了上述的接口。該類負(fù)責(zé)從數(shù)據(jù)源獲取數(shù)據(jù)，數(shù)據(jù)源可以是數(shù)據(jù)庫(kù)，也可以是 xml，或者是其他的存儲(chǔ)機(jī)制。
• 模型對(duì)象/數(shù)值對(duì)象（Model Object/Value Object） - 該對(duì)象是簡(jiǎn)單的 POJO，包含了 get/set 方法來(lái)存儲(chǔ)通過(guò)使用 DAO 類檢索到的數(shù)據(jù)。

實(shí)現(xiàn)

我們將創(chuàng)建一個(gè)作為模型對(duì)象或數(shù)值對(duì)象的 Student 對(duì)象。StudentDao 是數(shù)據(jù)訪問(wèn)對(duì)象接口。StudentDaoImpl 是實(shí)現(xiàn)了數(shù)據(jù)訪問(wèn)對(duì)象接口的實(shí)體類。DaoPatternDemo，我們的演示類使用 StudentDao 來(lái)演示數(shù)據(jù)訪問(wèn)對(duì)象模式的用法。

步驟 1

創(chuàng)建數(shù)值對(duì)象。

Student.java

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;
   }
}

步驟 2

創(chuàng)建數(shù)據(jù)訪問(wèn)對(duì)象接口。

StudentDao.java

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);
}

步驟 3

創(chuàng)建實(shí)現(xiàn)了上述接口的實(shí)體類。

StudentDaoImpl.java

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

public class StudentDaoImpl implements StudentDao {
    
   //列表是當(dāng)作一個(gè)數(shù)據(jù)庫(kù)
   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");
   }

   //從數(shù)據(jù)庫(kù)中檢索學(xué)生名單
   @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");
   }
}

步驟 4

使用 StudentDao 來(lái)演示數(shù)據(jù)訪問(wèn)對(duì)象模式的用法。

CompositeEntityPatternDemo.java

public class DaoPatternDemo {
   public static void main(String[] args) {
      StudentDao studentDao = new StudentDaoImpl();

      //輸出所有的學(xué)生
      for (Student student : studentDao.getAllStudents()) {
         System.out.println("Student: [RollNo : "
            +student.getRollNo()+", Name : "+student.getName()+" ]");
      }


      //更新學(xué)生
      Student student =studentDao.getAllStudents().get(0);
      student.setName("Michael");
      studentDao.updateStudent(student);

      //獲取學(xué)生
      studentDao.getStudent(0);
      System.out.println("Student: [RollNo : "
         +student.getRollNo()+", Name : "+student.getName()+" ]");     
   }
}

步驟 5

驗(yàn)證輸出。

Student: [RollNo : 0, Name : Robert ]
Student: [RollNo : 1, Name : John ]
Student: Roll No 0, updated in the database
Student: [RollNo : 0, Name : Michael ]