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。

实现

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。

步骤 1

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;
   }
}

步骤 2

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);
}

步骤 3

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 {

   //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");
   }
}

步骤 4

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();

      //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()+" ]");     
   }
}

步骤 5

验证输出。

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