标题：Java数据访问对象模式

数据访问对象模式或DAO模式用于将低级数据访问API或操作与高级业务服务分离。 以下是数据访问对象模式的参与者。

• 数据访问对象接口 - 此接口定义要对模型对象执行的标准操作。
• 数据访问对象具体类 - 此类实现上述接口。 这个类负责从数据源获取数据，数据源可以是数据库/xml或任何其他存储机制。
• 模型对象或值对象 - 此对象是简单的POJO，包含用于存储使用DAO类检索的数据的get/set方法。

实现实例

在这个将创建一个作为Model或Value对象的Student对象。 StudentDao是数据访问对象接口。 StudentDaoImpl是实现数据访问对象接口的具体类。 DaoPatternDemo这是一个演示类，将使用StudentDao演示使用数据访问对象模式。

数据访问对象模式示例的结构如下图所示 -

第1步

创建值对象，其代码如下 -

Student.java

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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;
   }
}
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;
   }
}
Java

第2步

创建数据访问对象接口，其代码如下 -

StudentDao.java

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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);
}
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);
}
Java

第3步

创建实现上面的接口的具体类，其代码如下 -

StudentDaoImpl.java

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import java.util.ArrayList;
import java.util.List;
 
public class StudentDaoImpl implements StudentDao {
 
   //list is working as a database
   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");
   }
 
   //retrive list of students from the 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");
   }
}
Java

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

public class StudentDaoImpl implements StudentDao {

   //list is working as a database
   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");
   }

   //retrive list of students from the 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");
   }
}
Java

第4步

使用StudentDao演示数据访问对象模式的使用。
DaoPatternDemo.java

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

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

      //print all students
      for (Student student : studentDao.getAllStudents()) {
         System.out.println("Student: [RollNo : " + student.getRollNo() + ", Name : " + student.getName() + " ]");
      }


      //update student
      Student student =studentDao.getAllStudents().get(0);
      student.setName("Michael");
      studentDao.updateStudent(student);

      //get the student
      studentDao.getStudent(0);
      System.out.println("Student: [RollNo : " + student.getRollNo() + ", Name : " + student.getName() + " ]");        
   }
}
Java

第5步

验证输出，执行上面的代码得到以下结果 -

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

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