DAO = Data Access Object
DAO模式将数据控制层和业务逻辑层分离。
DAO模式的实践，大抵可以分成三个部分：
1）数据实现（DAO）
2）Model
3）Service
这里的数据实现，可以直接通过JDBC的方式，或者Hibernate之类，数据控制与业务逻辑层的分离，道理也在这里。
Model可以理解为普通的POJO。
而Service则是就复杂情况而言的，包含了多个DAO的组合，加入了事务等处理。

看具体代码：
DAO部分：
1-1. com.cleversoft.lab.dao.CollegeDAO.java:
package com.cleversoft.lab.dao;

import java.util.List;
import java.util.Map;

public interface CollegeDAO {
    public List getAllColleges();

    public Map getCollegesMap();
}
1-2. com.cleversoft.lab.dao.CollegeDAOImp.java:
package com.cleversoft.lab.dao;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import com.cleversoft.lab.model.College;

public class CollegeDAOImp implements CollegeDAO {
    private static ArrayList<College> colleges;

    private static HashMap<Integer, College> collegesMap;

    static {
        colleges = new ArrayList<College>();
        colleges.add(new College(1, "常熟理工学院", "说明1"));
        colleges.add(new College(2, "苏州大学", "说明2"));
        colleges.add(new College(3, "北京大学", "说明3"));

        collegesMap = new HashMap<Integer, College>();
        Iterator i = colleges.iterator();
        while (i.hasNext()) {
            College college = (College) i.next();
            collegesMap.put(college.getCollegeID(), college);
        }
    }

    public List getAllColleges() {
        return colleges;
    }

    public Map getCollegesMap() {
        return collegesMap;
    }

}
2-1. com.cleversoft.lab.dao.DepartmentDAO.java:
package com.cleversoft.lab.dao;

import java.util.List;

import com.cleversoft.lab.model.Department;

public interface DepartmentDAO {
    public List getAllDepartments();

    public Department getDepartment(Integer id);

    public void update(Department dep);

    public void insert(Department dep);

    public void delete(Integer id);
}
2-2. com.cleversoft.lab.dao.DepartmentDAOImp.java:
package com.cleversoft.lab.dao;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;

import com.cleversoft.lab.model.College;
import com.cleversoft.lab.model.Department;

public class DepartmentDAOImp implements DepartmentDAO {
    private static ArrayList<Department> departments;

    @SuppressWarnings("unused")
    private static Map collegesMap;

    static {
        departments = new ArrayList<Department>();
        departments.add(new Department(1, "计算机系", "说明1", new College(1,
                "常熟理工学院", "说明1")));
        departments.add(new Department(2, "计算机系", "说明2", new College(2, "苏州大学",
                "说明2")));
        departments.add(new Department(3, "计算机系", "说明3", new College(3, "北京大学",
                "说明3")));

        CollegeDAO collegeDAO = new CollegeDAOImp();
        collegesMap = collegeDAO.getCollegesMap();
    }

    public void delete(Integer id) {
        for (int i = 0; i < departments.size(); i++) {
            Department department = (Department) departments.get(i);
            if (department.getDepartmentID().equals(id)) {
                departments.remove(i);
                break;
            }
        }
    }

    public List getAllDepartments() {
        return departments;
    }

    public Department getDepartment(Integer id) {
        Department dep = null;
        Iterator i = departments.iterator();
        while (i.hasNext()) {
            dep = (Department) i.next();
            if (dep.getDepartmentID().equals(id)) {
                break;
            }
        }
        return dep;
    }

    public void insert(Department dep) {
        int lastId = 0;
        Iterator i = departments.iterator();
        while (i.hasNext()) {
            Department department = (Department) i.next();
            if (department.getDepartmentID() > lastId) {
                lastId = department.getDepartmentID();
            }
        }
        dep.setCollege((College) collegesMap.get(dep.getCollege()
                .getCollegeID()));
        dep.setDepartmentID(lastId + 1);
        departments.add(dep);
    }

    public void update(Department dep) {
        Integer id = dep.getDepartmentID();
        for (int i = 0; i < departments.size(); i++) {
            Department department = (Department) departments.get(i);
            if (department.getDepartmentID().equals(id)) {
                dep.setCollege((College) collegesMap.get(dep.getCollege()
                        .getCollegeID()));
                departments.set(i, dep);
                break;
            }
        }
    }

}

Model部分：
1. com.cleversoft.lab.model.College.java:
package com.cleversoft.lab.model;

import java.io.Serializable;

@SuppressWarnings("serial")
public class College implements Serializable {
    Integer collegeID;

    String collegeName;

    String collegeDesc;

    public College() {
    }

    public College(Integer collegeID, String collegeName, String collegeDesc) {
        this.collegeID = collegeID;
        this.collegeName = collegeName;
        this.collegeDesc = collegeDesc;
    }

    public String getCollegeDesc() {
        return collegeDesc;
    }

    public void setCollegeDesc(String collegeDesc) {
        this.collegeDesc = collegeDesc;
    }

    public Integer getCollegeID() {
        return collegeID;
    }

    public void setCollegeID(Integer collegeID) {
        this.collegeID = collegeID;
    }

    public String getCollegeName() {
        return collegeName;
    }

    public void setCollegeName(String collegeName) {
        this.collegeName = collegeName;
    }
}
2. com.cleversoft.lab.model.Department.java:
package com.cleversoft.lab.model;

import java.io.Serializable;

@SuppressWarnings("serial")
public class Department implements Serializable {
    Integer departmentID;

    String departmentName;

    String departmentDesc;

    College college;

    public Department() {
    }

    public Department(Integer departmentID, String departmentName,
            String departmentDesc, College college) {
        this.departmentID = departmentID;
        this.departmentName = departmentName;
        this.departmentDesc = departmentDesc;
        this.college = college;
    }

    public College getCollege() {
        return college;
    }

    public void setCollege(College college) {
        this.college = college;
    }

    public String getDepartmentDesc() {
        return departmentDesc;
    }

    public void setDepartmentDesc(String departmentDesc) {
        this.departmentDesc = departmentDesc;
    }

    public Integer getDepartmentID() {
        return departmentID;
    }

    public void setDepartmentID(Integer departmentID) {
        this.departmentID = departmentID;
    }

    public String getDepartmentName() {
        return departmentName;
    }

    public void setDepartmentName(String departmentName) {
        this.departmentName = departmentName;
    }

}

Service:
1-1: com.cleversoft.lab.service.CollegeService.java:
package com.cleversoft.lab.service;

import java.util.List;
import java.util.Map;

public interface CollegeService {
    public List getAllColleges();
    
    public Map getCollegesMap();
}
1-2: com.cleversoft.lab.service.CollegeDAOService.java:
package com.cleversoft.lab.service;

import java.util.List;
import java.util.Map;

import com.cleversoft.lab.dao.CollegeDAO;
import com.cleversoft.lab.dao.CollegeDAOImp;

public class CollegeDAOService implements CollegeService {
    CollegeDAO dao;

    public CollegeDAOService() {
        this.dao = new CollegeDAOImp();
    }

    public List getAllColleges() {
        return dao.getAllColleges();
    }

    public Map getCollegesMap() {
        return dao.getCollegesMap();
    }
}
2-1: com.cleversoft.lab.service.DepartmentService.java:
package com.cleversoft.lab.service;

import java.util.List;

import com.cleversoft.lab.model.Department;

public interface DepartmentService {
    public List getAllDepartments();

    public Department getDepartment(Integer id);

    public void update(Department dep);

    public void insert(Department dep);

    public void delete(Integer id);
}
2-2: com.cleversoft.lab.service.DepartmentDAOService.java:
package com.cleversoft.lab.service;

import java.util.List;

import com.cleversoft.lab.dao.DepartmentDAO;
import com.cleversoft.lab.dao.DepartmentDAOImp;
import com.cleversoft.lab.model.Department;

public class DepartmentDAOService implements DepartmentService {
    DepartmentDAO dao;

    public DepartmentDAOService() {
        this.dao = new DepartmentDAOImp();
    }

    public void delete(Integer id) {
        dao.delete(id);
    }

    public List getAllDepartments() {
        return dao.getAllDepartments();
    }

    public Department getDepartment(Integer id) {
        return dao.getDepartment(id);
    }

    public void insert(Department dep) {
        dao.insert(dep);
    }

    public void update(Department dep) {
        dao.update(dep);
    }

}

通过实现来看，DAO模式，思路清晰，但是实现起来，还是比较麻烦的。
而ORM又帮助我们简化了什么，下回再续！

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