是否有适当的方法在DAO中实例化强类型对象？

问题描述

| 我正在尝试为包含几个原始数据类型和三个具有自己的DAO的强类型对象的类确定DAO编码的正确方法。 这是我要为其实施DAO的类，减去获取器/设置器：

public class Course
{
    private int id;
    private int number;
    private String title;
    private Subject subject;
    private School school;
    private Instructor instructor;
    ...
}

学科，学校和讲师类都有自己的DAO。因此，我最初的想法是使用这些DAO实例化Course对象中的Subject，School和Instructor对象（相关代码在try块中）：

public class CourseDAO
{
    public static Course selectCourse(int id)
    {
        ConnectionPool cp = ConnectionPool.getInstance();
        Connection c = cp.getConnection();
        PreparedStatement ps = null;
        ResultSet rs = null;

        Course course = null;

        String query = \"select id,number,title,subjectId,schoolId,instructorId \" +
                \"from course \" +
                \"where id = ? \" +
                \"limit 1\";

        try
        {
            ps = c.prepareStatement(query);
            ps.setInt(1,id);
            rs = ps.executeQuery();
            if (rs.next())
            {
                course = new Course();
                course.setId(rs.getInt(1));
                course.setNumber(rs.getInt(2));
                course.setTitle(rs.getString(3));
                course.setSubject(SubjectDAO.selectSubject(rs.getInt(4)));
                course.setSchool(SchoolDAO.selectSchool(rs.getInt(5)));
                course.setInstructor(InstructorDAO.selectInstructor(rs.getInt(6)));
            }
        }
        catch (sqlException e)
        {
            e.printstacktrace();
        }
        finally
        {
            DBUtil.closeResultSet(rs);
            DBUtil.closePreparedStatement(ps);
            cp.releaseConnection(c);
        }

        return course;
    }
}

其他DAO看起来与此非常相似。每个人都从ConnectionPool获取一个Connection，使用PreparedStatement执行简单查询，关闭ResultSet和PreparedStatement，然后将Connection释放回池中。 在以这种方式实现CourseDAO之后，我开始怀疑在查询中使用联接而不是调用SubjectDAO，SchoolDAO和InstructorDAO是否会更好/更高效，因为每个人都使用ConnectionPool中的Connection。查询和try块将更改如下：

String query = \"select course.id,course.number,course.title,\" +
        \"subject.id,subject.name,\" +
        \"school.id,school.name,school.abbreviation,school.type,school.city,school.region,school.country,\" +
        \"instructor.id,instructor.firstName,instructor.lastName \" +
        \"from course \" +
        \"inner join subject \" +
        \"on course.subjectId = subject.id \" +
        \"inner join school \" +
        \"on course.schoolId = school.id \" +
        \"inner join instructor \" +
        \"on course.instructorId = instructor.id \" +
        \"where course.id = ? \" +
        \"limit 1\";

try
{
    ps = c.prepareStatement(query);
    ps.setInt(1,id);
    rs = ps.executeQuery();
    if (rs.next())
    {
        course = new Course();
        course.setId(rs.getInt(1));
        course.setNumber(rs.getInt(2));
        course.setTitle(rs.getString(3));

        Subject subject = new Subject();
        subject.setId(rs.getInt(4));
        subject.setName(rs.getString(5));

        School school = new School();
        school.setId(rs.getInt(6));
        school.setName(rs.getString(7));
        school.setAbbreviation(rs.getString(8));
        school.setType(rs.getString(9));
        school.setCity(rs.getString(10));
        school.setRegion(rs.getString(11));
        school.setCountry(rs.getString(12));

        Instructor instructor = new Instructor();
        instructor.setId(rs.getInt(13));
        instructor.setFirstName(rs.getString(14));
        instructor.setLastName(rs.getString(15));

        course.setSubject(subject);
        course.setSchool(school);
        course.setInstructor(instructor);
    }
}

在这种情况下，哪个是更好的选择？使用一个数据库连接，联接三个表并返回15个字段？还是使用ConnectionPool中的四个连接（每个使用的DAO一个），编写更少的代码，并执行简单的查询？     

解决方法

        您必须将方法与联接一起使用。对每个属性进行查询的效率非常低，并且如果您有大量数据，由于n + 1选择问题的类型（在您的情况下为3n + 1），它将导致性能不佳。