[英]Is there a proper way to instantiate strongly typed objects in DAOs?
我试图确定哪种方法是为 class 编写 DAO 的正确方法,该 class 包含一些原始数据类型和三个具有自己的 DAO 的强类型对象。
这是我正在为其实现 DAO 的 class,减去 getter/setter:
public class Course
{
private int id;
private int number;
private String title;
private Subject subject;
private School school;
private Instructor instructor;
...
}
Subject、School 和 Instructor 类都有自己的 DAO。 所以我最初的想法是使用这些 DAO 来实例化 Course object 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 看起来与这个非常相似。 每个都从 ConnectionPool 中获取一个 Connection,使用 PreparedStatement 执行一个简单的查询,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连接而不是调用 SubjectDAO、SchoolDAO 和 InstructorDAO 是否会更好/更有效,因为每个都使用来自 ConnectionPool 的连接。 查询和尝试块将更改如下:
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,连接三个表,返回15个字段? 或者使用 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?
您绝对应该使用带有连接的方法。 对每个属性进行查询效率非常低,如果您有大量数据,由于 n+1 select 类型的问题(在您的情况下为 3n+1),它会导致性能不佳。
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