Hibernate学生选课，实现课程按照学生人数进行排序的方法
声明：编程新手，如有错漏，敬请指出。谢谢。
研究了一个星期hibernate，写了一些方法，一个学生可以选多门课，一门课有多个学生，我想把课程按照学生人数进行排序。
在Course类里，有一个attribute是student，是一个collection，通过查询Course，可以得到student这个 collection的size，也就是这个Course的人数。问题来了，hibernate里，可以在mapping文件里设置排序原则，即，在 hbm.xml文件里设置ordery by，但是无法order by一个collection。更不要说order by 一个collection的size了。
所以，我想了一个办法——自定义Comparator。
好了，开始吧。
POJO：Student
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import java.util.HashSet;
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import java.util.Set;
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public class Student
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{
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private int sid;
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private String sname;
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private String sage;
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private Set<Course> course=new HashSet<Course>();
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public Student()
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{
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}
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// getter/setter…
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}
POJO:Course
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import java.util.HashSet;
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import java.util.Set;
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public class Course
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{
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private int cid;
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private String cname;
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private Set<Student> student=new HashSet<Student>();
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public Course() {}
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//getter/setter…
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}
Student.hbm.xml
<?xml version=”1.0″ encoding=”UTF-8″?>
<!DOCTYPE hibernate-mapping
PUBLIC “-//Hibernate/Hibernate Mapping DTD 3.0//EN”
“http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd”>
<hibernate-mapping>
<class name=”com.test.Student” table=”student” >
<id name=”sid” >
<column name=”sid” sql-type=”int(10)” not-null=”true”/>
<generator/>
</id>
<property name=”sname”>
<column name=”sname”/>
</property>
<property name=”sage”>
<column name=”sage”/>
</property>
<set access=”property” name=”course”
table=”student_course_link” batch-size=”10″ fetch=”select”
lazy=”true” cascade=”save-update” inverse=”true”
>
<key column=”sid”/>
<many-to-many column=”cid”/>
</set>
</class>
</hibernate-mapping>
Course.hbm.xml
<?xml version=”1.0″ encoding=”UTF-8″?>
<!DOCTYPE hibernate-mapping
PUBLIC “-//Hibernate/Hibernate Mapping DTD 3.0//EN”
“http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd”>
<hibernate-mapping>
<class name=”com.test.Course” table=”course” >
<id name=”cid” >
<column name=”cid” sql-type=”int(10)” not-null=”true” />
<generator/>
</id>
<property name=”cname”>
<column name=”cname”/>
</property>
<set access=”property” name=”student” batch-size=”10″ fetch=”select”
table=”student_course_link” lazy=”true” cascade=”save-update”
inverse=”false”>
<key column=”cid”/>
<many-to-many column=”sid”/>
</set>
</class>
</hibernate-mapping>
数据库三个表，student，course，student_course_link(中间表)：
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CREATE TABLE `java2`.`student` (
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`sid` int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
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`sname` varchar(16) DEFAULT NULL,
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`sage` varchar(16) DEFAULT NULL,
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PRIMARY KEY (`sid`)
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) ENGINE=InnoDB DEFAULT CHARSET=latin1;
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DROP TABLE IF EXISTS `java2`.`course`;
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CREATE TABLE `java2`.`course` (
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`cid` int(10) UNSIGNED NOT NULL AUTO_INCREMENT,
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`cname` varchar(16) DEFAULT NULL,
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PRIMARY KEY (`cid`)
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) ENGINE=InnoDB DEFAULT CHARSET=latin1;
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DROP TABLE IF EXISTS `java2`.`student_course_link`;
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CREATE TABLE `java2`.`student_course_link` (
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`sid` int(10) NOT NULL,
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`cid` int(10) NOT NULL,
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PRIMARY KEY (`sid`,`cid`)
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) ENGINE=InnoDB DEFAULT CHARSET=latin1;
我们需要一个自定义Comparator：
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import java.util.Comparator;
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public final class HotCourseComparator implements Comparator {
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public int compare(Object arg0, Object arg1) {
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Course c1=(Course)arg0;
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Course c2=(Course)arg1;
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return c2.getStudent().size()-c1.getStudent().size(); // 按照选这门课的人数排序。
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}
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}
测试类：
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{
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public void rankCourse() {
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Session session=HibernateUtil.getSessionFactory().getCurrentSession();
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session.beginTransaction();
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Query query=session.createQuery("from Course");
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List l=query.list();
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Collections.sort(l,new HotCourseComparator()); //这里声明使用我们自定义的Comparator
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for(int j=0;j<l.size();j++) {
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Course c=(Course)l.get(j);
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int snum=c.getStudent().size();
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System.out.println(c.getCname()+" has "+snum+" students");
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}
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}
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public static void main(String[] args)
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{
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TestManyToMany t = new TestManyToMany();
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t.rankCourse();
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}
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}
运行结果：
C# has 4 students
Ruby on Rails has 4 students
PHP has 3 students
JAVA has 2 students
C has 2 students
PL/SQL has 2 students
MATLAB has 2 students
C++ has 1 students
Heskell has 1 students
Python has 1 students
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