Hibernate高级实体关联映射之多值的实体关联(多对多关联的两种思路之二)

Hibernate高级实体关联映射之多值的实体关联(多对多关联的两种思路之二)

hibernate反对多对多关联的使用，因为联结表中的额外列通常是不可能避免的。如果联结表有一些额外的列，而不只是两个外键列时，该怎么办？我们可以用一对多模拟多对多的关系。可以用两种常用的策略把这样一种结构映射到java类。第一种策略需要一个用于联结表的中间实体类，并通过一对多的关联而被映射。第二种策略通过给联结表使用一个值类型的类来利用组件的集合。

我们实现第二个策略：把联结表映射到组件的集合

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pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>hibernateTest</groupId> <artifactId>hibernateTest</artifactId> <version>1.0-SNAPSHOT</version> <packaging>jar</packaging> <name>hibernateTest</name> <url>http://maven.apache.org</url> <dependencies> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>3.8.1</version> <scope>test</scope> </dependency> <dependency> <groupId>org.hibernate</groupId> <artifactId>hibernate-core</artifactId> <version>3.3.1.GA</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-nop</artifactId> <version>1.5.2</version> </dependency> <dependency> <groupId>javassist</groupId> <artifactId>javassist</artifactId> <version>3.4.GA</version> </dependency> <dependency> <groupId>c3p0</groupId> <artifactId>c3p0</artifactId> <version>0.9.1</version> </dependency> <dependency> <groupId>com.oracle</groupId> <artifactId>ojdbc14</artifactId> <version>10.2.0.3.0</version> <scope>runtime</scope> </dependency> </dependencies> <build> <finalName>hibernateTest</finalName> <resources> <resource> <directory>src/main/resources</directory> </resource> <resource> <directory>src/main/java</directory> <excludes> <exclude>**/*.java</exclude> </excludes> </resource> </resources> <plugins> <plugin> <artifactId>maven-compiler-plugin</artifactId> <configuration> <source>1.6</source> <target>1.6</target> <encoding>UTF-8</encoding> </configuration> </plugin> </plugins> </build> </project>  

resources/hibernate.cfg.xml:

<?xml version="1.0" encoding="utf-8"?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory name="sessionFactory"> <!-- 指定连接数据库所用的驱动 --> <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property> <!-- 指定连接数据库的url，hibernate连接的数据库名 --> <property name="hibernate.connection.url">jdbc:oracle:thin:@localhost:1521:XE</property> <property name="hibernate.connection.useUnicode">true</property> <property name="hibernate.connection.characterEncoding">gbk</property> <!-- 指定连接数据库的用户名 --> <property name="hibernate.connection.username">system</property> <!-- 指定连接数据库的密码 --> <property name="hibernate.connection.password">password</property> <!-- 指定连接池里最大连接数 --> <property name="hibernate.c3p0.max_size">20</property> <!-- 指定连接池里最小连接数 --> <property name="hibernate.c3p0.min_size">5</property> <!-- 指定连接池里连接的超时时长,以秒为单位 --> <property name="hibernate.c3p0.timeout">120</property> <!-- 指定连接池里最大缓存多少个Statement对象 --> <property name="hibernate.c3p0.max_statements">100</property> <!-- 每隔XX秒检查连接池里的空闲连接 ，单位是秒 --> <property name="hibernate.c3p0.idle_test_period">120</property> <!-- 当连接池里面的连接用完的时候，C3P0一次获取的新的连接数 --> <property name="hibernate.c3p0.acquire_increment">2</property> <!-- 指定数据库方言 --> <property name="dialect">org.hibernate.dialect.OracleDialect</property> <!-- 显示Hibernate持久化操作所生成的SQL --> <property name="show_sql">true</property> <!-- 将SQL脚本进行格式化后再输出 --> <property name="hibernate.format_sql">true</property> <!-- 罗列所有的映射文件 --> <mapping resource="pojo/Student.hbm.xml" /> <mapping resource="pojo/Teacher.hbm.xml" /> </session-factory> </hibernate-configuration>  

pojo/Teacher.java:

package pojo;import java.io.Serializable;import java.util.ArrayList;import java.util.Collection;public class Teacher implements Serializable{private String teacher_id;private String teacher_name;private int teacher_age;private Collection<MyClass> myClassList = new ArrayList<MyClass>();public String getTeacher_id() {return teacher_id;}@SuppressWarnings("unused")private void setTeacher_id(String teacher_id) {this.teacher_id = teacher_id;}public String getTeacher_name() {return teacher_name;}public void setTeacher_name(String teacher_name) {this.teacher_name = teacher_name;}public int getTeacher_age() {return teacher_age;}public void setTeacher_age(int teacher_age) {this.teacher_age = teacher_age;}public Collection<MyClass> getMyClassList() {return myClassList;}public void setMyClassList(Collection<MyClass> myClassList) {this.myClassList = myClassList;}} 

pojo/Teacher.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="pojo"> <class name="Teacher" table="A_TEACHER" dynamic-insert="true" dynamic-update="true"> <id name="teacher_id" column="TEACHER_ID" type="string"> <generator class="uuid.hex" /> </id> <property name="teacher_name" column="TEACHER_NAME" type="string"/> <property name="teacher_age" column="TEACHER_AGE" type="integer"/> <bag name="myClassList" table="A_CLASS"> <key column="TEACHER_ID"/> <composite-element class="MyClass"> <parent name="teacher"/> <many-to-one name="student" column="STUDENT_ID" not-null="true" class="Student"/> <property name="className" column="CLASSNAME" type="string"/> <property name="schoolName" column="SCHOOLNAME" type="string"/> <property name="createTime" column="CREATETIME" type="calendar"/> </composite-element> </bag> </class> </hibernate-mapping>  

pojo/Student.java:

package pojo;import java.io.Serializable;import java.util.ArrayList;import java.util.Collection;public class Student implements Serializable{private String student_id;private String student_name;private int student_age;private Collection<MyClass> myClassList = new ArrayList<MyClass>();public String getStudent_id() {return student_id;}@SuppressWarnings("unused")private void setStudent_id(String student_id) {this.student_id = student_id;}public String getStudent_name() {return student_name;}public void setStudent_name(String student_name) {this.student_name = student_name;}public int getStudent_age() {return student_age;}public void setStudent_age(int student_age) {this.student_age = student_age;}public Collection<MyClass> getMyClassList() {return myClassList;}public void setMyClassList(Collection<MyClass> myClassList) {this.myClassList = myClassList;}} 

pojo/Student.hbm.xml:

<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="pojo"> <class name="Student" table="A_STUDENT" dynamic-insert="true" dynamic-update="true"> <id name="student_id" column="STUDENT_ID" type="string"> <generator class="uuid.hex" /> </id> <property name="student_name" column="STUDENT_NAME" type="string"/> <property name="student_age" column="STUDENT_AGE" type="integer"/> <bag name="myClassList" table="A_CLASS"> <key column="STUDENT_ID"/> <composite-element class="MyClass"> <parent name="student"/> <many-to-one name="teacher" column="TEACHER_ID" not-null="true" class="Teacher"/> <property name="className" column="CLASSNAME" type="string"/> <property name="schoolName" column="SCHOOLNAME" type="string"/> <property name="createTime" column="CREATETIME" type="calendar"/> </composite-element> </bag> </class> </hibernate-mapping>  

pojo/MyClass.java:

package pojo;import java.io.Serializable;import java.util.Calendar;public class MyClass implements Serializable {//值类型的组件类不需要标识符，也就是不用主键类private String className;private String schoolName;private Calendar createTime;private Student student;private Teacher teacher;public MyClass(){}public MyClass(String className,String schoolName,Student student,Teacher teacher){this.className = className;this.schoolName = schoolName;this.student = student;this.teacher = teacher;this.createTime = Calendar.getInstance();//不需要关联代码了}public String getClassName() {return className;}public void setClassName(String className) {this.className = className;}public String getSchoolName() {return schoolName;}public void setSchoolName(String schoolName) {this.schoolName = schoolName;}public Calendar getCreateTime() {return createTime;}public void setCreateTime(Calendar createTime) {this.createTime = createTime;}public Student getStudent() {return student;}public void setStudent(Student student) {this.student = student;}public Teacher getTeacher() {return teacher;}public void setTeacher(Teacher teacher) {this.teacher = teacher;}//如果是用set集合，这里还要实现equals()和hashCode()方法} 

util/HibernateUtil.java:

package util;import org.hibernate.SessionFactory;import org.hibernate.cfg.Configuration;public class HibernateUtil {private static SessionFactory sessionFactory;static{try {sessionFactory = new Configuration().configure().buildSessionFactory();} catch (Throwable e) {throw new ExceptionInInitializerError(e);}}public static SessionFactory getSessionFactory(){return sessionFactory;}public static void shutdown(){getSessionFactory().close();}} 

util/Manager.java:

package util;import org.hibernate.Session;import org.hibernate.Transaction;import pojo.MyClass;import pojo.Student;import pojo.Teacher;public class Manager {public static void main(String[] args) {Teacher t = new Teacher();t.setTeacher_name("fhd");t.setTeacher_age(35);Student s = new Student();s.setStudent_name("fuhaidong");s.setStudent_age(30);Session session = HibernateUtil.getSessionFactory().openSession();Transaction transaction = session.beginTransaction();session.save(t);session.save(s);MyClass my = new MyClass("class1111111", "schoolName1111111", s, t);//添加t.getMyClassList().add(my);System.out.println(t.getMyClassList().size());session.flush();//删除t.getMyClassList().remove(my);transaction.commit();session.close();}} 

表结构：

A_TEACHER表：TEACHER_ID,TEACHER_NAME,TEACHER_AGEA_STUDENT表：STUDENT_ID,STUDENT_NAME,STUDENT_AGEA_CLASS表：TEACHER_ID，STUDENT_ID,CLASSNAME,SCHOOLNAME,CREATETIME 

输出sql:

1Hibernate: insert into A_TEACHER (TEACHER_NAME, TEACHER_AGE, TEACHER_ID) values (?, ?, ?)Hibernate: insert into A_STUDENT (STUDENT_NAME, STUDENT_AGE, STUDENT_ID) values (?, ?, ?)Hibernate: insert into A_CLASS (TEACHER_ID, STUDENT_ID, CLASSNAME, SCHOOLNAME, CREATETIME) values (?, ?, ?, ?, ?)Hibernate: delete from A_CLASS where TEACHER_ID=? 

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要解除链接，就从集合中移除该元素。不需要额外的级联设置，如：

t.getMyClassList().add(my);

t.getMyClassList().remove(my);

注意：这种方法的缺点在于无法启用双向导航：根据定义，组件不可能有共享的引用。现在t可以导航到my,但s不可能做到，s不并知道my.但是，你可以编写一个查询来获取所需的对象。

这个例子是用bag实现的，使用set也可以实现。但不确定map,list是否可以，没测试！！！！！