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Java程序员从笨鸟到菜鸟之（五十六）细谈Hibernate（七）Hibernate自身一对多和多对多关系映射

2013-05-08 10:28 295 查看

一对多关系映射大家都明白，关系双方都一个含有对方多个引用，但自身一对多很多同学都不明白什么意思，那么首先我就说明一下什么是自身一对多，其实也很好理解，自身一对多就是自身含有本身的多个引用，例如新闻类别，新闻包含体育新闻和政治新闻，体育新闻内有含有足球新闻和篮球新闻，其实他们都属于新闻，只是名字不同而已，下面我们就以新闻类别为例来具体说明一下：

首先我们来看一下新闻类别的类图：

类图：category

从上面的图我们可以看出：每一个新闻类别都有一个父类别和一个孩子类别的set集合，这个父类别和孩子类别里面都是自身的引用，这样就行了自身一对多的对象关系

下面看一下具体的新闻实体类：Category.java

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public class Category { private Long id; private String name; private Category parentCategory; private Set<Category> childCategories; public Category(String name, Category parentCategory, Set<Category> childCategories) { this.name = name; this.parentCategory = parentCategory; this.childCategories = childCategories; } public Category() { } *******set、get方法省略 }

public class Category

{

private Long id;

private String name;

private Category parentCategory;

private Set<Category> childCategories;

public Category(String name, Category parentCategory,

Set<Category> childCategories)

{

this.name = name;

this.parentCategory = parentCategory;

this.childCategories = childCategories;

}

public Category()

{

}

*******set、get方法省略

}

看完具体的实体类之后我们下面看一下其具体的配置，其实他的配置中没什么特别的地方，仅仅只是他的配置中包含了一对多和多对一的共同标签存在而已：他即含有多的一方的<set>标签。也含有一的一方的<many-to-one>标签：

Category.hbm.xml配置文件

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<?xml version="1.0"?> <!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.shengsiyuan.hibernate.Category" table="categories"> <id name="id" column="id" type="long"> <generator class="increment"></generator> </id> <property name="name" type="string" > <column name="name" length="50" ></column> </property> <set name="childCategories" cascade="all" inverse="true"> <key column="category_id"></key> <one-to-many class="com.shengsiyuan.hibernate.Category"/> </set> <many-to-one name="parentCategory" column="category_id" class="com.shengsiyuan.hibernate.Category"> </many-to-one> </class> </hibernate-mapping>

<?xml version="1.0"?>

<!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.shengsiyuan.hibernate.Category" table="categories">

<id name="id" column="id" type="long">

<generator class="increment"></generator>

</id>

<property name="name" type="string" >

<column name="name" length="50" ></column>

</property>

<set name="childCategories" cascade="all" inverse="true">

<key column="category_id"></key>

<one-to-many class="com.shengsiyuan.hibernate.Category"/>

</set>

<many-to-one name="parentCategory" column="category_id" class="com.shengsiyuan.hibernate.Category">

</many-to-one>

</class>

</hibernate-mapping>

下面我们来看一下在自身一对多的关系下进行增删改查的示例：

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import java.util.HashSet; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; public class HibernateTest2 { private static SessionFactory sessionFactory; static { try { sessionFactory = new Configuration().configure() .buildSessionFactory(); } catch (Exception ex) { ex.printStackTrace(); } } public static void main(String[] args) { Session session = sessionFactory.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); Category category1 = new Category("level1", null, new HashSet<Category>()); Category category2 = new Category("level2", null, new HashSet<Category>()); Category category3 = new Category("level2", null, new HashSet<Category>()); Category category4 = new Category("level3", null, new HashSet<Category>()); Category category5 = new Category("level3", null, new HashSet<Category>()); Category category6 = new Category("level3", null, new HashSet<Category>()); Category category7 = new Category("level3", null, new HashSet<Category>()); category2.setParentCategory(category1); category3.setParentCategory(category1); category1.getChildCategories().add(category2); category1.getChildCategories().add(category3); category4.setParentCategory(category2); category5.setParentCategory(category2); category2.getChildCategories().add(category4); category2.getChildCategories().add(category5); category6.setParentCategory(category3); category7.setParentCategory(category3); category3.getChildCategories().add(category6); category3.getChildCategories().add(category7); Category category = (Category)session.get(Category.class, new Long(1)); System.out.println(category.getChildCategories().iterator().next().getName()); session.delete(category); tx.commit(); } catch(Exception ex) { if(null != tx) { tx.rollback(); } } finally { session.close(); } } }

import java.util.HashSet;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

public class HibernateTest2

{

private static SessionFactory sessionFactory;

static

{

try

{

sessionFactory = new Configuration().configure()

.buildSessionFactory();

}

catch (Exception ex)

{

ex.printStackTrace();

}

}

public static void main(String[] args)

{

Session session = sessionFactory.openSession();

Transaction tx = null;

try

{

tx = session.beginTransaction();

Category category1 = new Category("level1", null, new HashSet<Category>());

Category category2 = new Category("level2", null, new HashSet<Category>());

Category category3 = new Category("level2", null, new HashSet<Category>());

Category category4 = new Category("level3", null, new HashSet<Category>());

Category category5 = new Category("level3", null, new HashSet<Category>());

Category category6 = new Category("level3", null, new HashSet<Category>());

Category category7 = new Category("level3", null, new HashSet<Category>());

category2.setParentCategory(category1);

category3.setParentCategory(category1);

category1.getChildCategories().add(category2);

category1.getChildCategories().add(category3);

category4.setParentCategory(category2);

category5.setParentCategory(category2);

category2.getChildCategories().add(category4);

category2.getChildCategories().add(category5);

category6.setParentCategory(category3);

category7.setParentCategory(category3);

category3.getChildCategories().add(category6);

category3.getChildCategories().add(category7);

Category category = (Category)session.get(Category.class, new Long(1));

System.out.println(category.getChildCategories().iterator().next().getName());

session.delete(category);

tx.commit();

}

catch(Exception ex)

{

if(null != tx)

{

tx.rollback();

}

}

finally

{

session.close();

}

}

}

在很多实际开发过程中，多对多的映射关系也是比较常见的，最为明显的例子就是我们常用的学生选课示例，一个学生可以选多门课，一门课也可以由多个学生去选，这样就形成了多对多的映射关系，现在我们就以学生选课的实例来看一看多对多关系映射。由于在多对多映射中，双向多对多用的的比较多，并且单向多对多也比较简单，所以我们就以双向多对多进行讲解

我们先把必要的实体类和实体映射文件写好：

先简单看一下实体类：

student.java

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/** 学生实体类 */ public class Student { private Long id; //对象标识符(OID) private String name; //姓名 private String grade; //所在班级 private Set<Course> courses; //所有所选课程的集合 public Student(){} //无参数的构造方法 ******set、get方法省略 }

/** 学生实体类 */

public class Student {

private Long id;                //对象标识符(OID)

private String name;            //姓名

private String grade;           //所在班级

private Set<Course> courses;    //所有所选课程的集合

public Student(){}              //无参数的构造方法

******set、get方法省略

}

Course.java

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/** 课程实体类 */ public class Course { private Long id; //对象标识符(OID) private String name; //课程名 private Double creditHours; //课时数 private Set<Student> students; //选择了这门课程的学生的集合 public Course(){} //无参数的构造方法 ******set、get方法省略 }

/** 课程实体类 */

public class Course {

private Long id;                //对象标识符(OID)

private String name;            //课程名

private Double creditHours;     //课时数

private Set<Student> students;  //选择了这门课程的学生的集合

public Course(){}             //无参数的构造方法

******set、get方法省略

}

下一步编写实体映射文件：

Student.hbm.xml

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<?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.zxf.domain.Student" table="student">  <id name="id" column="id" type="long"> <generator class="native" /> </id>  <property name="name" /> <property name="grade" />  <set name="courses" table="student_course">  <key column="student_id" />  <many-to-many column="course_id" class="com.zxf.domain.Course" /> </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.zxf.domain.Course" table="course">  <id name="id" column="id" type="long"> <generator class="native" /> </id>  <property name="name" /> <property name="creditHours" column="credit_hours" />  <set name="students" table="student_course" inverse="true">  <key column="course_id" />  <many-to-many column="student_id" class="com.zxf.domain.Student" /> </set> </class> </hibernate-mapping>

<?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>

<!-- 映射持久化类 -->

<class name="com.zxf.domain.Student" table="student">

<!-- 映射对象标识符 -->

<id name="id" column="id" type="long">

<generator class="native" />

</id>

<!-- 映射普通属性 -->

<property name="name" />

<property name="grade" />

<!-- 映射集合属性,指定连接表 -->

<set name="courses" table="student_course">

<!-- 用key元素指定本持久类在连接表中的外键字段名 -->

<key column="student_id" />

<!-- 映射多对多关联类 -->

<many-to-many column="course_id"

class="com.zxf.domain.Course" />

</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>

<!-- 映射持久化类 -->

<class name="com.zxf.domain.Course" table="course">

<!-- 映射对象标识符 -->

<id name="id" column="id" type="long">

<generator class="native" />

</id>

<!-- 映射普通属性 -->

<property name="name" />

<property name="creditHours" column="credit_hours" />

<!-- 映射集合属性,指定连接表 -->

<set name="students" table="student_course" inverse="true">

<!-- 用key元素指定本持久类在连接表中的外键字段名 -->

<key column="course_id" />

<!-- 映射多对多关联类 -->

<many-to-many column="student_id" class="com.zxf.domain.Student" />

</set>

</class>

</hibernate-mapping>

下面具体看一下增删改查的具体测试的关键代码：

增加数据测试：

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<SPAN xmlns="http://www.w3.org/1999/xhtml"><SPAN xmlns="http://www.w3.org/1999/xhtml">tran = session.beginTransaction(); Student stu1 = new Student("xiaoli", "two"); Student stu2 = new Student("xiaoming", "two"); Student stu3 = new Student("xiaoqiang", "two"); Course course1 = new Course("java", 3.0); Course course2 = new Course("c#", 5.0); //stuset.add(stu1); //stuset.add(stu2); //course1.setStudents(stuset); //session.save(course1); couset.add(course1); couset.add(course2); stu1.setCourses(couset); session.save(stu1); tran.commit();</SPAN></SPAN>

tran = session.beginTransaction();

Student stu1 = new Student("xiaoli", "two");

Student stu2 = new Student("xiaoming", "two");

Student stu3 = new Student("xiaoqiang", "two");

Course course1 = new Course("java", 3.0);

Course course2 = new Course("c#", 5.0);

//stuset.add(stu1);

//stuset.add(stu2);

//course1.setStudents(stuset);

//session.save(course1);

couset.add(course1);

couset.add(course2);

stu1.setCourses(couset);

session.save(stu1);

tran.commit();

测试结论：如果想保存数据成功，不管是主控方还是被控方，如果想通过一次保存即可把双方数据保存，需要把实体配置中的cascade属性设置为all或者save-update，由于设置为all包含delete，在删除数据中，删除一条信息会导致相对应表的多条或者全部信息被删掉，所以一般配置save-update。

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