Learning Hibernate step by step - 04 多对一关联关系照射

Learning Hibernate step by step -- 04 多对一关联关系映射
Java对象有很多关联关系映射，如多对一、一对一、多对多等，在数据库中表之间有主外键关联，Java对象的关联关系如何与数据表形成映射呢？首先来看多对一的映射关系。
一、背景
1. 一所学校有很多班级（Group），一个班级有若干学生（Student），学生和班级之间的映射就是多对一的关联关系，如图所示：
[img][/img]
2. 数据库中分别存在t_group表和t_student表，分别对应班级和学生，t_student表有一个外键，引用t_group的id字段，ER关系如图所示：
[img][/img]
二、建立域对象
1. 先在项目中建立两个独立的域对象，Group和Student，两个对象包含基本的信息，先不对两个对象做关联。两外再分别建立两个对象的Dao对象，负责与数据库交互。
Persistent.java

public class Persistent implements Serializable {private static final long serialVersionUID = 1L;private long id;public long getId() {return id;}public void setId(long id) {this.id = id;}}

Group.java

public class Group extends Persistent {private static final long serialVersionUID = 1L;String grpName;public String getGrpName() {return grpName;}public void setGrpName(String grpName) {this.grpName = grpName;}}

Student.java

public class Student extends Persistent {private static final long serialVersionUID = 1L;private String studNo;private String studName;private String studSex;public String getStudNo() {return studNo;}public void setStudNo(String studNo) {this.studNo = studNo;}public String getStudName() {return studName;}public void setStudName(String studName) {this.studName = studName;}public String getStudSex() {return studSex;}public void setStudSex(String studSex) {this.studSex = studSex;}}

GroupDao.java

public class GroupDao {public long saveGroup(Object obj) {Session session = HibernateUtil.getSessionFactory().openSession();Transaction tx = null;try {tx = session.beginTransaction();Long retId = (Long)session.save(obj);tx.commit();return retId.longValue();} catch (Exception e) {if (tx != null) {tx.rollback();}// DO NOT swallow the Exception in actual projects.// throw e;e.printStackTrace();return 0L;} finally {session.close();}}}

StudentDao.java

public class StudentDao {public long saveStudent(Object obj) {Session session = HibernateUtil.getSessionFactory().openSession();Transaction tx = null;try {tx = session.beginTransaction();Long retId = (Long)session.save(obj);tx.commit();return retId.intValue();} catch (Exception e) {if (tx != null) {tx.rollback();}// DO NOT swallow the Exception in actual projects.// throw e;e.printStackTrace();return 0L;} finally {session.close();}}}

三、建立映射文件
1. 分别建立Group和Student的映射文件
Group.hbm.xml

<?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 package="com.foobar.domain"><class name="Group" table="`T_Group`"><comment>Groups infomation.</comment><id name="id"><generator   not-null="true"   column="`grp_name`"/></class></hibernate-mapping>

Student.hbm.xml

<?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 package="com.foobar.domain"><class name="Student" table="`T_Student`"><comment>Students infomation.</comment><id name="id"><generator   length="15"   column="`stud_name`"/><property name="studSex"   not-null="true"   length="1"  column="`stud_sex`"/><many-to-one name="group" column="`group_id`"/></class></hibernate-mapping>

2. 在Hibernate配置文件中引入映射文件

...<mapping resource="com/foobar/conf/hbm/Group.hbm.xml"/><mapping resource="com/foobar/conf/hbm/Student.hbm.xml"/>...

另外，我们加入如下属性，让程序在加载配置文件是自动创建表

<!-- mapping与ddl之间的关系--><property name="hibernate.hbm2ddl.auto">create</property>

四、建立测试程序，分别建立Group对象和Student对象，分别保存到数据库中

public class SchoolMgt {public static void main(String[] args) {GroupDao grpDao = new GroupDao();Group grp = new Group();StudentDao studDao = new StudentDao();Student stud = new Student();grp.setGrpName("Group A");if(grpDao.saveGroup(grp) != 0) {System.out.println("Group added successful!");}stud.setStudName("Zhangsan");stud.setStudNo("0001001");stud.setStudSex("F");if(studDao.saveStudent(stud) != 0) {System.out.println("Student added successful!");}}}

观察控制台输出：
Group added successful!
Student added successful!
五、建立关联关系
1. 先建立对象的关联关系，在Student对象中增加Group对象的引用，代码如下（省略了setter和getter）：

private Group group;// setter and getter 

2. 在Student.hbm.xml中增加many-to-one的属性映射：

<many-to-one name="group" column="`group_id`"/>

3. 修改测试代码，将Group对象加入Student中，运行程序，控制台输出成功信息

...stud.setGroup(grp);...

4. 在数据库中，我们发现t_student表多了一个字段group_id，这是外键，引用表t_group的id字段，由此，Hibernate自动实现了对象多对一关系到数据库表关联关系的映射。
六、总结
1. many-to-one属性的相关参数

<many-to-one        name="propertyName"                                          (1)        column="column_name"                                         (2)                                                   (3)        cascade="cascade_style"                                      (4)        fetch="join|select"                                          (5)        update="true|false"                                          (6)        insert="true|false"                                          (6)        property-ref="propertyNameFromAssociatedClass"               (7)        access="field|property|ClassName"                            (8)        unique="true|false"                                          (9)        not-null="true|false"                                        (10)        optimistic-lock="true|false"                                 (11)        lazy="proxy|no-proxy|false"                                  (12)        not-found="ignore|exception"                                 (13)        entity-name="EntityName"                                     (14)        formula="arbitrary SQL expression"                           (15)        node="element-name|@attribute-name|element/@attribute|."        embed-xml="true|false"        index="index_name"        unique_key="unique_key_id"        foreign-key="foreign_key_name"/>

解释如下：
(1). name: the name of the property.
(2). column (optional): the name of the foreign key column. This can also be specified by nested <column> element(s).
(3). class (optional - defaults to the property type determined by reflection): the name of the associated class.
(4). cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.
(5). fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.
(6). update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" association whose value is initialized from another property that maps to the same column(s), or by a trigger or other application.
(7). property-ref (optional): the name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.
(8). access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
(9). unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. By allowing this to be the target of a property-ref, you can make the association multiplicity one-to-one.
(10). not-null (optional): enables the DDL generation of a nullability constraint for the foreign key columns.
(11). optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
(12). lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation. lazy="false" specifies that the association will always be eagerly fetched.
(13). not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows will be handled. ignore will treat a missing row as a null association.
(14). entity-name (optional): the entity name of the associated class.
(15). formula (optional): an SQL expression that defines the value for a computed foreign key.
参考文档：http://docs.jboss.org/hibernate/core/3.3/reference/en/html/mapping.html#mapping-declaration-manytoone

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