Hibernate关联映射（二）一对多
一对多关联（单向）
其实一对多就是多对一的相反，所以这里我们还是使用上一篇中的实体类。
不过略有变化，是以一的一方为主控方。
Group.java

package org.jzkangta.hibernate.many2one1.entity;// Generated 2010-8-27 14:37:02 by Hibernate Tools 3.2.1.GAimport java.util.HashSet;import java.util.Set;/** * Group generated by hbm2java */public class Group  implements java.io.Serializable {     private int int_;     private String name;     private Set<User> users = new HashSet<User>();    public Set<User> getUsers() {        return users;    }    public void setUsers(Set<User> users) {        this.users = users;    }    public int getInt_() {        return int_;    }    public void setInt_(int int_) {        this.int_ = int_;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

User.java

package org.jzkangta.hibernate.many2one1.entity;// Generated 2010-8-27 14:33:39 by Hibernate Tools 3.2.1.GAimport java.util.Date;/** * User generated by hbm2java */public class User  implements java.io.Serializable {     private Integer id;     private String name;    public Integer getId() {        return this.id;    }        public void setId(Integer id) {        this.id = id;    }    public String getName() {        return this.name;    }        public void setName(String name) {        this.name = name;    }  }

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"><!-- Generated 2010-8-27 14:37:02 by Hibernate Tools 3.2.1.GA --><hibernate-mapping>    <class name="org.jzkangta.hibernate.many2one1.entity.Group" table="group" catalog="ibatisdb">        <id name="int_" type="int">            <column name="int" />            <generator />        </id>        <property name="name" type="string">            <column name="name" length="20" />        </property>        <Set name="users">            <key column="groupid_"/>            <one-to-many name="code">package org.jzkangta.hibernate.many2one1.entity;// Generated 2010-8-27 14:33:39 by Hibernate Tools 3.2.1.GAimport java.util.Date;/** * User generated by hbm2java */public class User  implements java.io.Serializable {     private Integer id;     private String name;     private Group group;    public Group getGroup() {        return group;    }    public void setGroup(Group group) {        this.group = group;    }    public Integer getId() {        return this.id;    }        public void setId(Integer id) {        this.id = id;    }    public String getName() {        return this.name;    }        public void setName(String name) {        this.name = name;    }  }

User.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"><!-- Generated 2010-8-27 14:33:42 by Hibernate Tools 3.2.1.GA --><hibernate-mapping>    <class name="org.jzkangta.hibernate.User" table="user" catalog="ibatisdb">        <id name="id" type="java.lang.Integer">            <column name="id_" />            <generator />        </id>        <property name="name" type="string">            <column name="name_" length="50" />        </property>        <many-to-one name="group" column="groupid_"/>    </class></hibernate-mapping>

<many-to-one name="group" column="groupid_" inverse="true"/>
这里的groupid必须和group.hbm.xml中的 <key column="groupid_"/>
一样。
这样，就完成双向关联。


注意：这种类型的关联，必须由多的一方来维护关系（User），不然会影响性能。（发出过多的update语句）
在group.hbm.xml中：
<Set name="users" inverse="true">
<key column="groupid_"/>
<one-to-many class="org.jzkangta.hibernate.entity.User"/>
</Set>
加上红色的部分，就可以翻转维护端。交给多的一端。