Hibernate: insert into t_group (name) values (?)

Hibernate: insert into t_user (group_id, name) values (?, ?)

?

7、实验2（仅保存Group对象的情况下，自动保存User）

（1）修改Group.java

package com.zgy.hibernate.model;

?

import java.util.HashSet;

import java.util.Set;

?

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.OneToMany;

import javax.persistence.Table;

import javax.persistence.CascadeType;

@Entity

@Table(name="t_group")

public class Group {

private int id;

private String name;

private Set users = new HashSet();

@Id

@GeneratedValue

public int getId() {

return id;

}

public void setId(int id) {

this.id = id;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

@OneToMany(mappedBy="group",

cascade={CascadeType.ALL}

)

public Set getUsers() {

return users;

}

public void setUsers(Set users) {

this.users = users;

}

?

?

}

（2）去除User.java中加入的cascade={CascadeType.ALL}这段Annotation

package com.zgy.hibernate.model;

?

import javax.persistence.CascadeType;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.ManyToOne;

import javax.persistence.Table;

?

@Entity

@Table(name="t_user")

public class User {

private int id;

private String name;

private Group group;

@Id

@GeneratedValue

public int getId() {

return id;

}

public void setId(int id) {

this.id = id;

}

?

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

@ManyToOne

public Group getGroup() {

return group;

}

public void setGroup(Group group) {

this.group = group;

}

?

}

（3）测试

要建立User和Group两者的双向关系，并且只保存Group对象

package com.zgy.hibernate.model;

?

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

import org.hibernate.cfg.Configuration;

import org.hibernate.service.ServiceRegistry;

import org.hibernate.tool.hbm2ddl.SchemaExport;

import org.junit.AfterClass;

import org.junit.BeforeClass;

import org.junit.Test;

?

public class ORMapppingTest {

?

public static SessionFactory sf = null;

@BeforeClass

public static void beforeClass(){

Configuration configure =new Configuration().configure();

new SchemaExport(configure).create(true, true);

sf = configure.buildSessionFactory();

}

@Test

public void testSaveUser() {

User user1 = new User();

user1.setName("user1");

User user2 = new User();

user2.setName("user2");

Group group = new Group();

group.setName("group1");

group.getUsers().add(user1);

group.getUsers().add(user2);

user1.setGroup(group);

user2.setGroup(group);

Session s = sf.openSession();

s.beginTransaction();

s.save(group);

//s.save(user);

s.getTransaction().commit();

}

?

@AfterClass

public static void afterClass(){

sf.close();

}

?

}

?

（4）观察结果

生成的SQL语句：

?

create table t_group (

id integer not null auto_increment,

name varchar(255),

primary key (id)

)

?

create table t_user (

id integer not null auto_increment,

name varchar(255),