Hibernate！！关联关系cascade

在写代码的时候，需要设置好关联，不然数据库中会没有相应的数据。

双向关系在程序中药设定双向关联，不然在数据库中会出问题

cascade翻译成中文叫：级联

cascade一般用在级联保存，级联更新，级联删除上

cascade可以取得值：

• ALL 在所有情况下，在持久化操作的增删改查，都会级联到另一个对象
• MERGE   合并的时候
• PERSIST   存储的时候
• REFRESH  刷新
• REMOVE   删除

如果两个对象之间有关联，那么就需要设置cascade这个属性。

@ManyToOne注解的这端，是多端

1.在注释@ManyToOne(cascade=CascadeType.ALL,optional=true)中将属性optional设置为true，这可以使得即使外键为空时仍可以向表中添加数据。

注意：导入包的时候是：import javax.persistence.CascadeType;

 

例子：

 

package com.hb.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="t_group")
public class Group {
    private  int gid ;
    private  String  gname ;
    
    @Id
    @GeneratedValue
    public int getGid() {
        return gid;
    }
    public void setGid(int gid) {
        this.gid = gid;
    }
    public String getGname() {
        return gname;
    }
    public void setGname(String gname) {
        this.gname = gname;
    }
    

}

 

package com.hb.model;


import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
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;
    }
    

    //cascade属性是一个数组，cascade是级联
    @ManyToOne(cascade={CascadeType.ALL})
    public Group getGroup() {
        return group;
    }
    public void setGroup(Group group) {
        this.group = group;
    }
    
    

}

package com.hb.model;

import static org.junit.Assert.*;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class hibernateCRUDTest {
    
private  static  SessionFactory  sessionFactory ;
    
    @BeforeClass
    public  static  void  beforeClass(){
        new  SchemaExport(new AnnotationConfiguration().configure()).create(true, true);
        sessionFactory = new  AnnotationConfiguration().configure().buildSessionFactory();
        
    }
    
    @AfterClass
    public  static  void  afterClass(){
        sessionFactory.close();
    }
    

    @Test
    public void testSaveUser() {
        User  u = new User();
        u.setName("u1");
        Group  g = new  Group();
        u.setGroup(g);
        g.setGname("g1");
        Session  session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        //session.save(g);
        session.save(u);
        session.getTransaction().commit();
        
        
        
    }
    
    public static void main(String[] args) {
        beforeClass();
    }

}

 

 

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
          "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
          "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<!-- Generated by MyEclipse Hibernate Tools.                   -->
<hibernate-configuration>

<session-factory>
    <property name="dialect">
        org.hibernate.dialect.MySQLDialect
    </property>
    <property name="connection.url">
        jdbc:mysql://localhost:3306/hb
    </property>
    <property name="connection.username">root</property>
    <property name="connection.password">root</property>
    <property name="connection.driver_class">
        com.mysql.jdbc.Driver
    </property>
    <property name="myeclipse.connection.profile">Mysql</property>
    <property name="show_sql">true</property>
    <property name="format_sql">true</property>

    <property name="current_session_context_class">thread</property>
    
    <mapping class="com.hb.model.Group" />
    <mapping class="com.hb.model.User" />
</session-factory>

</hibernate-configuration>

 

 在 public class hibernateCRUDTest 类中，测试保存Group

    @Test
    public void testSaveGroup() {
        User  u1= new User();
        u1.setName("u1");
        User  u2 = new User();
        u2.setName("u2");
        Group  g = new  Group();
        g.setGname("g1");
        
        g.getUsers().add(u1);
        g.getUsers().add(u2);
        
        u1.setGroup(g);
        u2.setGroup(g);

        Session  session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        session.save(g);
        session.getTransaction().commit();
    }

由上面的 g.getUsers().add(u1); 和 g.getUsers().add(u2) 可知现在是双向关联，所以对应的Group类中需要加入@OneToMany属性

双向关系在程序中要设定双向关联，不然在数据库中会出问题

 

双向关系在程序中要设定mappedBy="group"，不然在数据库中会出问题

代码如下：

 

 

package com.hb.model;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="t_group")
public class Group {
    private  int gid ;
    private  String  gname ;
    private  Set<User>  users = new HashSet<User>();
    
    @OneToMany(mappedBy="group",cascade={CascadeType.ALL})
    public Set<User> getUsers() {
        return users;
    }
    public void setUsers(Set<User> users) {
        this.users = users;
    }
    @Id
    @GeneratedValue
    public int getGid() {
        return gid;
    }
    public void setGid(int gid) {
        this.gid = gid;
    }
    public String getGname() {
        return gname;
    }
    public void setGname(String gname) {
        this.gname = gname;
    }
    

}