hibernate---关联关系的 crud_cascade_fetch

CRUD怎么写??

存user信息, 自动存group信息

user.java

package com.bjsxt.hibernate;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
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;
	@ManyToOne(cascade={CascadeType.ALL})
	public Group getGroup() {
		return group;
	}
	public void setGroup(Group group) {
		this.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;
	}
}

group.java:

package com.bjsxt.hibernate;

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

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

@Entity
@Table(name="t_group")
public class Group {
	private int id;
	private String name;
	private Set<User> users = new HashSet<User>();
	@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<User> getUsers() {
		return users;
	}
	public void setUsers(Set<User> users) {
		this.users = users;
	}
}

hibernate.cfg.xml:

<?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">

<hibernate-configuration>

    <session-factory>

         
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost/hibernate</property>
        <property name="connection.username">root</property>
        <property name="connection.password">linda0213</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
       <!--
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:SXT</property>
        <property name="connection.username">scott</property>
        <property name="connection.password">tiger</property>
      	<property name="dialect">org.hibernate.dialect.OracleDialect</property>
       -->

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>
        <property name="format_sql">true</property>

        <!-- Drop and re-create the database schema on startup
        <property name="hbm2ddl.auto">update</property>
		 -->
        <!--  
        
		<mapping resource="com/bjsxt/hibernate/Group.hbm.xml"/>
		 <mapping resource="com/bjsxt/hibernate/User.hbm.xml"/>
		 -->
		 <mapping class="com.bjsxt.hibernate.Group"/>
		<mapping class="com.bjsxt.hibernate.User"/>
		
    </session-factory>

</hibernate-configuration>

test文件:

package com.bjsxt.hibernate;

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 HibernateORMappingTest {
	private static SessionFactory sessionFactory;
	
	@BeforeClass
	public static void beforeClass() {
		new SchemaExport(new AnnotationConfiguration().configure()).create(false, 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();
		g.setName("g1");
		u.setGroup(g);
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		//s.save(g);
		s.save(u);
		s.getTransaction().commit();
	}
	
	@Test
	public void testSchemaExport() {
		new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
	}
	
	public static void main(String[] args) {
		beforeClass();
	}
}

如果设定group同时更新user的话:

@Test
	public void testSaveGroup() {
		User u1 = new User();
		u1.setName("u1");
		User u2 = new User();
		u2.setName("u2");
		Group g = new Group();
		g.setName("g1");
		g.getUsers().add(u1);
		g.getUsers().add(u2);
		u1.setGroup(g);
		u2.setGroup(g);
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		//s.save(g);
		s.save(g);
		s.getTransaction().commit();
	}

铁律:双向关系,必须设置双向关联.　双向一定要设置MappedBy

get/load:

从多里会自动被一取出来,即, 取user, 会取出group

遵循原则: 默认, 多对一 eager, 一对多 lazy.

@Test
	public void testGetUser() {
		
		testSaveGroup();
		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.get(User.class, 1);
		
		s.getTransaction().commit();
		System.out.println(u.getGroup().getName());
	}

从一取, 不会取, 因为cascade的all不影响读取. 只管CUD, 增删改

fetch影响读取.　管load,get等读取

想取多的话: 可以改group.java:

fetch=FetchType.EAGER.  LAZY是默认不取.

所以从多取一 默认是EAGER

从一取多, 默认是lazy, 因为数据多.

@OneToMany(mappedBy="group",
		cascade={CascadeType.ALL},
	 	fetch=FetchType.EAGER
		)
	
	public Set<User> getUsers() {
		return users;
	}

test.java:

@Test
	public void testGetGroup() {
		
		testSaveGroup();
		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		Group g = (Group)s.get(Group.class, 1);
		s.getTransaction().commit();
		for(User u : g.getUsers()) {
			System.out.println(u.getName());
		}
	}

　　

lazy加载的报错问题: 因为commit后session结束, 不能再链接数据库获取信息了, 得把下面访问group数据库的信息放在commit上面.

@Test
	public void testLoadUser() {
		
		testSaveGroup();
		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.load(User.class, 1);
		
		s.getTransaction().commit();
		System.out.println(u.getGroup().getName());
		
	}

Load的lazy和eager的区别:

test代码:

@Test
	public void testLoadUser() {
		
		testSaveGroup();
		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.load(User.class, 1);
		System.out.println(u.getGroup().getName());
		s.getTransaction().commit();
		
	}

user.java如果改成lazy的话:

@ManyToOne(cascade={CascadeType.ALL},fetch=FetchType.LAZY)
	public Group getGroup() {
		return group;
	}

运行时:　

1. user对group是 lazy时: 先select user, 再join user 和group

2. user对group是eager时, 结果是先 　join user 和group, 再select user.

因为是eager会先join, 由于group里又有一个eager,所以又取了一遍user, 所以取的时候, 只要一边设置eager就可以了. 所以默认的话, 一对多默认是 lazy, 多对一默认是eager.

XML也有对上面问题的控制, 在set里设置一个inverse="true"

<?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.bjsxt.hibernate.Group" table="t_group">
		<id name="id">
			<generator class="native"></generator>
		</id>
		
		<property name="name"></property>
		<set name="users" inverse="true">
			<key column="groupId"></key>
			<one-to-many class="com.bjsxt.hibernate.User"/>
		</set>
    </class>
	
</hibernate-mapping>

　　

 update:

会同时更新user和group

之所以级联更新, 是因为user.java里有@ManyToOne(cascade={CascadeType.ALL})

@Test
	public void testUpdateUser() {
		
		testSaveGroup();
		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.get(User.class, 1);
		
		s.getTransaction().commit();
		
		u.setName("user");
		u.getGroup().setName("group");
		
		Session s2 = sessionFactory.getCurrentSession();
		s2.beginTransaction();
		s2.update(u);
		
		s2.getTransaction().commit();
	}

　　

delete: user里有u1, u2, group里是g1, u1,u2 都是g1.

结果两个表的数据都被删了. 因为删除u1时关联group 的g1, 删除g1的时候又级联的删除了u2.

@Test
	public void testDeleteUser() {		
		testSaveGroup();		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.load(User.class, 2);
		s.delete(u);
		s.getTransaction().commit();		
	}

所以现在的O/R mapping的编程模型有

a)映射模型 : jpa annotation (最常用), hibernate annotation extension, hibernate xml, jpa xml

b)编程接口: 增删改查的接口,  jpa 接口 , hibernate 接口

c)数据查询:

   1. HQL

   2. EJBQL(JP QL)

所以现在开发模式是jpa annotation和hibernate的编程接口.

解决方法: 先设定关系为null, 再删除:

@Test
	public void testDeleteUser() {		
		testSaveGroup();		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.load(User.class, 2);
		u.setGroup(null);
		s.delete(u);		
		s.getTransaction().commit();		
	}

第二种方法:用createQuery.

@Test
	public void testDeleteUser() {		
		testSaveGroup();		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		User u = (User)s.load(User.class, 2);
		s.createQuery("delete from User u where u.id = 2").executeUpdate();
		s.getTransaction().commit();		
	}

删除group的情况: cascade all所以级联全部删除

@Test
	public void testDeleteGroup() {
		
		testSaveGroup();
		
		Session s = sessionFactory.getCurrentSession();
		s.beginTransaction();
		//User u = (User)s.load(User.class, 1);
		//u.setGroup(null);
		//s.delete(u);
		Group g = (Group)s.load(Group.class, 1);
		s.delete(g);
		//s.createQuery("delete from User u where u.id = 1").executeUpdate();
		s.getTransaction().commit();
		
	}