Hibernate 1+N 问题

1+N问题的描述：举例，多个主题（Topic）属于一个帖子（Category），一个帖子含有多个主题。当只需要查询Topic时不要查询Category时，如果@ManyToOne的属性fetch=FetchType.EAGER,这时查询所有Topic时，每查询一个Topic就会多产生一个SQL语句查询相关的Category表的数据，这样要是有N条Topic数据，就会产生1+N条SQL语句。同样的在@OneToMany的情况下，要是在Many方设置fetch=FetchType.EAGER，同样也会产生1+N的问题。

 

解决方案有三种：

1. fetch=FetchType.LAZY，设为懒加载

2. @BatchSize（size=5）代表一次取5条数据，这样取5条数据只要发出一条SQL语句，注意是用在被关联类上的（不建议用）

3. 迫切左外连接检索 join fetch(Criteria 查询默认就是join fetch)

下面请看代码：

Category类

package com.lbx.model;

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

import org.hibernate.annotations.BatchSize;

@Entity
@BatchSize(size=2)
public class Category {
	
	private int id;
	private String name;
	
	@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;
	}
	
}

 

Topic类

package com.lbx.model;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Topic {

	private int id;
	private String title;
	private Category category;
	
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
	
	//@ManyToOne(fetch=FetchType.LAZY)
	@ManyToOne
	public Category getCategory() {
		return category;
	}
	public void setCategory(Category category) {
		this.category = category;
	}
	
}

 

 

hibernate.cfg.xml文件配置

<?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>

        <!-- Database connection settings -->
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/testhib</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>

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

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>

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

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

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">update</property>

		<mapping class="com.lbx.model.Category"/>
		<mapping class="com.lbx.model.Topic"/>
		<mapping class="com.lbx.model.Msg"/>
		

    </session-factory>

</hibernate-configuration>

 

 

测试代码

package com.lbx.model.test;

import java.util.List;

import javax.persistence.FetchType;
import javax.persistence.ManyToOne;

import junit.framework.TestCase;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.lucene.Text;

import com.lbx.hibernate.Util.HibUtil;
import com.lbx.model.Category;
import com.lbx.model.Msg;
import com.lbx.model.Topic;

public class Test extends TestCase {
	
	@Text
	public void save(){
		Session session = HibUtil.getSession();
		session.beginTransaction();
		
		for (int i = 0; i < 3; i++) {
			Category c = new Category();
			c.setName("c" + i);
			Topic t = new Topic();
			t.setTitle("t" + i);
			t.setCategory(c);
			session.save(c);
			session.save(t);
		}
		
		session.beginTransaction().commit();
		session.close();
	}
	
	//1+N问题(这里我只要取出Topic就可以了)
	@Text
	public void testHQL_01(){
		Session session = HibUtil.getSession();
		session.beginTransaction();
		
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		/**
		 * 这里要是不把Topic类中不设 @ManyToOne(fetch=FetchType.LAZY)，这里就要发很多SQL语句，关联的表都会查
		 * 但是设了@ManyToOne(fetch=FetchType.LAZY) 之后就不会发出查询相关表的查询语句，用到的时候才发出
		 */
		Query q = session.createQuery("from Topic");
		List<Topic> topics = (List<Topic>)q.list();
		System.out.println(topics.size());
		for (int i = 0; i < topics.size(); i++) {
			System.out.println(topics.get(i).getId() + "  " + topics.get(i).getTitle());
		}
		session.beginTransaction().commit();
		session.close();
	}
	
	//用到被关联表的信息
	@Text
	public void testHQL_02(){
		Session session = HibUtil.getSession();
		session.beginTransaction();
		
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		/**
		 * 这里要是不把Topic类中不设 @ManyToOne(fetch=FetchType.LAZY)，这里就要发很多SQL语句，关联的表都会查
		 * 但是设了@ManyToOne(fetch=FetchType.LAZY) 之后就不会发出查询相关表的查询语句，用到的时候才发出
		 */
		Query q = session.createQuery("from Topic");
		List<Topic> topics = (List<Topic>)q.list();
		System.out.println(topics.size());
		for (int i = 0; i < topics.size(); i++) {
			System.out.println(topics.get(i).getId() + "  " + topics.get(i).getTitle());
			/**
			 * 注意，在这里要用到Category类的信息,所以就会发出相关的查询信息
			 */
			System.out.println(topics.get(i).getCategory().getId() + "  " + 
					topics.get(i).getCategory().getName());
		}
		session.beginTransaction().commit();
		session.close();
	}
	
	//@BatchSize的使用，其属性size=5就代表一次取5个
	@Text
	public void testHQL_03(){
		Session session = HibUtil.getSession();
		session.beginTransaction();
		
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
		/**
		 * 这里要是不把Topic类中不设 @ManyToOne(fetch=FetchType.LAZY)，这里就要发很多SQL语句，关联的表都会查
		 * 但是设了@ManyToOne(fetch=FetchType.LAZY) 之后就不会发出查询相关表的查询语句，用到的时候才发出
		 */
		Query q = session.createQuery("from Topic");
		List<Topic> topics = (List<Topic>)q.list();
		System.out.println(topics.size());
		for (int i = 0; i < topics.size(); i++) {
			System.out.println(topics.get(i).getId() + "  " + topics.get(i).getTitle());
			/**
			 * 注意，在这里要用到Category类的信息,所以就会发出相关的查询信息
			 */
			System.out.println(topics.get(i).getCategory().getId() + "  " + 
					topics.get(i).getCategory().getName());
		}
		session.beginTransaction().commit();
		session.close();
	}
	
	// join fetch,迫切左外连接检索
	@Text
	public void testHQL_04(){
		Session session = HibUtil.getSession();
		session.beginTransaction();
		//Criteria 查询默认就是join fetch
		//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();  
		/**
		 * 这里要是不把Topic类中不设 @ManyToOne(fetch=FetchType.LAZY)，这里就要发很多SQL语句，关联的表都会查
		 * 但是设了@ManyToOne(fetch=FetchType.LAZY) 之后就不会发出查询相关表的查询语句，用到的时候才发出
		 */
		Query q = session.createQuery("from Topic t left join fetch t.category c");
		List<Topic> topics = (List<Topic>)q.list();
		System.out.println(topics.size());
		for (int i = 0; i < topics.size(); i++) {
			System.out.println(topics.get(i).getId() + "  " + topics.get(i).getTitle());
		}
		session.beginTransaction().commit();
		session.close();
	}
	
}