Hibernate Annotation 基于外键的单向一对多关联

基于外键关联的单向一对多关联是一种很少见的情况，并不推荐使用。
基于连接表的单向一对多关联 应该优先被采用。请注意，通过指定unique="true"，我们可以把多样性从多对多改变为一对多。

摘自Hibernate Reference

Product与Category是多对一的关系，Product对象维护着对Category对象的参考，如果由Category对象维护对多个Product对象的管理，就是一对多单向关联。
多对一就是从Product端来看，多个产品属于一个分类，每个产品都需要维护各自的Category对象。
而一对多就是从Category端来看，一个Category下有多个分类，Category需要维护多个Product信息。

 

嘿，不好意思，各位看官，因为恢复系统把之前做的物理数据模型图弄丢了，本来打算一并附上的，现在搞不到了，呵呵。

 

 

SQL脚本：（附件中可下载）

 

-- MySQL dump 10.13  Distrib 5.1.55, for Win32 (ia32)
--
-- Host: localhost    Database: hibernate_demo
-- ------------------------------------------------------
-- Server version	5.1.55-community

/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40101 SET @OLD_CHARACTER_SET_RESULTS=@@CHARACTER_SET_RESULTS */;
/*!40101 SET @OLD_COLLATION_CONNECTION=@@COLLATION_CONNECTION */;
/*!40101 SET NAMES utf8 */;
/*!40103 SET @OLD_TIME_ZONE=@@TIME_ZONE */;
/*!40103 SET TIME_ZONE='+00:00' */;
/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;

--
-- Table structure for table `category`
--

DROP TABLE IF EXISTS `category`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `category` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) NOT NULL,
  `description` varchar(30) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=9 DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `category`
--

LOCK TABLES `category` WRITE;
/*!40000 ALTER TABLE `category` DISABLE KEYS */;
INSERT INTO `category` VALUES (1,'fruit2','fruit category2');
/*!40000 ALTER TABLE `category` ENABLE KEYS */;
UNLOCK TABLES;

--
-- Table structure for table `product`
--

DROP TABLE IF EXISTS `product`;
/*!40101 SET @saved_cs_client     = @@character_set_client */;
/*!40101 SET character_set_client = utf8 */;
CREATE TABLE `product` (
  `id` int(11) NOT NULL,
  `name` varchar(30) NOT NULL,
  `price` int(11) NOT NULL,
  `description` varchar(30) NOT NULL,
  `category_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `category_id` (`category_id`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
/*!40101 SET character_set_client = @saved_cs_client */;

--
-- Dumping data for table `product`
--

LOCK TABLES `product` WRITE;
/*!40000 ALTER TABLE `product` DISABLE KEYS */;
/*!40000 ALTER TABLE `product` ENABLE KEYS */;
UNLOCK TABLES;
/*!40103 SET TIME_ZONE=@OLD_TIME_ZONE */;

/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40101 SET CHARACTER_SET_CLIENT=@OLD_CHARACTER_SET_CLIENT */;
/*!40101 SET CHARACTER_SET_RESULTS=@OLD_CHARACTER_SET_RESULTS */;
/*!40101 SET COLLATION_CONNECTION=@OLD_COLLATION_CONNECTION */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;

-- Dump completed on 2012-08-16 22:20:47

 

 

//Category.java

 

package com.zyp.examples;

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

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

import org.hibernate.annotations.GenericGenerator;

/**
 * Category entity. @author MyEclipse Persistence Tools
 */

@Entity
@Table(name="category")
public class Category implements java.io.Serializable {

	// Fields

	private static final long serialVersionUID = 411764056858459654L;
	
	@Id
	@GenericGenerator(name="incrementGenerator", strategy="increment")
	@GeneratedValue(generator="incrementGenerator", strategy=GenerationType.IDENTITY)
	@Column(name="id")
	private Integer id;
	
	@Column(name="name", nullable=false)
	private String name;
	
	@Column(name="description", nullable=false)
	private String description;
	
	@OneToMany(targetEntity=Product.class, cascade=CascadeType.ALL)
	@JoinColumn(name="category_id")
	private Set<Product> products = new HashSet<Product>();

	// Constructors

	/** default constructor */
	public Category() {
	}

	/** full constructor */
	public Category(String name, String description) {
		this.name = name;
		this.description = description;
	}

	// Property accessors

	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;
	}

	public String getDescription() {
		return this.description;
	}

	public void setDescription(String description) {
		this.description = description;
	}

	public Set<Product> getProducts() {
		return products;
	}

	public void setProducts(Set<Product> products) {
		this.products = products;
	}
	
	

}

 

 

//Product.java

package com.zyp.examples;

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

import org.hibernate.annotations.GenericGenerator;

/**
 * Product entity. @author MyEclipse Persistence Tools
 */

@Entity
@Table(name="product")
public class Product implements java.io.Serializable {

	// Fields

	private static final long serialVersionUID = -3046977533702585304L;

	@Id
	@GenericGenerator(name="incrementGenerator", strategy="increment")
	@GeneratedValue(generator="incrementGenerator", strategy=GenerationType.IDENTITY)
	@Column(name="id")
	private Integer id;
	
	@Column(name="name", nullable=false)
	private String name;
	
	@Column(name="price", nullable=false)
	private Integer price;
	
	@Column(name="description",nullable=false)
	private String description;
	
	
	// Constructors

	/** default constructor */
	public Product() {
	}


	// Property accessors

	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;
	}

	public Integer getPrice() {
		return this.price;
	}

	public void setPrice(Integer price) {
		this.price = price;
	}

	public String getDescription() {
		return this.description;
	}

	public void setDescription(String description) {
		this.description = description;
	}


}

 

 

 

//hibernate.cfg.xml

这个就不弄了，都一样

 

//HibernateUtil.java

也不弄了

 

 

//HibernateTest.java

 

package com.zyp.examples;

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

import org.hibernate.Session;
import org.hibernate.Transaction;

public class HibernateTest {

	public static void main(String[] args) {
		
		//同时保存分类和产品
		addAll();
		
		//根据已有分类保存产品
		//先添加分类
//		addCategory();
//		addProduct();
		
		//级联删除
//		deleteByCategory();

	}
	
	public static void deleteByCategory()
	{
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction tx = session.beginTransaction();
		Category cat = (Category) session.load(Category.class, new Integer(1));
		session.delete(cat);

		tx.commit();
		session.close();
		HibernateUtil.shutdown();
	}
	
	public static void addAll()
	{
		Product product = new Product();
		product.setDescription("apple2");
		product.setName("apple2");
		product.setPrice(10);
		
		
		Product product1 = new Product();
		product1.setDescription("orige2");
		product1.setName("orige2");
		product1.setPrice(20);
		
		Category category = new Category();
		category.setDescription("fruit category2");
		category.setName("fruit2");
		
		
		Set<Product> products = new HashSet<Product>();
		products.add(product);
		products.add(product1);
		category.setProducts(products);
		
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction tx = session.beginTransaction();
		session.save(category);
		//启用级联操作
//		session.save(product);
//		session.save(product1);
		tx.commit();
		session.close();
		HibernateUtil.shutdown();
		
	}
	
	public static void addCategory()
	{
		Category category = new Category();
		category.setName("fruit");
		category.setDescription("fruit category");
		
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction tx = session.beginTransaction();
		session.save(category);
		tx.commit();
		HibernateUtil.shutdown();
	}
	
	
	public static void addProduct()
	{
		Product product = new Product();
		product.setDescription("banana");
		product.setName("banana");
		product.setPrice(10);
		
		Product product1 = new Product();
		product1.setDescription("tom");
		product1.setName("tom");
		product1.setPrice(20);		
		
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction tx = session.beginTransaction();
		Category category = (Category)session.load(Category.class, new Integer(1));
		
		Set<Product> products = new HashSet<Product>();
		products.add(product);
		products.add(product1);
		
		category.setProducts(products);
		
		session.save(category);
		//启用级联操作
//		session.save(product);
//		session.save(product1);
		tx.commit();
		session.close();
		HibernateUtil.shutdown();
		
		
	}
}

 

 

=================== 悲剧的分界线 =========================

 

中间出了一个错误，困扰了我一个晚上，本来昨晚就应该发布这个文章的。哎，找了这么大一个互联网，就没一篇帖子给出我想要的答案，我晕，难道你们只知道双向关联，不知道单向关联么。。。。

 

仔细一想，之前的xml配置的时候貌似隐隐的出现了这样的错误，马上解决了。看起来，学习注解的时候，参考一下xml的配置还是有必要的。

 

Hibernate:
    insert
    into
        product
        (description, name, price, id)
    values
        (?, ?, ?, ?)
22:16:41,718 DEBUG AbstractBatcher:66 - Executing batch size: 2
22:16:41,718 DEBUG AbstractBatcher:418 - about to close PreparedStatement (open PreparedStatements: 1, globally: 1)
22:16:41,718 DEBUG JDBCExceptionReporter:92 - Could not execute JDBC batch update [insert into product (description, name, price, id) values (?, ?, ?, ?)]
java.sql.BatchUpdateException: Field 'category_id' doesn't have a default value
    at com.mysql.jdbc.PreparedStatement.executeBatchSerially(PreparedStatement.java:2007)
    at com.mysql.jdbc.PreparedStatement.executeBatch(PreparedStatement.java:1443)
    at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:70)
    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:268)
    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:266)
    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:167)
    at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
    at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)
    at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1028)
    at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:366)
    at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:137)
    at com.zyp.examples.HibernateTest.addAll(HibernateTest.java:67)
    at com.zyp.examples.HibernateTest.main(HibernateTest.java:14)
22:16:41,718  WARN JDBCExceptionReporter:100 - SQL Error: 1364, SQLState: HY000
22:16:41,718 ERROR JDBCExceptionReporter:101 - Field 'category_id' doesn't have a default value
22:16:41,718 ERROR AbstractFlushingEventListener:324 - Could not synchronize database state with session
org.hibernate.exception.GenericJDBCException: Could not execute JDBC batch update
    at org.hibernate.exception.SQLStateConverter.handledNonSpecificException(SQLStateConverter.java:126)
    at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:114)
    at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:275)
    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:266)
    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:167)
    at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
    at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)
    at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1028)
    at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:366)
    at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:137)
    at com.zyp.examples.HibernateTest.addAll(HibernateTest.java:67)
    at com.zyp.examples.HibernateTest.main(HibernateTest.java:14)
Caused by: java.sql.BatchUpdateException: Field 'category_id' doesn't have a default value
    at com.mysql.jdbc.PreparedStatement.executeBatchSerially(PreparedStatement.java:2007)
    at com.mysql.jdbc.PreparedStatement.executeBatch(PreparedStatement.java:1443)
    at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:70)
    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:268)
    ... 9 more
Exception in thread "main" org.hibernate.exception.GenericJDBCException: Could not execute JDBC batch update
    at org.hibernate.exception.SQLStateConverter.handledNonSpecificException(SQLStateConverter.java:126)
    at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:114)
    at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)
    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:275)
    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:266)
    at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:167)
    at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)
    at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)
    at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1028)
    at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:366)
    at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:137)
    at com.zyp.examples.HibernateTest.addAll(HibernateTest.java:67)
    at com.zyp.examples.HibernateTest.main(HibernateTest.java:14)
Caused by: java.sql.BatchUpdateException: Field 'category_id' doesn't have a default value
    at com.mysql.jdbc.PreparedStatement.executeBatchSerially(PreparedStatement.java:2007)
    at com.mysql.jdbc.PreparedStatement.executeBatch(PreparedStatement.java:1443)
    at org.hibernate.jdbc.BatchingBatcher.doExecuteBatch(BatchingBatcher.java:70)
    at org.hibernate.jdbc.AbstractBatcher.executeBatch(AbstractBatcher.java:268)
    ... 9 more

这个问题和之前的一样，都是因为外键设置了非空导致，外键是可以为空的。
为什么呢？
我们不是通过外键关联来保存Product的么，先保存category，然后保存Product，我们在保存product的时候，没有设置主键值，而是从category的id获取，而此时的product 外键要求非空，因此报错。

mysql> alter table product
    -> modify category_id int not null;

可以使用上面的语句来重现这个错误。