hibernate关联关系 - [多对多]

原创 于 2019-07-17 19:57:23 发布 · 121 阅读 · 0 ·
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本文介绍Hibernate框架中多对多关系的实现方法，包括实体类定义、映射文件配置及DAO层操作等关键步骤，并通过具体示例展示了如何在数据库与Hibernate之间建立和维护多对多关联。

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hibernate关联关系 - [多对多]

数据库与hibernate的多对多

数据库中不能直接映射多对多

处理：创建一个桥接表(中间表)，将一个多对多关系转换成两个一对多

注意：
① 数据库多表联接查询
永远就是二个表的联接查询
② 交叉连接
③ 外连接：left(左)/right(右)/full(左右)
主从表：连接条件不成立时，主表记录永远保留，与null匹配

hibernate的多对多

hibernate可以直接映射多对多关联关系(看作两个一对多)

在hibernate中，你只管查询当前表对象即可，
hibernate会自动关联桥表以及关联表查询出关联对象

流程

以查询book_id=8圣墟这本书为例
1、通过建模反射自动生成sql,可以拿到book_id=8这条记录的基本信息{book_id=8,book_name=圣墟,price=40}
2、book_id=8->bid=8去查询中间表t_hibernate_book_category
拿到了cid=8,9
3、cid=8,9->t_hibernate_category的category_id=8,9
4、拿到了当前book实例对应的category的集合
5、最终{book_id=8,book_name=圣墟,price=40}
->{book_id=8,book_name=圣墟,price=40，categories=[Category [categoryId=1, categoryName=古典]]}

实现多对多

我们要先准备三个实体类
Book

package com.lst.four.entity;

import java.io.Serializable;
import java.util.HashSet;
import java.util.Set;

public class Book implements Serializable{
//	book_id int primary key auto_increment,
//	   book_name varchar(50) not null,
//	   price float not null
	private Integer bookId;
	private String bookName;
	private Float price;
	
	private Set<Category> categories = new HashSet<Category>();
	private Integer initCategories = 0;

	public Integer getInitCategories() {
		return initCategories;
	}

	public void setInitCategories(Integer initCategories) {
		this.initCategories = initCategories;
	}

	public Integer getBookId() {
		return bookId;
	}

	public void setBookId(Integer bookId) {
		this.bookId = bookId;
	}

	public String getBookName() {
		return bookName;
	}

	public void setBookName(String bookName) {
		this.bookName = bookName;
	}

	public Float getPrice() {
		return price;
	}

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

	public Set<Category> getCategories() {
		return categories;
	}

	public void setCategories(Set<Category> categories) {
		this.categories = categories;
	}

	@Override
	public String toString() {
		return "Book [bookId=" + bookId + ", bookName=" + bookName + ", price=" + price + "]";
	}

	public Book(Integer bookId, String bookName) {
		super();
		this.bookId = bookId;
		this.bookName = bookName;
	}

	public Book() {
		super();
	}
}

Category

public class Category implements Serializable{
//	category_id int primary key auto_increment,
//	   category_name varchar(50) not null
	private Integer categoryId;
	private String categoryName;
	private Set<Book> books = new HashSet<Book>();
	public Integer getCategoryId() {
		return categoryId;
	}
	public void setCategoryId(Integer categoryId) {
		this.categoryId = categoryId;
	}
	public String getCategoryName() {
		return categoryName;
	}
	public void setCategoryName(String categoryName) {
		this.categoryName = categoryName;
	}
	public Set<Book> getBooks() {
		return books;
	}
	public void setBooks(Set<Book> books) {
		this.books = books;
	}
	@Override
	public String toString() {
		return "Category [categoryId=" + categoryId + ", categoryName=" + categoryName + "]";
	}
	
}

TreeNode

public class TreeNode {
	private Integer nodeId;
	private String nodeName;
	private Integer treeNodeType;
	private Integer position;
	private String url;
	private TreeNode parent;
	private Set<TreeNode> children = new HashSet<TreeNode>();
	private Integer initChildren = 0;

	public Integer getNodeId() {
		return nodeId;
	}

	public void setNodeId(Integer nodeId) {
		this.nodeId = nodeId;
	}

	public String getNodeName() {
		return nodeName;
	}

	public void setNodeName(String nodeName) {
		this.nodeName = nodeName;
	}

	public Integer getTreeNodeType() {
		return treeNodeType;
	}

	public void setTreeNodeType(Integer treeNodeType) {
		this.treeNodeType = treeNodeType;
	}

	public Integer getPosition() {
		return position;
	}

	public void setPosition(Integer position) {
		this.position = position;
	}

	public String getUrl() {
		return url;
	}

	public void setUrl(String url) {
		this.url = url;
	}

	public TreeNode getParent() {
		return parent;
	}

	public void setParent(TreeNode parent) {
		this.parent = parent;
	}

	public Set<TreeNode> getChildren() {
		return children;
	}

	public void setChildren(Set<TreeNode> children) {
		this.children = children;
	}

	public Integer getInitChildren() {
		return initChildren;
	}

	public void setInitChildren(Integer initChildren) {
		this.initChildren = initChildren;
	}

	@Override
	public String toString() {
		return "TreeNode [nodeId=" + nodeId + ", nodeName=" + nodeName + ", treeNodeType=" + treeNodeType
				+ ", position=" + position + ", url=" + url + "]";
	}
}

准备实体类的映射文件
Book.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.lst.four.entity.Book" table="t_hibernate_book">
		<cache usage="read-only" region="com.zking.five.entity.Book"/>
		<id name="bookId" type="java.lang.Integer" column="book_id">
			<generator class="increment" />
		</id>
		<property name="bookName" type="java.lang.String"
			column="book_name">
		</property>
		<property name="price" type="java.lang.Float"
			column="price">
		</property>
		
		<!-- 
			table:代表的是中间表
			name:书籍类的关联属性
			inverse：中间表交于对方维护
			key：当前类对应的表列段在中间表（t_hibernate_book_category）的列键bid
			many-to-many：
				column：对应的是上面key查出来中间表（t_hibernate_book_category）的另一个字段cid，
				当做关联表的主键（category_id）进行查询
				class:上述查出来的主键对应的实体类
		 -->
		
		<set table="t_hibernate_book_category" name="categories" cascade="save-update" inverse="false">
			<!-- one -->
			<key column="bid"></key>
			<!-- many -->
			<many-to-many column="cid" class="com.lst.four.entity.Category"></many-to-many>
		</set>
	</class>
</hibernate-mapping>

category.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.lst.four.entity.Category" table="t_hibernate_category">
		<id name="categoryId" type="java.lang.Integer" column="category_id">
			<generator class="increment" />
		</id>
		<property name="categoryName" type="java.lang.String"
			column="category_name">
		</property>
		
		<set table="t_hibernate_book_category" name="books" cascade="save-update" inverse="true">
			<key column="cid"></key>
			<many-to-many column="bid" class="com.lst.four.entity.Book"></many-to-many>
		</set>
	</class>
</hibernate-mapping>

TreeNode.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.lst.four.entity.TreeNode" table="t_hibernate_sys_tree_node">
		<id name="nodeId" type="java.lang.Integer" column="tree_node_id">
			<generator class="increment" />
		</id>
		<property name="nodeName" type="java.lang.String"
			column="tree_node_name">
		</property>
		<property name="treeNodeType" type="java.lang.Integer"
			column="tree_node_type">
		</property>
		<property name="position" type="java.lang.Integer"
			column="position">
		</property>
		<property name="url" type="java.lang.String"
			column="url">
		</property>
		
		<many-to-one name="parent" class="com.lst.four.entity.TreeNode" column="parent_node_id"/>
		
		<set name="children" cascade="save-update" inverse="true">
			<key column="parent_node_id"></key>
			<one-to-many class="com.lst.four.entity.TreeNode"/>
		</set>
	</class>
</hibernate-mapping>

准备dao方法
BookDao

public class BookDao{
	public Integer addBook(Book book) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Integer bid = (Integer) session.save(book);
		transaction.commit();
		session.close();
		return bid;
	}
	
	public Integer addCategory(Category category) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Integer cid = (Integer) session.save(category);
		transaction.commit();
		session.close();
		return cid;
	}
	
	public Category getCategory(Category category) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Category c = session.get(Category.class, category.getCategoryId());
		transaction.commit();
		session.close();
		return c;
	}
	
	public Book getBook(Book book) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Book b = session.get(Book.class, book.getBookId());
		if (b != null && new Integer(1).equals(book.getInitCategories())) {
			Hibernate.initialize(b.getCategories());
		}
		transaction.commit();
		session.close();
		return b;
	}
	
	public void delBook(Book book) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		session.delete(book);
		transaction.commit();
		session.close();
	}
	
	public void delCategory(Category category) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		Category c = session.get(Category.class, category.getCategoryId());
		if(c!=null) {
			for (Book b : c.getBooks()) {
//				通过在被控方通过主控方来解除关联关系，最后被控方再做删除
				b.getCategories().remove(c);
			}
		}
		session.delete(c);
		transaction.commit();
		session.close();
	}
}

TreeNodeDao

public class TreeNodeDao {
	public TreeNode load(TreeNode treeNode) {
		Session session = SessionFactoryUtils.openSession();
		Transaction transaction = session.beginTransaction();
		TreeNode t = session.load(TreeNode.class, treeNode.getNodeId());
		if(t != null && new Integer(1).equals(treeNode.getInitChildren())) {
			Hibernate.initialize(t.getChildren());
			Hibernate.initialize(t.getParent());
		}
		transaction.commit();
		session.close();
		return t;
	}
}

测试junit
BookDaoTest

public class BookDaoTest {
	private BookDao bookDao = new BookDao();

	@Test
	public void testGetBook() {
		Book book = new Book();
		book.setBookId(2);
		book.setInitCategories(1);
		Book b = this.bookDao.getBook(book);
		System.out.println(b.getBookName());
		System.out.println(b.getCategories());
	}
	
	/** 
	 * book.hbm.xml	inverse=fasle 
	 * category.hbm.xml inverse=true 
	 * 数据添加正常 
	 * 书籍表、桥接表各新增一条数据 
	 */
	@Test
	public void test1() {
		Book book = new Book();
		book.setBookName("仙武帝尊");
		book.setPrice(10f);
		Category category = new Category();
		category.setCategoryId(5);
//		直接将category对象加入到新建的book中是错误的，因为此时的category是临时态的，hibernate是不会管理的
//		book.getCategories().add(category);
		Category c = this.bookDao.getCategory(category);
		
//		c.getBooks().add(book);
		book.getCategories().add(c);
		this.bookDao.addBook(book);
	}

	/**
	 * book.hbm.xml	inverse=true
	 * category.hbm.xml inverse=true
	 * 只增加书籍表数据 
	 * 桥接表不加数据 
	 * 原因：双方都没有去维护关系 
	 */
	@Test
	public void test2() {
		Book book = new Book();
		book.setBookName("荒莽纪");
		book.setPrice(25f);
		Category category = new Category();
		category.setCategoryId(5);
		Category c = this.bookDao.getCategory(category);
		
		book.getCategories().add(c);
		this.bookDao.addBook(book);
//		c.getBooks().add(book);
	}
}

TreeNodeTest

public class TreeNodeDaoTest {
	private TreeNodeDao treeNodeDao = new TreeNodeDao();

	@Test
	public void testLoad() {
		TreeNode treeNode = new TreeNode();
		treeNode.setNodeId(6);
		treeNode.setInitChildren(1);
		
		TreeNode t = this.treeNodeDao.load(treeNode);
		System.out.println(t);
		System.out.println(t.getParent());
		System.out.println(t.getChildren());
	}
}

效果展示：
书籍表
在这里插入图片描述
中间表
在这里插入图片描述
书籍类别表
在这里插入图片描述

小结

多对多关系注意事项
1、 一定要定义一个主控方
2、 多对多删除
① 主控方直接删除
② 被控方先通过主控方解除多对多关系，再删除被控方
③ 禁用级联删除
3、 关联关系编辑，不需要直接操作桥接表，hibernate的主控方会自动维护

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