数据库的多对多：

      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：数据库多表联接查询，永远就是二个表的联接查询

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) 

多对多关系注意事项：一定要定义一个主控方     

案例：

书本表：t_hibernate_book

书本类型表：t_hibernate_category

关联表：t_hibernate_book_category

表结构：

book表：

category表：

关联表：

1.书本：Book.java

package com.zking.five.entity;

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

public class Book {
	private Integer bookId;
	private String bookName;
	private Float price;
	
	private Set<Category> categorys=new HashSet<Category>();
	
	private Integer initCagetorys=0;
	
	public Integer getInitCagetorys() {
		return initCagetorys;
	}
	public void setInitCagetorys(Integer initCagetorys) {
		this.initCagetorys = initCagetorys;
	}
	public Set<Category> getCategorys() {
		return categorys;
	}
	public void setCategorys(Set<Category> categorys) {
		this.categorys = categorys;
	}
	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;
	}
	
	

}

2.书本类型：category.java

package com.zking.five.entity;

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

public class Category {
	
	private Integer categoryId;
	private String categoryName;
	
	//创建关联
	private Set<Book> books=new HashSet<Book>();
	
	//强制立即加载
	private Integer initBooks=0;
	
	public Integer getInitBooks() {
		return initBooks;
	}
	public void setInitBooks(Integer initBooks) {
		this.initBooks = initBooks;
	}
	public Set<Book> getBooks() {
		return books;
	}
	public void setBooks(Set<Book> books) {
		this.books = books;
	}
	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;
	}
	
	

}

3.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 table="t_hibernate_book" name="com.zking.five.entity.Book">
    		<id name="bookId" type="java.lang.Integer" column="book_id ">
    			<generator class="increment"></generator>
    		</id>
    		<property name="bookName" type="java.lang.String" column="book_name "></property>
    		<property name="price" type="java.lang.Float" column="price "></property>
    		
    		<!-- table:中间连接表
    		name="categorys"：Book实体类里的类属性
    		 -->
    	    <set table="t_hibernate_book_category" name="categorys" cascade="save-update" inverse="false">
    	    	<!-- one -->
    			<key column="bid"></key>
    			<!-- many -->
    			<many-to-many column="cid" class="com.zking.five.entity.Category"></many-to-many>
    		</set> 
    		
    		
    	</class>
    </hibernate-mapping>

4.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 table="t_hibernate_category" name="com.zking.five.entity.Category">
    		<id name="categoryId" type="java.lang.Integer" column="category_id ">
    			<generator class="increment"></generator>
    		</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.zking.five.entity.Book"></many-to-many>
    		</set> 
    	
    	</class>
    </hibernate-mapping>

5.dao方法测试：

package com.zking.five.dao;

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

import com.zking.five.entity.Book;
import com.zking.two.util.SessionFactoryUtils;

public class BookDao {
	
	public Book get(Book book) {
		Session session = SessionFactoryUtils.getSession();
		Transaction transaction = session.beginTransaction();
		Book b = session.get(Book.class, book.getBookId());
		if(b!=null&&new Integer(1).equals(book.getInitCagetorys())) {
			Hibernate.initialize(b.getCategorys());
		}
		transaction.commit();
		session.close();
		return b;
	}

}




package com.zking.five.dao;

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

import com.zking.five.entity.Category;
import com.zking.two.util.SessionFactoryUtils;

public class CategoryDao {
	
	public Category getCategory(Category category) {
		Session session = SessionFactoryUtils.getSession();
		Transaction transaction = session.beginTransaction();
		Category c = session.get(Category.class, category.getCategoryId());
		if(c!=null&&new Integer(1).equals(category.getInitBooks())) {
			Hibernate.initialize(c.getBooks());
		}
		transaction.commit();
		session.close();
		return c;
	}

}

6.测试类：

package com.zking.five.dao;

import static org.junit.Assert.*;

import org.junit.Test;

import com.zking.five.entity.Book;
import com.zking.five.entity.Category;

public class BookDaoTest {
	private BookDao bookDao=new BookDao();
	private Book book=new Book();
	private CategoryDao categoryDao=new CategoryDao();
	private Category category=new Category();

	/**
	 * 通过一本书能够查询到多个类别
	 * jdbc:三表联查
	 * hibernate：只需要查询单个对象即可，它会自动关联查询，交给映射文件即可
	 */
	@Test
	public void testGet() {
		book.setBookId(4);
		book.setInitCagetorys(1);
		Book b = this.bookDao.get(book);
		System.out.println(b.getBookName());
		for (Category ba : b.getCategorys()) {
			System.out.println(ba.getCategoryName());
		}
	}

	/**
	 * 通过一个列查到多本书
	 */
	@Test
	public void testGetCategory() {
		category.setCategoryId(1);
		category.setInitBooks(1);
		Category c = this.categoryDao.getCategory(category);
		System.out.println(c.getCategoryName());
		for (Book b : c.getBooks()) {
			System.out.println(b.getBookName());
		}
	}
	
	
}

结果展示：