数据库的多对多 1.1 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1：数据库多表联接查询 永远就是两个表的联接查询 注2：交叉连接 注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
A B AB select * from A,B,AB WHERE A.aID=AB.aID and b.bid = AB.bid where 在hibernate中，你只管查询当前表对象即可,hibernate会自动关联桥接表(通过映射文件找到桥接表)以及关联表,查询出关联对象
2. hibernate的多对多 2.1 hibernate可以直接映射多对多关联关系(看作两个一对多)
案例： 多对多级联查询 书籍表、书籍类别表
实体类: 书 :book 类型:category 一本书对应多种类型 ,一种类型有多本书
Book
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> categroies=new HashSet<>(); private Integer initCategroies=0; public Integer getInitCategroies() { return initCategroies; } public void setInitCategroies(Integer initCategroies) { this.initCategroies = initCategroies; } 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> getCategroies() { return categroies; } public void setCategroies(Set<Category> categroies) { this.categroies = categroies; } }Category
package com.zking.five.entity; import java.util.HashSet; import java.util.Set; public class Category { private Integer categoryId; private String categoryName; private Set<Book> books=new HashSet<>(); private Integer initbooks=0; public Integer getInitbooks() { return initbooks; } public void setInitbooks(Integer initbooks) { this.initbooks = initbooks; } 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; } }映射类 (体现关系) Book.hbml.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> <!-- 通过查询一本书 查出关联的所有类别 的原理: session.get(Book.class, 1); =select * from t_hibernate_book where book_id=?(1) resultSet -> 1 西游记 50 Book b=Class.forName("com.zking.five.entity.Book").newInstance(); //空的 b.setBookId(1); b.setBookName(西游记); b.setPrice(50); hibernate处理关联关系 1 .通过set找到桥接表 (table属性), 2.找到 当前实体类对应表 的主键 在桥接表中的外键(key 标签中的column属性) 3.查出关联表(t_hibernate_category)的主键 (category_id) select cid from t_hibernate_book_category where bid=? (1) resultSet -> 1 2 list<String>1/2 4.查出来的外键关联了一个实体类 (class="com.zking.five.entity.Category"),他可以找到这个类的映射文件( class标签的name="com.zking.five.entity.Category"), 从而找到了对应的实体类对应的表的主键id标签中的column字段 ->category_id select * from t_hibernate_category where t_hibernate_category in (1,2) resultSet-> 1 古典 2 神话 Category c=Class.forName("com.zking.five.entity.Category").newInstance(); 原理参照EntityBaseDao ->List<c> -> categories 5.b.setcategoyies(categories); --> <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 桥接表 --> <set table="t_hibernate_book_category" name="categroies" cascade="save-update" inverse="false"> <!-- one 书--> <key column="bid"></key> <!-- many 类别--> <many-to-many column="cid" class="com.zking.five.entity.Category"></many-to-many> </set> </class> </hibernate-mapping>Category.hbml.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> <!--table 桥接表 --> <set table="t_hibernate_book_category" name="books" cascade="save-update" inverse="true"> <!-- one 类别--> <key column="cid"></key> <!-- many 书--> <many-to-many column="bid" class="com.zking.five.entity.Book"></many-to-many> </set> </class> </hibernate-mapping>查询的方法
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.SessionFactoryUtil; public class BookDao { public Book getBook(Book book) { Session session = SessionFactoryUtil.getSession(); Transaction transaction = session.beginTransaction(); Book b= session.get(Book.class, book.getBookId()); if(b!=null&&new Integer(1).equals(book.getInitCategroies())) { Hibernate.initialize(b.getCategroies()); } transaction.commit(); SessionFactoryUtil.closeSession(); 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.SessionFactoryUtil; public class CategoryDao { public Category getCategory(Category category) { Session session = SessionFactoryUtil.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(); SessionFactoryUtil.closeSession(); return c; } }测试
package com.zking.five.dao; import org.junit.Test; import com.zking.five.entity.Book; import com.zking.five.entity.Category; public class BookDaoTest { private BookDao bookDao=new BookDao(); /** * 通过查询一本书 查出关联的所有类别 */ @Test public void testGetBook() { Book book=new Book(); book.setInitCategroies(1);//强制加载 book.setBookId(1); Book b = this.bookDao.getBook(book); System.out.println(b.getBookName());//书名 for (Category c : b.getCategroies()) { System.out.println(c.getCategoryName());//类别 } } } 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 CategoryDaoTest { private CategoryDao categoryDao=new CategoryDao(); /** * 通过查询一个类别 查出关联的所有书籍 */ @Test public void testGetCategory() { Category category=new Category(); category.setCategoryId(3); category.setInitbooks(1); Category c = this.categoryDao.getCategory(category); System.out.println(c.getCategoryName()); for (Book b : c.getBooks()) { System.out.println(b.getBookName()); } } }