A Product corresponds to a Category
One Category corresponds to multiple Products
First set up a many-to-one relationship , first prepare Category.java, Category.hbm.xml and Category, and add Category mapping in hibernate.cfg.xml .
Add Category property to Product.java:
package com.how2java.pojo;
public class Product {
int id;
String name;
float price;
Category category;//Add category attribute and add get and set methods
public Category getCategory() {
return category;
}
public void setCategory(Category category) {
this.category = category;
}
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;
}
public float getPrice() {
return price;
}
public void setPrice(float price) {
this.price = price;
}
}
Set up the Category many-to-one relationship in Product.hbm.xml:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.how2java.pojo">
<class name="Product" table="product_">
<id name="id" column="id">
<generator class="native">
</generator>
</id>
<property name="name" />
<property name="price" />
<many-to-one name="category" class="Category" column="cid" /><!--Use the many-to-one tag to set a many-to-one relationship name="category" corresponds to the category in the Product class The attribute class="Category" represents the corresponding Category class column="cid" represents the foreign key to the category_ table -->
</class>
</hibernate-mapping>
TestHibernate tests many-to-one relationships:
package com.how2java.test;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import com.how2java.pojo.Category;
import com.how2java.pojo.Product;
public class TestHibernate {
public static void main(String[] args) {
SessionFactory sf = new Configuration().configure().buildSessionFactory();
Session s = sf.openSession();
s.beginTransaction();
//The core code, in this test example, adds a new Category object "c1" and sets it to the category of the product with id=8
Category c =new Category();
c.setName("c1");
s.save(c);
Product p = (Product) s.get(Product.class, 8);
p.setCategory(c);
s.update(p);
s.getTransaction().commit();
s.close();
sf.close();
}
}
Then implement a one-to-many relationship: Category and Product are one-to-many relationships.
Add a Set collection to Category:
package com.how2java.pojo;
import java.util.Set;
public class Category {
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;
}
int id;
String name;
Set<Product> products;//Core code, add a set collection
public Set<Product> getProducts() {
return products;
}
public void setProducts(Set<Product> products) {
this.products = products;
}
}
Add one-to-many mapping to Category.hbm.xml:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.how2java.pojo">
<class name="Category" table="category_">
<id name="id" column="id">
<generator class="native">
</generator>
</id>
<property name="name" />
<!--Core code-->
<set name="products" lazy="false"><!--set is used to set a one-to-many (many-to-many) relationship. You can also use a list. The setting is a little more complicated. Here, a simple set is used to get started. . name="products" corresponds to the products attribute in the Category class. lazy="false" means not to use lazy loading. -->
<key column="cid" not-null="false" /><!-- Indicates that the foreign key is cid and can be empty -->
<one-to-many class="Product" /><!-- Indicates that the class corresponding to one-to-many is Product-->
</set>
</class>
</hibernate-mapping>
TestHibernate tests one-to-many relationships
package com.how2java.test;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import com.how2java.pojo.Category;
import com.how2java.pojo.Product;
public class TestHibernate {
public static void main(String[] args) {
SessionFactory sf = new Configuration().configure().buildSessionFactory();
Session s = sf.openSession();
s.beginTransaction();
//core code
Category c = (Category) s.get(Category.class, 1);
Set<Product> ps = c.getProducts();
for (Product p : ps) {
System.out.println(p.getName());
}
s.getTransaction().commit();
s.close();
sf.close();
}
}
Settings for many-to-many relationships :
A Product can be purchased by multiple Users
A User can purchase multiple Products
So the relationship between Product and User is many-to-many many-to-many
To achieve a many-to-many relationship, there must be an intermediate table user_product to maintain the relationship between User and Product
Prepare User.java and User.hbm.xml first.
package com.how2java.pojo;
import java.util.Set;
public class User {
int id;
String name;
Set<Product> products;
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;
}
public Set<Product> getProducts() {
return products;
}
public void setProducts(Set<Product> products) {
this.products = products;
}
}
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.how2java.pojo">
<class name="User" table="user_">
<id name="id" column="id">
<generator class="native">
</generator>
</id>
<property name="name" />
<!-- The core code, products are provided by USer, and then in the user_product table, user corresponds to uid, product corresponds to pid-->
<set name="products" table="user_product" lazy="false">
<key column="uid" />
<many-to-many column="pid" class="Product" />
</set>
</class>
</hibernate-mapping>
Product.java adds a collection of corresponding Users
package com.how2java.pojo;
import java.util.Set;
public class Product {
int id;
String name;
float price;
Category category;
Set<User> users;//Add the corresponding Users and set methods and get methods
public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}
public Category getCategory() {
return category;
}
public void setCategory(Category category) {
this.category = category;
}
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;
}
public float getPrice() {
return price;
}
public void setPrice(float price) {
this.price = price;
}
}
The settings of Product.hbm.xml are the same:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.how2java.pojo">
<class name="Product" table="product_">
<id name="id" column="id">
<generator class="native">
</generator>
</id>
<property name="name" />
<property name="price" />
<many-to-one name="category" class="Category" column="cid" />
<!-- Core code-->
<set name="users" table="user_product" lazy="false">
<key column="pid" />
<many-to-many column="uid" class="User" />
</set>
</class>
</hibernate-mapping>
Add the mapping of User in hibernate.cfg.xml.
TestHibernate tests many-to-many relationships
First add 3 users
Then demo product 1 is purchased by users 1, 2, 3.
package com.how2java.test;
import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import com.how2java.pojo.Product;
import com.how2java.pojo.User;
public class TestHibernate {
public static void main(String[] args) {
SessionFactory sf = new Configuration().configure().buildSessionFactory();
Session s = sf.openSession();
s.beginTransaction();
//add 3 users
Set<User> users = new HashSet();
for (int i = 0; i < 3; i++) {
User u =new User();
u.setName("user"+i);
users.add(u);
s.save(u);
}
// Product 1 was purchased by users 1, 2, 3
Product p1 = (Product) s.get(Product.class, 1);
p1.setUsers(users);
s.save(p1);
s.getTransaction().commit();
s.close();
sf.close();
}
}