Hibernate Entity Mapping: May-to-many relationship with a join table

Map a many-to-many relationship between 2 entities with a link table in the database schema.

1) Create the Category and Item entity persistent classes.

Category.java

package info.icontraining.hibernate;

import java.util.*;

public class Category {
 
   private Long id;
   private String categoryName;
   private Set<Item> items = new HashSet<Item>() ;
 
   public Long getId() {
      return id;
   }
   public void setId(Long id) {
      this.id = id;
   }
   public String getCategoryName() {
      return categoryName;
   }
   public void setCategoryName(String categoryName) {
      this.categoryName = categoryName;
   }
   public Set<Item> getItems() {
      return items;
   }
   public void setItems(Set<Item> items) {
      this.items = items;
   }
}

Item.java

package info.icontraining.hibernate;

import java.util.*;

public class Item {

   private Long id;
   private String itemName;
   private Set<Category> categories = new HashSet<Category>();
 
   public Long getId() {
      return id;
   }
   public void setId(Long id) {
      this.id = id;
   }
   public String getItemName() {
      return itemName;
   }
   public void setItemName(String itemName) {
      this.itemName = itemName;
   }
   public Set<Category> getCategories() {
      return categories;
   }
   public void setCategories(Set<Category> categories) {
      this.categories = categories;
   }
}

2) Create the Mapping files,

Category.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 
<hibernate-mapping> 
   <class name="info.icontraining.hibernate.Category" table="CATEGORY_TABLE"> 
      <id name="id" column="CATEGORY_ID"> 
         <generator class="native" /> 
      </id> 
      <property name="categoryName" column="CATEGORY_NAME" type="string" />
      <set name="items" table="CATEGORY_ITEM" cascade="save-update">
         <key column="CATEGORY_ID" />
         <many-to-many class="info.icontraining.hibernate.Item" column="ITEM_ID" />
      </set>
   </class> 
</hibernate-mapping>

Item.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
   "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
   "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> 
<hibernate-mapping> 
   <class name="info.icontraining.hibernate.Item" table="ITEM_TABLE"> 
      <id name="id" column="ITEM_ID"> 
         <generator class="native" /> 
      </id> 
      <property name="itemName" column="ITEM_NAME" type="string" />
      <set name="categories" table="CATEGORY_ITEM" cascade="save-update" inverse="true">
         <key column="ITEM_ID" />
         <many-to-many class="info.icontraining.hibernate.Category" column="CATEGORY_ID" />
      </set>
   </class> 
</hibernate-mapping>

3) In the hibernate.cfg.xml file, add the following configuration,

<mapping resource="info/icontraining/hibernate/Item.hbm.xml" />
<mapping resource="info/icontraining/hibernate/Category.hbm.xml" />

4) Create the client code in a servlet, as follows,

package info.icontraining.servlets;

import java.io.*; 
import java.util.*; 
import javax.servlet.*; 
import javax.servlet.http.* ; 
import org.hibernate.*; 
import info.icontraining.hibernate.*;

public class CategoryItem extends HttpServlet { 
  
   public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException{ 
  
      PrintWriter out = res.getWriter(); 

      Session session = HibernateUtil.getSessionFactory().openSession(); 
      Transaction tx = session.beginTransaction(); 
   
      Category category1 = new Category();
      category1.setCategoryName("Category-1");
   
      Category category2 = new Category();
      category2.setCategoryName("Category-2");
   
      Item item1 = new Item();
      item1.setItemName("Item-1");
   
      Item item2 = new Item();
      item2.setItemName("Item-2");
 
      category1.getItems().add(item1);
      category1.getItems().add(item2);
   
      category2.getItems().add(item1);
      category2.getItems().add(item2);
  
      item1.getCategories().add(category1);
      item1.getCategories().add(category2);
   
      item2.getCategories().add(category1);
      item2.getCategories().add(category2);
   
      session.save(category1);
      session.save(category2);
   
      tx.commit(); 
      session.close(); 

      Session newSession = HibernateUtil.getSessionFactory().openSession(); 
      Transaction newTx = newSession.beginTransaction(); 
   
      List<Category> categories = newSession.createQuery("from Category").list(); 
      out.println( categories.size() + " categories found:" ); 
      for ( Iterator<Category> iter = categories.iterator(); iter.hasNext(); ) { 
         Category cat = iter.next(); 
         out.println( "Name of Category: " + cat.getCategoryName()); 
         out.println( "Number of Items in Category: " + cat.getItems().size());
      } 
   
      newTx.commit(); 
      newSession.close();
   } 
}

5) Configure the servlet in a web.xml file,

<servlet>
   <servlet-name>CategoryItem</servlet-name>
   <servlet-class>info.icontraining.servlets.CategoryItem</servlet-class>
</servlet>
<servlet-mapping>
   <servlet-name>CategoryItem</servlet-name>
   <url-pattern>/categoryItem</url-pattern>
</servlet-mapping>

6) Enter the following URL in the browser,

http://localhost:8080/WebAppName/categoryItem

7) Check the database for the created tables and the data in those tables