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Hibernate – Many-to-many relationship example

февруари 8th, 2010 admin

“Many-to-many” example

This is a many-to-many relationship table design, a STOCK table has more than one CATEGORY, and CATEGORY can belong to more than one STOCK, the relationship is linked with a third table called STOCK_CATEGORY.

many-to-many-relationship

Hibernate implementation

In Hibernate, this “many-to-many” relationship can implemented in two ways :

  • Hibernate XML Mapping file
  • Hibernate Annotation

1. Hibernate XML mapping file

In XML mapping way, you need three Java classes to hold the table data,

  • Stock.java -> STOCK table
  • Category.java -> CATEGORY table
  • StockCategory.java -> STOCK_CATEGORY table

and three XML mapping files to describe the table relationship.

  • Stock.hbm.xml -> STOCK table
  • Category.hbm.xml -> CATEGORY table
  • StockCategory.hbm.xml -> STOCK_CATEGORY table
Stock.java
public class Stock implements java.io.Serializable {
   private Set<StockCategory> stockCategories = new HashSet<StockCategory>(0);
   ...
   public Set<StockCategory> getStockCategories() {
	return this.stockCategories;
   }

   public void setStockCategories(Set<StockCategory> stockCategories) {
	this.stockCategories = stockCategories;
   }
Category.java
public class Category implements java.io.Serializable {
   private Set<StockCategory> stockCategories = new HashSet<StockCategory>(0);
   ...
   public Set<StockCategory> getStockCategories() {
	return this.stockCategories;
   }

   public void setStockCategories(Set<StockCategory> stockCategories) {
	this.stockCategories = stockCategories;
   }
StockCategory.java
public class StockCategory implements java.io.Serializable {
   private Integer stockCategoryId;
   private Stock stock;
   private Category category;
   ...
Stock.hbm.xml
...
<class name="com.mkyong.common.Stock" table="stock" ...>
   <id name="stockId" type="java.lang.Integer">
      <column name="STOCK_ID" />
      <generator class="identity" />
   </id>
   ...
   <set name="stockCategories" inverse="true" lazy="true" 
       table="stock_category" ...>
       <key>
         <column name="STOCK_ID" not-null="true" />
       </key>
          <one-to-many class="com.mkyong.common.StockCategory" />
   </set>
...

Set inverse=”true” in set variable, Stock does not want to maintain the relationship.

Category.hbm.xml
...
<class name="com.mkyong.common.Category" table="category" ...>
   <id name="categoryId" type="java.lang.Integer">
       <column name="CATEGORY_ID" />
       <generator class="identity" />
   </id>
   ...
   <set name="stockCategories" inverse="true" lazy="true" 
       table="stock_category" ...>
      <key>
         <column name="CATEGORY_ID" not-null="true" />
      </key>
         <one-to-many class="com.mkyong.common.StockCategory" />
   </set>
...

Set inverse=”true” in set variable, Category does not want to maintain the relationship also.

StockCategory.hbm.xml
...
<class name="com.mkyong.common.StockCategory" table="stock_category" ...>
   <id name="stockCategoryId" type="java.lang.Integer">
      <column name="STOCK_CATEGORY_ID" />
      <generator class="identity" />
   </id>
   <many-to-one name="stock" class="com.mkyong.common.Stock" ...>
       <column name="STOCK_ID" not-null="true" />
   </many-to-one>
   <many-to-one name="category" class="com.mkyong.common.Category" ...>
        <column name="CATEGORY_ID" not-null="true" />
    </many-to-one>
</class>
...

StockCategory maintain the relationship between Stock and Category, see inverse=”true” explanation.

2. Hibernate annotation

In annotation way, three classes are required, all the relationships are declared inside the Java classes.

  • Stock.java -> STOCK table
  • category.java -> CATEGORY table
  • StockCategory.java -> STOCK_CATEGORY table
Stock.java
...
@Entity
@Table(name = "stock", catalog = "mkyong", uniqueConstraints = {
		@UniqueConstraint(columnNames = "STOCK_NAME"),
		@UniqueConstraint(columnNames = "STOCK_CODE") })
public class Stock implements java.io.Serializable {
	private Set<StockCategory> stockCategories =
                                         new HashSet<StockCategory>(0);
	...
	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "STOCK_ID", unique = true, nullable = false)
	public Integer getStockId() {
		return this.stockId;
	}
        ...
	@OneToMany(fetch = FetchType.LAZY, mappedBy = "stock")
	public Set<StockCategory> getStockCategories() {
		return this.stockCategories;
	}

	public void setStockCategories(Set<StockCategory> stockCategories) {
		this.stockCategories = stockCategories;
	}
...
Category.java
...
@Entity
@Table(name = "category", catalog = "mkyong")
public class Category implements java.io.Serializable {
	private Set<StockCategory> stockCategories =
                                       new HashSet<StockCategory>(0);
        ...
	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "CATEGORY_ID", unique = true, nullable = false)
	public Integer getCategoryId() {
		return this.categoryId;
	}
	...
	@OneToMany(fetch = FetchType.LAZY, mappedBy = "category")
	public Set<StockCategory> getStockCategories() {
		return this.stockCategories;
	}

	public void setStockCategories(Set<StockCategory> stockCategories) {
		this.stockCategories = stockCategories;
	}
...
StockCategory.java
@Entity
@Table(name = "stock_category", catalog = "mkyong",
        uniqueConstraints = @UniqueConstraint(columnNames = {
        "STOCK_ID", "CATEGORY_ID" }))
public class StockCategory implements java.io.Serializable {
	private Integer stockCategoryId;
	private Stock stock;
	private Category category;
	...
	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "STOCK_CATEGORY_ID", unique = true, nullable = false)
	public Integer getStockCategoryId() {
		return this.stockCategoryId;
	}

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "STOCK_ID", nullable = false)
	public Stock getStock() {
		return this.stock;
	}

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "CATEGORY_ID", nullable = false)
	public Category getCategory() {
		return this.category;
	}
...

The code is self-explanatory, all the @annotation are the XML mapping tag substitution.

Test It

Run the example, both annotation and XML mapping file will generate the same output. Hibernate inserts a row into the STOCK table, a row into the CATEGORY table, and both relationship in STOCK_CATEGORY table.

        Stock stock = new Stock();
        Category category = new Category();
        ...
        session.save(stock);
        session.save(category);
        ...
        StockCategory stockCategory = new StockCategory(stock, category);
        session.save(stockCategory);
Output
Hibernate:
    insert into mkyong.stock
    (STOCK_CODE, STOCK_NAME)
    values (?, ?)

Hibernate:
    insert into mkyong.category
    (`DESC`, NAME)
    values (?, ?)

Hibernate:
    insert into mkyong.stock_category
    (CATEGORY_ID, STOCK_ID)
    values (?, ?)

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