many-to-many

//-----------------------------关联表---------------------------------
@Entity
@Table(name = "CATEGORY_ITEM")
@org.hibernate.annotations.Immutable
public class CategorizedItem {
//使用符合主键
@Embeddable
public static class Id implements Serializable {
@Column(name = "CATEGORY_ID")
protected Long categoryId;
@Column(name = "ITEM_ID")
protected Long itemId;
public Id() {
}
public Id(Long categoryId, Long itemId) {
this.categoryId = categoryId;
this.itemId = itemId;
}
public boolean equals(Object o) {
if (o != null && o instanceof Id) {
Id that = (Id) o;
return this.categoryId.equals(that.categoryId)
&& this.itemId.equals(that.itemId);
}
return false;
}
public int hashCode() {
return categoryId.hashCode() + itemId.hashCode();
}
@Column(updatable = false)
@NotNull
protected String addedBy;
@Column(updatable = false)
@NotNull
protected Date addedOn = new Date(); F Maps timestamp

//映射为OneToMany
@ManyToOne
@JoinColumn(
name = "CATEGORY_ID",insertable = false, updatable = false)
protected Category category; G Maps category
@ManyToOne
@JoinColumn(
name = "ITEM_ID",insertable = false, updatable = false)
protected Item item; H Maps item

//值id,由程序生成
@EmbeddedId
protected Id id = new Id();
public CategorizedItem(
String addedByUsername,
Category category,
Item item) {
this.addedBy = addedByUsername;
this.category = category;
this.item = item;
this.id.categoryId = category.getId();
this.id.itemId = item.getId();
category.getCategorizedItems().add(this);
item.getCategorizedItems().add(this);
}
// ...
}

//-------------------------两个多对多实体-----------------------------------
@Entity
public class Category {
//放弃管理
@OneToMany(mappedBy = "category")
protected Set<CategorizedItem> categorizedItems = new HashSet<>();
// ...
}
@Entity
public class Item {
@OneToMany(mappedBy = "item")
protected Set<CategorizedItem> categorizedItems = new HashSet<>();
// ...
}

//-------------------------------持久化---------------------------------

Category someCategory = new Category("Some Category");
Category otherCategory = new Category("Other Category");
em.persist(someCategory);
em.persist(otherCategory);
Item someItem = new Item("Some Item");
Item otherItem = new Item("Other Item");
em.persist(someItem);
em.persist(otherItem);
CategorizedItem linkOne = new CategorizedItem(
"johndoe", someCategory, someItem
);
CategorizedItem linkTwo = new CategorizedItem(
"johndoe", someCategory, otherItem
);
CategorizedItem linkThree = new CategorizedItem(
"johndoe", otherCategory, someItem
);
em.persist(linkOne);
em.persist(linkTwo);
em.persist(linkThree);