JPA笔记4 ManyToMany

 

package many_to_many;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Table(name = "JPA_CATEGORY")
@Entity
public class Category {

    private Integer id;
    private String categoryName;
    private Set<Item> items = new HashSet<>();

    @GeneratedValue(strategy = GenerationType.AUTO)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    @ManyToMany(mappedBy = "categories")
    public Set<Item> getItems() {
        return items;
    }

    public void setItems(Set<Item> items) {
        this.items = items;
    }

}

 

package many_to_many;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

@Table(name = "JPA_ITEM")
@Entity
public class Item {

    private Integer id;
    private String itemName;
    private Set<Category> categories = new HashSet<>();

    @GeneratedValue(strategy = GenerationType.AUTO)
    @Id
    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    @Column(name = "ITEM_NAME")
    public String getItemName() {
        return itemName;
    }

    public void setItemName(String itemName) {
        this.itemName = itemName;
    }

    /*
     * 使用@ManyToMany注解来映射多对多关联关系
     * 使用@JoinTable来映射中间表
     * 1.name指向中间表的名字
     * 2.j joinColumns映射当前类所在的表在中间表中的外键
     * 2.1 name指定外键列的列名
     * 2.2 referencedColumnName指定外键列关联当前表的哪一列
     * 3. inverseJoinColumns映射关联的类所在中间表的外键
     * */
    @JoinTable(name = "ITEM_CATEGORY", joinColumns = { @JoinColumn(name = "ITEM_ID", referencedColumnName = "ID") }, inverseJoinColumns = { @JoinColumn(name = "CATEGORY_ID", referencedColumnName = "ID") })
    @ManyToMany
    public Set<Category> getCategories() {
        return categories;
    }

    public void setCategories(Set<Category> categories) {
        this.categories = categories;
    }

}

 

package many_to_many;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.EntityTransaction;
import javax.persistence.Persistence;

public class Main4 {

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        String persistenceUnitName = "jpa-1";
        EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory(persistenceUnitName);

        EntityManager entityManager = entityManagerFactory.createEntityManager();

        EntityTransaction transaction = entityManager.getTransaction();
        transaction.begin();

        insert(entityManager);

        transaction.commit();
        entityManager.close();
        entityManagerFactory.close();
    }

    // 1.多对多的保存
    private static void insert(EntityManager entityManager) {
        Item i1 = new Item();
        i1.setItemName("i-1");
        Item i2 = new Item();
        i2.setItemName("i-2");

        Category c1 = new Category();
        c1.setCategoryName("c-1");

        Category c2 = new Category();
        c2.setCategoryName("c-2");

        i1.getCategories().add(c1);
        i1.getCategories().add(c2);

        i2.getCategories().add(c1);
        i2.getCategories().add(c2);

        c1.getItems().add(i1);
        c1.getItems().add(i2);

        c2.getItems().add(i1);
        c2.getItems().add(i2);

        entityManager.persist(i1);
        entityManager.persist(i2);
        entityManager.persist(c1);
        entityManager.persist(c2);
    }

}