JPA系列九：映射关联关系-双向多对多

╰半夏微凉° 2022-02-09 11:09 299阅读 0赞

1、创建实体类Category

@Table(name="JPA_CATEGORIES")
    @Entity
    public class Category {
    
    	private Integer id;
    	private String categoryName;
    	
    	private Set<Item> items = new HashSet<>();
    
    	@GeneratedValue
    	@Id
    	public Integer getId() {
    		return id;
    	}
    
    	public void setId(Integer id) {
    		this.id = id;
    	}
    
    	@Column(name="CATEGORY_NAME")
    	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;
    	}
    }

2、创建实体类Item

@Table(name="JPA_ITEMS")
    @Entity
    public class Item {
    
    	private Integer id;
    	private String itemName;
    	
    	private Set<Category> categories = new HashSet<>();
    
    	@GeneratedValue
    	@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. 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;
    	}
    }

3、在persistence.xml中配置

<class>com.atguigu.jpa.helloworld.Item</class>
     <class>com.atguigu.jpa.helloworld.Category</class>

4、测试：

@Test
    public void testManyToManyPersist(){
    	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");
    	//设置关联关系
    	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);
    	//执行保存
    	entityManager.persist(i1);
    	entityManager.persist(i2);
    	entityManager.persist(c1);
    	entityManager.persist(c2);
    }

控制台输出：

Hibernate: 
        insert 
        into
            JPA_ITEMS
            (ITEM_NAME) 
        values
            (?)
    Hibernate: 
        insert 
        into
            JPA_ITEMS
            (ITEM_NAME) 
        values
            (?)
    Hibernate: 
        insert 
        into
            JPA_CATEGORIES
            (CATEGORY_NAME) 
        values
            (?)
    Hibernate: 
        insert 
        into
            JPA_CATEGORIES
            (CATEGORY_NAME) 
        values
            (?)
    Hibernate: 
        insert 
        into
            ITEM_CATEGORY
            (ITEM_ID, CATEGORY_ID) 
        values
            (?, ?)
    Hibernate: 
        insert 
        into
            ITEM_CATEGORY
            (ITEM_ID, CATEGORY_ID) 
        values
            (?, ?)
    Hibernate: 
        insert 
        into
            ITEM_CATEGORY
            (ITEM_ID, CATEGORY_ID) 
        values
            (?, ?)
    Hibernate: 
        insert 
        into
            ITEM_CATEGORY
            (ITEM_ID, CATEGORY_ID) 
        values
            (?, ?)

//对于关联的集合对象, 默认使用懒加载的策略.
    //使用维护关联关系的一方获取, 还是使用不维护关联关系的一方获取, SQL 语句相同. 
    @Test
    public void testManyToManyFind(){
    	Item item = entityManager.find(Item.class, 2);
    	System.out.println(item.getItemName());
    	System.out.println(item.getCategories().size());
    	
    	Category category = entityManager.find(Category.class, 3);
    	System.out.println(category.getCategoryName());
    	System.out.println(category.getItems().size());
    }

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