JPA学习笔记九多对多(单向/双向)

单向
学生就正常来

package com.yjj.entity2;

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

@Entity
@Table(name = "t_student")
public class Student {
    @Id
    @GeneratedValue
    private Long id;
    private String sname;

    public Student() {

    }

    public Student(String sname) {
        this.sname = sname;
    }

    public Long getId() {
        return id;
    }

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

    public String getSname() {
        return sname;
    }

    public void setSname(String sname) {
        this.sname = sname;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", sname=" + sname + "]";
    }
}

老师里维护一个学生集合

package com.yjj.entity2;

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

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;


@Entity
@Table(name = "t_teacher")
public class Teacher {
    @Id
    @GeneratedValue
    private Long id;
    private String tname;
    // @ManyToMany注释表示Teacher是多对多关系的一端。
    // @JoinTable描述了多对多关系的数据表关系。name属性指定中间表名称，joinColumns定义中间表与Teacher表的外键关系。
    // 中间表Teacher_Student的Teacher_ID列是Teacher表的主键列对应的外键列，inverseJoinColumns属性定义了中间表与另外一端(Student)的外键关系。
    @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
    @JoinTable(name = "t_teacher_student", joinColumns = { @JoinColumn(name = "teacher_id") }, inverseJoinColumns = {
            @JoinColumn(name = "student_id") })
    private Set<Student> students = new HashSet<Student>();

    public Teacher() {

    }

    public Teacher(String tname) {
        this.tname = tname;
    }

    public Long getId() {
        return id;
    }

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

    public String getTname() {
        return tname;
    }

    public void setTname(String tname) {
        this.tname = tname;
    }

    public Set<Student> getStudents() {
        return students;
    }

    public void setStudents(Set<Student> students) {
        this.students = students;
    }

    @Override
    public String toString() {
        return "Teacher [id=" + id + ", tname=" + tname + "]";
    }
}

测试类

  Teacher t1 = new Teacher("t1");
        Teacher t2 = new Teacher("t2");

        Student s1 = new Student("s1");
        Student s2 = new Student("s2");
        Student s3 = new Student("s3");

        entityManager.persist(t1);
        entityManager.persist(t2);

        entityManager.persist(s1);
        entityManager.persist(s2);
        entityManager.persist(s3);// 全部发出5条insert单表

        // 添加中间表
        // t1老师教2个学生s1,s2
        // t2老师教3个学生s1,s2,s3
        t1.getStudents().add(s1);
        t1.getStudents().add(s2);

        t2.getStudents().add(s1);
        t2.getStudents().add(s2);
        t2.getStudents().add(s3);
        entityManager.persist(s1);
        entityManager.persist(s2);
        entityManager.persist(s3);
        entityManager.persist(t1);
        entityManager.persist(t2);

双向
产品和类目中都维护一个对方的集合，注意mappedBy=什么

package com.yjj.entity2;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;
/**
 * author:殷俊杰
 */
@Table(name="product")
@Entity
public class Product {
    @Id
    @GeneratedValue
    @Column(name="id")
    private Integer id;
    @Column(name="productName")
    private String productName;

    @JoinTable(name = "Category_Product",
            joinColumns = {@JoinColumn(name = "product_id",referencedColumnName = "id")},
            inverseJoinColumns = {@JoinColumn(name="category_id",referencedColumnName = "id")}
    )
    @ManyToMany
    private List<Category> categories=new ArrayList<>();

    @Override
    public String toString() {
        return "Product{" +
                "id=" + id +
                ", productName='" + productName + '\'' +
                '}';
    }

    public Integer getId() {
        return id;
    }

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

    public String getProductName() {
        return productName;
    }

    public void setProductName(String productName) {
        this.productName = productName;
    }

    public List<Category> getCategories() {
        return categories;
    }

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

package com.yjj.entity2;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;

@Table(name="category")
@Entity
public class Category {
    @Id
    @GeneratedValue
    @Column(name="id")
    private Integer id;
    @Column(name="categoryName")
    private String categoryName;
    @ManyToMany(mappedBy = "categories")
    private List<Product> products=new ArrayList<>();

    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;
    }

    public List<Product> getProducts() {
        return products;
    }

    public void setProducts(List<Product> products) {
        this.products = products;
    }

    @Override
    public String toString() {
        return "Category{" +
                "id=" + id +
                ", categoryName='" + categoryName + '\'' +
                ", products=" + products +
                '}';
    }
}

测试类

Product product=new Product();
       product.setProductName("手机");
       Product product1=new Product();
       product1.setProductName("电脑");

       Category category=new Category();
       category.setCategoryName("数码");
       Category category1=new Category();
       category1.setCategoryName("玩具");

       category.getProducts().add(product);
       category.getProducts().add(product1);

       product.getCategories().add(category);
       product.getCategories().add(category1);

       entityManager.persist(category);
       entityManager.persist(category1);
       entityManager.persist(product);
       entityManager.persist(product1);

product.png

category.png

category_product.png

JPA帮我们创建的三张表

JPA学习笔记九 多对多(单向/双向)

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