JPA 菜鸟教程 3 单向多对一

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JPA中的@ManyToOne

单向多对一(@ManyToOne)关联是最常见的单向关联关系。假设多种商品(Product)可以有一个商品类型(ProductType),只关心商品(Product)实体找到对应的商品类型(ProductType)实体，而无须关心从某个商品类型(ProductType)找到全部商品(Product).

单向多对一表的ddl语句

CREATE TABLE `t_product_type` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8;

CREATE TABLE `t_product` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
`type_id` bigint(20) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `FK_oyt6r2g6hwbyee5adel4yj59e` (`type_id`),
CONSTRAINT `FK_oyt6r2g6hwbyee5adel4yj59e` FOREIGN KEY (`type_id`) REFERENCES `t_product_type` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;

Product

package com.jege.jpa.many2one;

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

/**
* @author JE哥
* @email 1272434821@qq.com
* @description:单向：多个产品属于同一个产品类型
*/
@Entity
@Table(name = "t_product")
public class Product {
@Id
@GeneratedValue
private Long id;
private String name;
// 多对一：optional=false表示外键type_id不能为空
@ManyToOne(optional = true)
@JoinColumn(name = "type_id")
private ProductType type;

public Product() {
}

public Product(String name, ProductType type) {
this.name = name;
this.type = type;
}

public Long getId() {
return id;
}

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

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public ProductType getType() {
return type;
}

public void setType(ProductType type) {
this.type = type;
}

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

}

ProductType

package com.jege.jpa.many2one;

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

/**
* @author JE哥
* @email 1272434821@qq.com
* @description:单向
*/
@Entity
@Table(name = "t_product_type")
public class ProductType {
@Id
@GeneratedValue
private Long id;
private String name;

public Long getId() {
return id;
}

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

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

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

}

Many2OneTest

package com.jege.jpa.many2one;

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

import org.junit.After;
import org.junit.AfterClass;
import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Test;
/**
* @author JE哥
* @email 1272434821@qq.com
* @description:单向多对一Test
*/
public class Many2OneTest {
private static EntityManagerFactory entityManagerFactory = null;
private EntityManager entityManager = null;

@BeforeClass
public static void setUpBeforeClass() throws Exception {
entityManagerFactory = Persistence.createEntityManagerFactory("com.jege.jpa");
}

// 要求：一次性保存1个产品类型，保存2个产品
// 单向多对一保存的时候必须先保存一方，否则会出现多余的update语句，从而影响性能
@Before
public void persist() throws Exception {
entityManager = entityManagerFactory.createEntityManager();
entityManager.getTransaction().begin();

ProductType type = new ProductType();
type.setName("产品类型1");

// 传入type的本质是处理数据库product表的type_id外键
Product product1 = new Product("产品1", null);
Product product2 = new Product("产品2", type);

System.out.println("保存之前：" + type);
entityManager.persist(type);// JPA会自动把保存后的主键放到当前对象的id里面
System.out.println("保存之后：" + type);
entityManager.persist(product1);
entityManager.persist(product2);
System.out.println("++++++++++++++++++++");
}

// 可以通过多方Product获取一方ProductType是否为null，来判断是否有产品类型
@Test
public void find() throws Exception {
Product product = entityManager.find(Product.class, 2L);
System.out.println(product);
ProductType type = product.getType();
if (type == null) {
System.out.println("当前产品是没有产品类型的");
} else {
System.out.println("当前产品是有产品类型的");
}
}

@After
public void tearDown() throws Exception {
entityManager.getTransaction().commit();
if (entityManager != null && entityManager.isOpen())
entityManager.close();
}

@AfterClass
public static void tearDownAfterClass() throws Exception {
if (entityManagerFactory != null && entityManagerFactory.isOpen())
entityManagerFactory.close();
}

}

其他关联项目

源码地址