@OneToMany或@ManyToOne的用法-annotation关系映射篇（上)

例如我们用一个例子来开启JPA的一对多和多对一的学习。

比如你去当当网上买书籍，当当网就给你一个订单。

通过上面的业务活动描述你可以分析得出：一个订单可以包含一个或多个订单项.那么我们将将订单和订单项设计关系为(1:N)一对多的关系(排除0)。得出关系我们就使用JPA来实现这个关系(关于建立JPA和基本的配置我就不再说了，如果不明白请看JPA入门篇)。

首先建立订单实体类

Java代码



import java.util.HashSet;


import java.util.Set;



import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.Id;

import javax.persistence.OneToMany;

import javax.persistence.Table;



@Entity

@Table(name="orders")


public class Order {




private String orderid;


private Float amount = 0f;


private Set<OrderItem> items = new HashSet<OrderItem>();




@Id

@Column(length = 12)


public String getOrderid() {


return orderid;


}



public void setOrderid(String orderid) {


this.orderid = orderid;


}



@Column(nullable = false)


public Float getAmount() {


return amount;


}



public void setAmount(Float amount) {


this.amount = amount;


}



@OneToMany(cascade = { CascadeType.REFRESH, CascadeType.PERSIST,CascadeType.MERGE, CascadeType.REMOVE },mappedBy ="order") //这里配置关系，并且确定关系维护端和被维护端。mappBy表示关系被维护端，只有关系端有权去更新外键。这里还有注意OneToMany默认的加载方式是赖加载。当看到设置关系中最后一个单词是Many，那么该加载默认为懒加载

public Set<OrderItem> getItems() {


return items;


}



public void setItems(Set<OrderItem> items) {


this.items = items;


}



/**

*该方法用于向order中加order项

/*

public void addOrderItem(OrderItem orderItem){


orderItem.setOrder(this);//用关系维护端来维护关系

this.items.add(orderItem);


}



}

订单项的实体类

Java代码



import javax.persistence.CascadeType;


import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.ManyToOne;



@Entity

public class OrderItem {


private Integer id;


private String productName;


private Float sellPrice = 0f;


private Order order;




@Id

@GeneratedValue

public Integer getId() {


return id;


}



public void setId(Integer id) {


this.id = id;


}



@Column(length = 40, nullable = false)


public String getProductName() {


return productName;


}



public void setProductName(String productName) {


this.productName = productName;


}



@Column(nullable = false)


public Float getSellPrice() {


return sellPrice;


}



public void setSellPrice(Float sellPrice) {


this.sellPrice = sellPrice;


}



@ManyToOne(cascade = {CascadeType.MERGE,CascadeType.REFRESH }, optional = true)


@JoinColumn(name="order_id")//这里设置JoinColum设置了外键的名字，并且orderItem是关系维护端

public Order getOrder() {


return order;


}



public void setOrder(Order order) {


this.order = order;


}



}

下面看看测试类

Java代码



import javax.persistence.EntityManager;


import javax.persistence.EntityManagerFactory;

import javax.persistence.Persistence;



import org.junit.BeforeClass;

import org.junit.Test;



import cn.itcast.bean.Order;

import cn.itcast.bean.OrderItem;



public class OneToManyTest {




@BeforeClass

public static void setUpBeforeClass() throws Exception {


}

@Test public void addOrder(){


EntityManagerFactory factory = Persistence.createEntityManagerFactory("test");


EntityManager em = factory.createEntityManager();

em.getTransaction().begin(); // start transaction



Order order = new Order();

order.setAmount(34f);

order.setOrderid("00001");




//order中包含的OrderItem项OrderItem1，OrderItem2

OrderItem orderItem1 = new OrderItem();


orderItem1.setProductName("书");


orderItem1.setSellPrice(22f);

order.addOrderItem(orderItem1); //add orderitem in order



OrderItem orderItem2 = new OrderItem();


orderItem2.setProductName("篮球");


orderItem2.setSellPrice(100f);

order.addOrderItem(orderItem2);



em.persist(order); //persist order object

em.getTransaction().commit(); //commit transaction

em.close();

factory.close();

}

}