转@ManyToMany- annotation关系映射篇（下）

原文：http://blog.sina.com.cn/s/blog_6fef491d0100obdd.html

终于要说ManyToMany了

场景：Product和Customer。

先看TestProduct.java

 package net.paoding.forum.domain;   

 import java.util.ArrayList;
 import java.util.List;   

 import javax.persistence.Entity;
 import javax.persistence.Id;
 import javax.persistence.ManyToMany;   

 @Entity
 public class TestProduct
 {
     private String             id;
     private String             name;
     private float              price;
     private List<TestCustomer> customers = new ArrayList<TestCustomer>();   

     @Id
     public String getId()
     {
         return id;
     }   

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

     public String getName()
     {
         return name;
     }   

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

     public float getPrice()
     {
         return price;
     }   

     public void setPrice(float price)
     {
         this.price = price;
     }   

     @ManyToMany
     public List<TestCustomer> getCustomers()
     {
         return customers;
     }   

     public void setCustomers(List<TestCustomer> customers)
     {
         this.customers = customers;
     }   

 }  

注意这里的ManyToMany什么都没有写。

再看TestCustomer.java

 package net.paoding.forum.domain;   

 import java.util.ArrayList;
 import java.util.List;   

 import javax.persistence.Entity;
 import javax.persistence.Id;
 import javax.persistence.ManyToMany;   

 @Entity
 public class TestCustomer
 {
     private String            id;
     private String            tel;
     private List<TestProduct> products = new ArrayList<TestProduct>();   

     @Id
     public String getId()
     {
         return id;
     }   

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

     public String getTel()
     {
         return tel;
     }   

     public void setTel(String tel)
     {
         this.tel = tel;
     }   

     @ManyToMany(mappedBy = "customers")
     public List<TestProduct> getProducts()
     {
         return products;
     }   

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

这里的ManyToMany我写了mappedBy这个attribute。

然后看hib产生的sql:

  drop table test_customer cascade constraints;
 drop table test_product cascade constraints;
 drop table test_product_customers cascade constraints;   

 create table test_customer (
         id varchar2(255 char) not null,
         tel varchar2(255 char),
         primary key (id)
 );   

 create table test_product (
     id varchar2(255 char) not null,
     price float not null,
     name varchar2(255 char),
     primary key (id)
 );   

 create table test_product_customers (
     products_id varchar2(255 char) not null,
     customers_id varchar2(255 char) not null
 );  

ok! 非常好。hib终于在ManyToMany上没有犯白痴了。

上面强调了mappedBy这个属性。其实，在annotation 系列中。都有提到mappedBy这个东西。只是，我没有说到底是什么意思。其实很简单：这个东西就相当于xml配置中的inverse。写了mappedBy就代表这个方法的返回值是被维护方