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

 1 package net.paoding.forum.domain;   
 2   
 3 import java.util.ArrayList;   
 4 import java.util.List;   
 5   
 6 import javax.persistence.Entity;   
 7 import javax.persistence.Id;   
 8 import javax.persistence.ManyToMany;   
 9   
10 @Entity  
11 public class TestProduct   
12 {   
13     private String             id;   
14     private String             name;   
15     private float              price;   
16     private List<TestCustomer> customers = new ArrayList<TestCustomer>();   
17   
18       
19     @Id  
20     public String getId()   
21     {   
22         return id;   
23     }   
24   
25       
26     public void setId(String id)   
27     {   
28         this.id = id;   
29     }   
30   
31       
32     public String getName()   
33     {   
34         return name;   
35     }   
36   
37       
38     public void setName(String name)   
39     {   
40         this.name = name;   
41     }   
42   
43       
44     public float getPrice()   
45     {   
46         return price;   
47     }   
48   
49       
50     public void setPrice(float price)   
51     {   
52         this.price = price;   
53     }   
54   
55       
56     @ManyToMany  
57     public List<TestCustomer> getCustomers()   
58     {   
59         return customers;   
60     }   
61   
62       
63     public void setCustomers(List<TestCustomer> customers)   
64     {   
65         this.customers = customers;   
66     }   
67   
68 }

注意这里的ManyToMany什么都没有写。

再看TestCustomer.java

 1 package net.paoding.forum.domain;   
 2   
 3 import java.util.ArrayList;   
 4 import java.util.List;   
 5   
 6 import javax.persistence.Entity;   
 7 import javax.persistence.Id;   
 8 import javax.persistence.ManyToMany;   
 9   
10 @Entity  
11 public class TestCustomer   
12 {   
13     private String            id;   
14     private String            tel;   
15     private List<TestProduct> products = new ArrayList<TestProduct>();   
16   
17       
18     @Id  
19     public String getId()   
20     {   
21         return id;   
22     }   
23   
24       
25     public void setId(String id)   
26     {   
27         this.id = id;   
28     }   
29   
30       
31     public String getTel()   
32     {   
33         return tel;   
34     }   
35   
36       
37     public void setTel(String tel)   
38     {   
39         this.tel = tel;   
40     }   
41   
42       
43     @ManyToMany(mappedBy = "customers")   
44     public List<TestProduct> getProducts()   
45     {   
46         return products;   
47     }   
48   
49       
50     public void setProducts(List<TestProduct> products)   
51     {   
52         this.products = products;   
53     }   
54 }

这里的ManyToMany我写了mappedBy这个attribute。

然后看hib产生的sql:

 1  drop table test_customer cascade constraints;   
 2 drop table test_product cascade constraints;   
 3 drop table test_product_customers cascade constraints;   
 4   
 5 create table test_customer (   
 6         id varchar2(255 char) not null,   
 7         tel varchar2(255 char),   
 8         primary key (id)   
 9 );   
10   
11 create table test_product (   
12     id varchar2(255 char) not null,   
13     price float not null,   
14     name varchar2(255 char),   
15     primary key (id)   
16 );   
17   
18 create table test_product_customers (   
19     products_id varchar2(255 char) not null,   
20     customers_id varchar2(255 char) not null  
21 );

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