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在EJB3中需要使用@ManyToMany對(duì)封裝多對(duì)多關(guān)系的字段或getter方法進(jìn)行注釋。先看看下面的表：

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圖1  t_addresses表

t_addresses表和t_customers表是多對(duì)多的關(guān)系。需要使用一個(gè)關(guān)聯(lián)表來(lái)描述這種關(guān)系，關(guān)聯(lián)表的結(jié)構(gòu)如下圖所示。

t_customers_addresses

在Customer類(lèi)中定義一個(gè)Collection < ADDRESS>類(lèi)型的字段（addresses），用于保存與該Customer對(duì)象相對(duì)應(yīng)的多個(gè)Address對(duì)象，代碼如下：

 
 
 
 

  
  
  
  
package entity;  
  
  
  
  
 
  
  
  
  
import java.util.Collection;  
  
  
  
  
 
  
  
  
  
import javax.persistence.CascadeType;  
  
  
  
  
import javax.persistence.Entity;  
  
  
  
  
import javax.persistence.FetchType;  
  
  
  
  
import javax.persistence.GeneratedValue;  
  
  
  
  
import javax.persistence.GenerationType;  
  
  
  
  
import javax.persistence.Id;  
  
  
  
  
import javax.persistence.JoinColumn;  
  
  
  
  
import javax.persistence.JoinTable;  
  
  
  
  
import javax.persistence.ManyToMany;  
  
  
  
  
import javax.persistence.OneToMany;  
  
  
  
  
import javax.persistence.OneToOne;  
  
  
  
  
import javax.persistence.PrimaryKeyJoinColumn;  
  
  
  
  
import javax.persistence.Table;  
  
  
  
  
 
  
  
  
  
@Entity 
  
  
  
  
@Table(name = "t_customers")  
  
  
  
  
public class Customer  
  
  
  
  
{  
  
  
  
  
    private int id;  
  
  
  
  
    private String name;  
  
  
  
  
    private Referee referee;  
  
  
  
  
    private Collection< Order> orders;  
  
  
  
  
 
  
  
  
  
    private Collection< Address> addresses;  
  
  
  
  
 
  
  
  
  
    @ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)  
  
  
  
  
    @JoinTable(name = "t_customers_addresses", joinColumns = @JoinColumn(name = "customer_id",   
  
  
  
  
     referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "address_id", referencedColumnName = "id"))  
  
  
  
  
    public Collection< Address> getAddresses()  
  
  
  
  
    {  
  
  
  
  
        return addresses;  
  
  
  
  
    }  
  
  
  
  
 
  
  
  
  
    ... ...  
  
  
  
  
}  
  
  
  
  
 
  
  
  
  
    @JoinTable注釋用于指定連接表和t_customers及t_addresses表的連接字段關(guān)系。  
  
  
  
  
    Address類(lèi)的代碼如下：  
  
  
  
  
package entity;  
  
  
  
  
 
  
  
  
  
import java.util.Collection;  
  
  
  
  
 
  
  
  
  
import javax.persistence.CascadeType;  
  
  
  
  
import javax.persistence.Entity;  
  
  
  
  
import javax.persistence.FetchType;  
  
  
  
  
import javax.persistence.GeneratedValue;  
  
  
  
  
import javax.persistence.GenerationType;  
  
  
  
  
import javax.persistence.Id;  
  
  
  
  
import javax.persistence.ManyToMany;  
  
  
  
  
import javax.persistence.Table;  
  
  
  
  
@Entity 
  
  
  
  
@Table(name="t_addresses")  
  
  
  
  
public class Address  
  
  
  
  
{  
  
  
  
  
    private int id;  
  
  
  
  
    private String addressLine;  
  
  
  
  
    private String country;  
  
  
  
  
    private String postCode;  
  
  
  
  
    private Collection< Customer> customers;  
  
  
  
  
    @Id 
  
  
  
  
    @GeneratedValue(strategy=GenerationType.IDENTITY)  
  
  
  
  
    public int getId()  
  
  
  
  
    {  
  
  
  
  
        return id;  
  
  
  
  
    }  
  
  
  
  
    public void setId(int id)  
  
  
  
  
    {  
  
  
  
  
        this.id = id;  
  
  
  
  
    }  
  
  
  
  
    public String getAddressLine()  
  
  
  
  
    {  
  
  
  
  
        return addressLine;  
  
  
  
  
    }  
  
  
  
  
    public void setAddressLine(String addressLine)  
  
  
  
  
    {  
  
  
  
  
        this.addressLine = addressLine;  
  
  
  
  
    }  
  
  
  
  
    public String getCountry()  
  
  
  
  
    {  
  
  
  
  
        return country;  
  
  
  
  
    }  
  
  
  
  
    public void setCountry(String country)  
  
  
  
  
    {  
  
  
  
  
        this.country = country;  
  
  
  
  
    }  
  
  
  
  
    public String getPostCode()  
  
  
  
  
    {  
  
  
  
  
        return postCode;  
  
  
  
  
    }  
  
  
  
  
    public void setPostCode(String postCode)  
  
  
  
  
    {  
  
  
  
  
        this.postCode = postCode;  
  
  
  
  
    }  
  
  
  
  
    @ManyToMany(mappedBy="addresses")  
  
  
  
  
    public Collection< Customer> getCustomers()  
  
  
  
  
    {  
  
  
  
  
        return customers;  
  
  
  
  
    }  
  
  
  
  
    public void setCustomers(Collection< Customer> customers)  
  
  
  
  
    {  
  
  
  
  
        this.customers = customers;  
  
  
  
  
    }      
  
  
  
  
}  
 
 
 
 

由于是多對(duì)多的關(guān)系，因此，在Address類(lèi)中需要定義一個(gè)Collection類(lèi)型的字段(customers）用來(lái)保存與該Address對(duì)象相對(duì)應(yīng)的Customer對(duì)象。getCustomers方法也需要使用@ManyToMany進(jìn)行注釋?？梢允褂孟旅娲a進(jìn)行測(cè)試：

 
 
 
 

  
  
  
  
Customer customer = new Customer();  
  
  
  
  
customer.setName("微軟11");  
  
  
  
  
List< Address> addresses = new ArrayList< Address>();  
  
  
  
  
Address address = new entity.Address();  
  
  
  
  
address.setAddressLine("address1");  
  
  
  
  
address.setCountry("中國(guó)");   
  
  
  
  
address.setPostCode("12345678");  
  
  
  
  
addresses.add(address);  
  
  
  
  
address = new entity.Address();  
  
  
  
  
address.setAddressLine("address2");  
  
  
  
  
address.setCountry("美國(guó)");   
  
  
  
  
address.setPostCode("4321");  
  
  
  
  
addresses.add(address);  
  
  
  
  
customer.setAddresses(addresses);  
  
  
  
  
em.persist(customer); 
 
 
 
 

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