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上一篇： eclipse + JBoss 5 + EJB3開發指南（8）：實現Entity Bean的一對多（one-to-many）映射

在EJB3中需要使用@ManyToMany對封裝多對多關系的字段或getter方法進行注釋。先看看下面的表：

圖1 t_addresses表

t_addresses表和t_customers表是多對多的關系。需要使用一個關聯表來描述這種關系，關聯表的結構如下圖所示。

圖2 t_customers_addresses

在Customer類中定義一個Collection<Address>類型的字段（addresses），用于保存與該Customer對象相對應的多個Address對象，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 Stringname;
private Refereereferee;
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表的連接字段關系。
Address類的代碼如下：

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 StringaddressLine;
private Stringcountry;
private StringpostCode;
private Collection < Customer > customers;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public int getId()
{
return id;
}
public void setId( int id)
{
this .id = id;
}
public StringgetAddressLine()
{
return addressLine;
}
public void setAddressLine(StringaddressLine)
{
this .addressLine = addressLine;
}
public StringgetCountry()
{
return country;
}
public void setCountry(Stringcountry)
{
this .country = country;
}
public StringgetPostCode()
{
return postCode;
}
public void setPostCode(StringpostCode)
{
this .postCode = postCode;
}
@ManyToMany(mappedBy = " addresses " )
public Collection < Customer > getCustomers()
{
return customers;
}
public void setCustomers(Collection < Customer > customers)
{
this .customers = customers;
}
}

由于是多對多的關系，因此，在Address類中需要定義一個Collection<Customer>類型的字段(customers）用來保存與該Address對象相對應的Customer對象。getCustomers方法也需要使用@ManyToMany進行注釋。可以使用下面代碼進行測試：

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

下一篇： eclipse + JBoss 5 + EJB3開發指南（9）：實現Entity Bean的多對多（many-to-many）映射

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