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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 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 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：
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");
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);
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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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