在EJB3中需要使用@ManyToMany对封装多对多关系的字段或getter方法进行注释。先看看下面的表：
图1 t_addresses表
t_addresses表和t_customers表是多对多的关系。需要使用一个关联表来描述这种关系，关联表的结构如下图所示。
图2 t_customers_addresses
在Customer类中定义一个Collection<Address>类型的字段（addresses），用于保存与该Customer对象相对应的多个Address对象，代码如下：
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;
}
... ...
}
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注释用于指定连接表和t_customers及t_addresses表的连接字段关系。
Address类的代码如下：
Address类的代码如下：
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;
}
}
由于是多对多的关系，因此，在Address类中需要定义一个Collection<Customer>类型的字段(customers）用来保存与该Address对象相对应的Customer对象。getCustomers方法也需要使用@ManyToMany进行注释。可以使用下面代码进行测试：
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;
}
}
Customer customer
=
new
Customer();
customer.setName( " 微软11 " );
List < Address > addresses = new ArrayList < Address > ();
Address address = new entity.Address();
address.setAddressLine( " address1 " );
address.setCountry( " 中国 " );
address.setPostCode( " 12345678 " );
addresses.add(address);
address = new entity.Address();
address.setAddressLine( " address2 " );
address.setCountry( " 美国 " );
address.setPostCode( " 4321 " );
customer.setName( " 微软11 " );
List < Address > addresses = new ArrayList < Address > ();
Address address = new entity.Address();
address.setAddressLine( " address1 " );
address.setCountry( " 中国 " );
address.setPostCode( " 12345678 " );
addresses.add(address);
address = new entity.Address();
address.setAddressLine( " address2 " );
address.setCountry( " 美国 " );
address.setPostCode( " 4321 " );
addresses.add(address);
customer.setAddresses(addresses);
em.persist(customer);
customer.setAddresses(addresses);
em.persist(customer);
本文转自 androidguy 51CTO博客，原文链接：http://blog.51cto.com/androidguy/214431，如需转载请自行联系原作者
