实现Entity Bean的多对多（many-to-many）映射
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; } ... ...}
@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：
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：
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");addresses.add(address);customer.setAddresses(addresses);em.persist(customer);
绊脚石乃是进身之阶。
