說明
??????? 在HibernateAnnotations中通過@ManyToMany注解可定義多對多關聯。同時，也需要通過注解@JoinTable描述關聯表和關聯條件。對于雙向關聯，其中一端必須定義為owner，另一端必須定義為inverse（在對關聯表進行更性操作時這一端將被忽略）。被關聯端不必也不能描述物理映射，只需要一個簡單的mappedBy參數，該參數包含了主體端的屬性名，這樣就綁定了雙方的關系。

??????上周六去電影院圓了兒時的夢想，看了變形金剛，超棒的一部片子 ^_^。那么就以劇院和觀眾為例講解吧。

如何制作PO

1）找到CUBE--需要引入哪些類：

import ?java.util.ArrayList;
import ?java.util.List;
import ?javax.persistence.CascadeType;
import ?javax.persistence.Entity;
import ?javax.persistence.FetchType;
import ?javax.persistence.JoinColumn;
import ?javax.persistence.JoinTable;
import ?javax.persistence.ManyToMany;
import ?javax.persistence.Table;
import ?org.hibernate.annotations.Cache;
import ?org.hibernate.annotations.CacheConcurrencyStrategy;

/**?*//**
?*?Theater
?*?@author?allen
?*/
@SuppressWarnings("serial")
@Entity
@Table(name?=?"THEATER")
@Cache(usage?=?CacheConcurrencyStrategy.READ_WRITE)
public?class?Theater?implements?Serializable?{
????
????@ManyToMany(
????????????targetEntity=net.allen.domain.Audience.class,
????????????cascade?={CascadeType.PERSIST,CascadeType.MERGE},
????????????fetch=FetchType.LAZY
????)
????@JoinTable(
????????????name="THEATER_AUDIENCE",
????????????joinColumns={@JoinColumn(name="THEATER_ID")},
????????????inverseJoinColumns={@JoinColumn(name="AUDIENCE_ID")}
????)
????@Cache(usage?=?CacheConcurrencyStrategy.NONSTRICT_READ_WRITE)
????private?List<Audience>?audiences?=?new?ArrayList<Audience>();

????/**?*//**
?????*?@return?Returns?the?audiences.
?????*/
????public?List<Audience>?getAudiences()?{
????????return?audiences;
????}

????/**?*//**
?????*?@param?audiences?The?audiences?to?set.
?????*/
????public?void?setAudiences(List<Audience>?audiences)?{
????????this.audiences?=?audiences;
????}????
}

功能說明：
@ManyToMany注解
???? targetEntity屬性：指向被關聯端的實體對象
???? cascade屬性：與Hibernate xml配置文件中的意思一樣，這里選用兩種方式
????????????CascadeType.PERSIST：若實體是處于被管理狀態，或當persist()方法被調用時，觸發級聯創建（create）操作。???
????????????CascadeType.MERGE：若實體是處于被管理狀態，或當merge)方法被調用時，觸發級聯合并（merge）操作。
?????????????其它屬性如CascadeType.REMOVE、CascadeType.REFRESH、CascadeType.ALL等屬性可參考Hibernate Annotations Reference。
???? fetch屬性：關聯關系獲取方式
?????????????? LAZY（默認值）在第一次訪問關聯對象時才觸發相應的查詢操作。
?????????????? 另一個值EAGER是通過out join select直接獲取關聯對象
????
@JoinTable注解
???? name屬性：指定關聯表名 若不指定Hibernate可以根據既定的規則自動生成（具體規則見reference）
???? joinColumns屬性：指定主體端的外鍵
???? inverseJoinColumns屬性：指定被關聯端的外鍵

@Cache注解
???? usage屬性：給定了緩存的并發策略


3）找到霸天虎--被關聯端：

/**?*//**
?*?Audience
?*?@author?allen
?*/

@SuppressWarnings("serial")
@Entity
@Table(name?=?"AUDIENCE")
@Cache(usage?=?CacheConcurrencyStrategy.READ_WRITE)
public?class?Audience?implements?Serializable?{
????
????@ManyToMany(
???????????cascade={CascadeType.PERSIST,CascadeType.MERGE},
???????????mappedBy="audiences"
????)
????/**?*//**?所在的劇院?*/
????private?List<Theater>?theaters?=?new?ArrayList<Theater>();

????/**?*//**
?????*?@return?Returns?the?theaters.
?????*/
????public?List<Theater>?getTheaters()?{
????????return?theaters;
????}

????/**?*//**
?????*?@param?theaters?The?theaters?to?set.
?????*/
????public?void?setTheaters(List<Theater>?theaters)?{
????????this.theaters?=?theaters;
????}
}

/**?*//**
?????*?select?transformers?wathers?from?ShowMax?Theater
?????*/
????protected?void?selectWathers()?{
????????//1)?get?current?theater
????????Theater?theater?=?findTheaterById("showMax");
????????//2)?clear?theater's?audiences
????????theater.getAudiences().clear();
????????//3)?get?audiences?who?want?to?watch?transformers
????????List<Audience>?audiences?=?findAudiencesByMovie("transformers");
????????for?(Audience?a:?audiences)?{
????????????//4)?mountain?relations
????????????a.getTheaters().add(theater);
????????????theater.getAudiences().add(a);
????????}
????????//5)?do?save?main?entity
????????doSaveEntity(theater);
????}

tips：注意第二步的操作。

好了，大功告成！說回電影，紅蜘蛛這小子跑得還挺快，期待續集！


PS：找到一份中文的hiberante?annotations reference，與大家共享。點擊下載


??????????????????????????????????????????????????????????????????????????????THE END
????????????????