Hibernate中ManyToOne,OneToMany,OneToOne的使用例

有3个表：

News：新闻表

NewsResource：新闻附件表

Resource：资源表

表关系：

1个新闻（News）对应多条新闻附件（NewsResource）

News：ID <---> NewsResource：news_id

1个新闻附件（NewsResource）对应1个资源（Resource）。

NewsResource：resource_id <---> Resource：ID

实体类如下：

News：

@Entity
@Table(name="news")
public class NewsEntity implements Serializable{
private static final long serialVersionUID = 48L;

@Id
@GeneratedValue
@Column(name="ID")
private Long id;

@OneToMany(cascade=CascadeType.ALL,fetch=FetchType.LAZY,mappedBy="news")
private Set<NewsResourceEntity> newsResource;

//其他内容省略
}

NewsResource:

关于NewsResource表，有两种写法：

方法1：

这种写法用于不想和news表做级联更新，只想做级联查询的情况。news_id和resource_id都定义了2次，上面@Column用于自己表的更新，下面的@JoinColumn用于级联查询的时候的Join列。

@Entity
@Table(name="newsresource")
public class NewsResourceEntity implements Serializable{
private static final long serialVersionUID = 48L;

@Id
@GeneratedValue
@Column(name="ID")
private Long id;

@Column(name = "news_id")
private Long newsId;

@Column(name = "resource_id")
private Long resourceId;

@ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
@JoinColumn(name="news_id", insertable = false,updatable = false)
private NewsEntity news;

@OneToOne(optional = false, cascade = CascadeType.MERGE)
@JoinColumn(name = "resource_id", referencedColumnName = "id", unique = true, insertable = false,updatable = false)
private ResourceEntity resource;

//其他内容省略
}

方法2：

这种写法就是典型的级联查询+级联更新。

<pre name="code" class="java">@Entity
@Table(name="newsresource")
public class NewsResourceEntity implements Serializable{
private static final long serialVersionUID = 48L;

@Id
@GeneratedValue
@Column(name="ID")
private Long id;

@ManyToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
@JoinColumn(name="news_id")
private NewsEntity news;

@OneToOne(optional = false, cascade = CascadeType.MERGE)
@JoinColumn(name = "resource_id", referencedColumnName = "id", unique = true)
private ResourceEntity resource;

//其他内容省略
}

Resource：

@Entity
@Table(name="resource")
public class ResourceEntity implements Serializable {
private static final long serialVersionUID = 48L;

@Id
@GeneratedValue
@Column(name="ID")
private Long id;

@OneToOne(optional = true, cascade = CascadeType.ALL, fetch=FetchType.LAZY, mappedBy = "resource")
private NewsResourceEntity newsResource;

//其他内容省略
}

另外，懒加载（LAZY）如果遇到问题，请参照相关文章：

http://blog.csdn.net/sunroyi666/article/details/51890571