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