自己总结一下关联关系映射中的OneToMany,ManyToOne,单向，双向注意的事项！ps:例子

来源我的百度空间http://hi.baidu.com/nyistzengpan

（1）ManyToOne（多对一）单向：不产生中间表，但可以用@Joincolumn（name="  "）来指定生成外键的名字，外键在多的一方表中产生！

（2）OneToMany（一对多）单向：会产生中间表，此时可以用@onetoMany @Joincolumn（name=" "）避免产生中间表，并且指定了外键的名字（别看@joincolumn在一中写着，但它存在在多的那个表中）

（3）OneToMany ,ManyToOne 双向（两个注解一起用的）：如果不在@OneToMany中加mappedy属性就会产生中间表，此时通常在@ManyToOne的注解下再添上注解@Joincolumn(name=" ")来指定外键的名字(说明：多的一方为关系维护端，关系维护端负责外键记录的更新，关系被维护端没有权利更新外键记录)！（@OneToMany(mappedBy="一对多中，多中一的属性")出现mapby为被维护端|||默认为延迟加载)

ps:举例说明：以下依次是一对多的关系（course----testtopic，chapter----testtopic），一共四个表（搭的都是双向关系），时间关系最后一个不上代码了

@Entity //科目表
public  class  Course {
    private  int  id;
    private   String name;
    private  Set<Chapter> chapters =  new  HashSet<Chapter>();
    public  void  addChapters(Chapter chapter){
        if (! this .chapters.contains(chapter)){
            this .chapters.add(chapter);
            chapter.setCourse( this );
        }
    }
    @OneToMany (cascade=CascadeType.REFRESH,fetch=FetchType.LAZY,mappedBy= "course" )
    public  Set<Chapter> getChapters() {
        return  chapters;
    }
    public  void  setChapters(Set<Chapter> chapters) {
        this .chapters = chapters;
    }
    @Id
    @GeneratedValue (strategy=GenerationType.AUTO)
    public  int  getId() {
        return  id;
    }
    public  void  setId( int  id) {
        this .id = id;
    }
    @Column (length =  50 )
    public  String getName() {
        return  name;
    }
    public  void  setName(String name) {
        this .name = name;
    }
}

@Entity //章节表
public  class  Chapter {
    private  int  id;
    private  String name;

    private  Set<TestTopic> topics = new  HashSet<TestTopic>();

    private  Course course;

    @Id  @GeneratedValue (strategy =GenerationType.AUTO )
    public  int  getId() {
        return  id;
    }

    public  void  setId( int  id) {
        this .id = id;
    }
    @Column (length =  150 )
    public  String getName() {
        return  name;
    }
    public  void  setName(String name) {
        this .name = name;
    }
    @ManyToOne
    @JoinColumn (name= "courseId" )
    public  Course getCourse() {
        return  course;
    }

    public  void  setCourse(Course course) {
        this .course = course;
    }

    @OneToMany (cascade=CascadeType.REFRESH,mappedBy= "chapter" ,fetch=FetchType.EAGER)
    public  Set<TestTopic> getTopics() {
        return  topics;
    }
    public  void  setTopics(Set<TestTopic> topics) {
        this .topics = topics;
    }
}

@Entity //考试套题表
public  class  TestTopic {
    private  int  id;
    private  String name;
    private  Set<Question> questions =  new  HashSet<Question>();
    private  Chapter chapter;
    @Id  @GeneratedValue (strategy =GenerationType.AUTO )
    public  int  getId() {
        return  id;
    }
    public  void  setId( int  id) {
        this .id = id;
    }
    @Column (length =  150 )
    public  String getName() {
        return  name;
    }
    public  void  setName(String name) {
        this .name = name;
    }

    @OneToMany (cascade=CascadeType.ALL,mappedBy= "testTopic" ,fetch=FetchType.EAGER)
    public  Set<Question> getQuestions() {
        return  questions;
    }
    public  void  setQuestions(Set<Question> questions) {
        this .questions = questions;
    }
    @ManyToOne
    @JoinColumn (name= "chapterId" )
    public  Chapter getChapter() {
        return  chapter;
    }
    public  void  setChapter(Chapter chapter) {
        this .chapter = chapter;
    }
}