hibernate学习-两表关联（一对多，多对多，自关联）注解配置-第6天

痛定思痛。 2022-01-30 03:49 303阅读 0赞

关联关系，是使用最多的一种关系，非常重要。在内存中反映为实体关系，映射到 DB  
中为主外键关系。实体间的关联，即对外键的维护。关联关系的发生，即对外键数据的改  
变。外键：外面的主键，即，使用其它表的主键值作为自已的某字段的取值。

## 一、基本概念 ##

前4个请查看 [Hibernate5讲义][Hibernate5]  
5、关联方向  
（1）单向关联  
指具有关联关系的实体对象间的加载与访问关系是单向的。即，只有一个实体对象可  
以加载和访问对方，但对方是看不到另一方的。  
（2）双向关联  
指具有关联关系的实体对象间的加载与访问关系是双向的。即，任何一方均可加载和  
访问另一方。  
6、关联数量  
实体对象间的关系，从数量上可以划分为：1:1，1:n，n:1，m:n 。

## 二、关系映射 ##

demo代码路径：[https://github.com/MisterChegy/hibernate][https_github.com_MisterChegy_hibernate]

### （1）1:n-单向关联 ###

model类 (get,set方法略过)

/**
     * 一对多单向关联
     */
    @Entity
    @Table(name = "minister", catalog = "test")
    public class Minister implements java.io.Serializable {
    
    	// Fields
    
    	private Integer mid;
    	private String mname;

/**
     *一对多单向关联
     */
    @Entity
    @Table(name = "country", catalog = "test")
    public class Country implements java.io.Serializable {
    
    	@Id
    	@Column(name = "cid", unique = true, nullable = false)
    	private Integer cid;
    	
    	@Column(name = "cname", length = 45)
    	private String cname;
    	//关联属性
    	@OneToMany(targetEntity = Minister.class, cascade=CascadeType.ALL)
    	//用来指定与所操作实体或实体集合相关联的数据库表中的列字段
    	@JoinColumn(name="cid")
    	private Set<Minister> ministers = new HashSet<Minister>(0);

测试

/*
    	 * 去掉cascade = CascadeType.ALL和mappedBy =
    	 * "country"，即取消级联保存和多端维护，然后添加@JoinColumn(name="cid")
    	 * 添加@JoinColumn(name="cid")可以避免生成中间表
    	 *
    	 * 一对多单向关联
    	 */
    	@Test
    	public void test01() {
    		try {
    			session.beginTransaction();
    
    			model2.Minister minister1 = new model2.Minister(1, "aaa");
    			Set<model2.Minister> ministers = new HashSet<model2.Minister>();
    			ministers.add(minister1);
    
    			model2.Country country = new model2.Country(1, "USA");
    			country.setMinisters(ministers);
    
    			System.out.println("country = " + country);
    
    			session.save(country);
    			session.getTransaction().commit();
    		} catch (Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}
    	}

结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70]

### （2）1:n-双向关联 ###

model类

/**
     * 一对多双向关联
     */
    @Entity
    @Table(name = "minister", catalog = "test")
    public class Minister implements java.io.Serializable {
    
    	@Id
    	@Column(name = "mid", unique = true, nullable = false)
    	private Integer mid;
    	
    	@Column(name = "mname", length = 45)
    	private String mname;
    	// 关联属性
    	@ManyToOne(cascade = CascadeType.ALL)
    	@JoinColumn(name = "cid")
    	private Country country;

/**
     *一对多双向关联
     */
    @Entity
    @Table(name = "country", catalog = "test")
    public class Country implements java.io.Serializable {
    
        @Id
    	@Column(name = "cid", unique = true, nullable = false)
    	private Integer cid;
        @Column(name = "cname", length = 45)
    	private String cname;
    	//关联属性
        @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY , mappedBy = "country")
    	private Set<Minister> ministers = new HashSet<Minister>(0);

测试

/*
    	 * 在 country 的映射文件中的关联属性映射中增加控制反转设置mappedBy =
    	 * "country",注册要删除@JoinColumn(name="cid") 在 country
    	 * 的映射文件中的关联属性映射中增加设置级联操作，或测试类中进行save(minister)。
    	 *
    	 * 一对多双向关联
    	 */
    	@Test
    	public void test02() {
    		try {
    			session.beginTransaction();
    
    			model.Minister minister1 = new model.Minister(2, "aaa");
    			model.Country country = new model.Country(2, "USA");
    			minister1.setCountry(country);
    
    			HashSet<model.Minister> ministers = new HashSet<model.Minister>();
    			ministers.add(minister1);
    
    			country.setMinisters(ministers);
    
    			session.save(country);
    			session.getTransaction().commit();
    		} catch (Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}
    	}

结果  
![在这里插入图片描述][20190517224321730.png]

### （3）自关联 ###

model类

/**
     * 自关联
     */
    @Entity
    @Table(name = "column", catalog = "test")
    public class Column implements java.io.Serializable {
    
    	@Id
    	@javax.persistence.Column(name = "id", unique = true, nullable = false)
    	private Integer id;
    	// 父栏目
    	// 多方关联属性，多对1映射
    	@ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    	@JoinColumn(name = "cid")
    	private Column parentColumn;
    	@javax.persistence.Column(name = "name", length = 45)
    	private String name;
    	// 子栏目
    	// 1方关联属性
    	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "parentColumn")
    	private Set<Column> childrenColumns = new HashSet<Column>(0);

测试

/*
    	 * 自关联 在多端添加cascade = CascadeType.ALL
    	 */
    	@Test
    	public void test03() {
    		try {
    			session.beginTransaction();
    			Column sportsColumn = new Column(1, "体育栏目");
    
    			Column footballColumn = new Column(2, "足球栏目");
    			footballColumn.setParentColumn(sportsColumn);
    
    			Column basketballColumn = new Column(3, "篮球栏目");
    			basketballColumn.setParentColumn(sportsColumn);
    
    			System.out.println("footballColumn = " + footballColumn);
    			System.out.println("basketballColumn = " + basketballColumn);
    
    			session.save(footballColumn);
    			session.save(basketballColumn);
    
    			session.getTransaction().commit();
    		} catch (Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}
    	}

结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 1]

### （4）n:1-单向关联 ###

model类

/**
     *多对一单向关联
     */
    @Entity
    @Table(name = "minister", catalog = "test")
    public class Minister implements java.io.Serializable {
    	@Id
    	@Column(name = "mid", unique = true, nullable = false)
    	private Integer mid;
    	@Column(name = "mname", length = 45)
    	private String mname;
    	// 关联属性
    	@ManyToOne(cascade = CascadeType.ALL)
    	@JoinColumn(name = "cid")
    	private Country country;

/**
     * 多对一单向关联
     */
    @Entity
    @Table(name = "country", catalog = "test")
    public class Country implements java.io.Serializable {
    	
    	@Id
    	@Column(name = "cid", unique = true, nullable = false)
    	private Integer cid;
    	@Column(name = "cname", length = 45)
    	private String cname;

测试

/*
    	 * 去掉cascade = CascadeType.ALL和mappedBy =
    	 * "country"，即取消级联保存和多端维护，然后添加@JoinColumn(name="cid")
    	 * 添加@JoinColumn(name="cid")可以避免生成中间表
    	 *
    	 * 多对一单向关联
    	 */
    	@Test
    	public void test04() {
    		try {
    			session.beginTransaction();
    
    			model3.Minister minister1 = new model3.Minister(3, "aaa");
    
    			model3.Country country = new model3.Country(3, "USA");
    
    			minister1.setCountry(country);
    			System.out.println("minister1 = " + minister1);
    
    			session.save(minister1);
    			session.getTransaction().commit();
    		} catch (Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}
    	}

结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 2]

### （5）n:m-单向关联 ###

model类

/**
     * 多对多单向关联
     */
    @Entity
    @Table(name = "course", catalog = "test")
    public class Course implements java.io.Serializable {
    
    	@Id
    	@Column(name = "cid", unique = true, nullable = false)
    	private Integer cid;
    	
    	@Column(name = "cname", length = 45)
    	private String cname;

/**
     * 多对多单向关联
     */
    @Entity
    @Table(name = "student", catalog = "test")
    public class Student implements java.io.Serializable {
    
    	@Id
    	@Column(name = "sid", unique = true, nullable = false)
    	private Integer sid;
    	@Column(name = "sname", length = 45)
    	private String sname;
    	@ManyToMany
    	@JoinTable(name = "student_course",
    	joinColumns = {@JoinColumn(name = "s_id", referencedColumnName = "sid")},
    	inverseJoinColumns = {@JoinColumn(name = "c_id", referencedColumnName ="cid")})
    	private Set<Course> courses;

测试

/*
    	 * n:m-单向关联 ,只维护student,student_course;course不更新（需要提前手动更新）
    	 */
    	@Test
    	public void test06() {
    		try {
    			session.beginTransaction();
    
    			model3.Course course1 = new model3.Course(1, "struts2");
    			model3.Course course2 = new model3.Course(2, "Hibernate");
    
    			model3.Student student = new model3.Student(1, "bbb");
    			student.getCourses().add(course1);
    			student.getCourses().add(course2);
    			
    			System.out.println("student = "+student);
    			session.save(course1);
    			session.save(course2);
    			
    			session.save(student);
    			session.getTransaction().commit();
    		} catch (Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}
    	}

结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 3]

### （5）n:m-双向关联 ###

model类

/**
     * 多对多双向关联
     */
    @Entity
    @Table(name = "course", catalog = "test")
    public class Course implements java.io.Serializable {
    	
    	@Id
    	@Column(name = "cid", unique = true, nullable = false)
    	private Integer cid;
    	@Column(name = "cname", length = 45)
    	private String cname;
    	@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    	@JoinTable(name = "student_course",
    		joinColumns = {@JoinColumn(name = "c_id", referencedColumnName = "cid")},
    		inverseJoinColumns = {@JoinColumn(name = "s_id", referencedColumnName ="sid")})	
    	private Set<Student> students;

/**
     * 多对多双向关联
     */
    @Entity
    @Table(name = "student", catalog = "test")
    public class Student implements java.io.Serializable {
    
    	@Id
    	@Column(name = "sid", unique = true, nullable = false)
    	private Integer sid;
    	@Column(name = "sname", length = 45)
    	private String sname;
    	@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    	@JoinTable(name = "student_course",
    		joinColumns = {@JoinColumn(name = "s_id", referencedColumnName = "sid")},
    		inverseJoinColumns = {@JoinColumn(name = "c_id", referencedColumnName ="cid")})
    	private Set<Course> courses;

测试

/*
    	 * n:m-双向关联
    	 */
    	@Test
    	public void test05() {
    		try {
    			session.beginTransaction();
    
    			Course course1 = new Course(1, "struts2");
    			Course course2 = new Course(2, "Hibernate");
    
    			Student student = new Student(2, "aaa");
    			student.getCourses().add(course1);
    			student.getCourses().add(course2);
    			
    			System.out.println("student = "+student);
    			
    			session.save(student);
    			session.getTransaction().commit();
    		} catch (Exception e) {
    			e.printStackTrace();
    			session.getTransaction().rollback();
    		}
    	}

结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 4]

[Hibernate5]: https://github.com/MisterChegy/hibernate/blob/master/Hibernate5%E8%AE%B2%E4%B9%89.pdf
[https_github.com_MisterChegy_hibernate]: https://github.com/MisterChegy/hibernate
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[20190517224321730.png]: /images/20220130/94f810cb08c543e199057375f16edb08.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 1]: /images/20220130/82514ce456be4effb48d9e9e8b681dd0.png
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[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 3]: /images/20220130/26b08df91ef144eeb50feb53be6c3f9f.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZWd5MjE4_size_16_color_FFFFFF_t_70 4]: /images/20220130/b2bc730f097e454baf3d364a6c45300d.png