one2many &&many2many-白红宇

one2many &&many2many

阅读量：4964 次

发布时间：2019-06-12

本文共 8447 字，大约阅读时间需要 28 分钟。

只记录双向的情况(双向是单向的一种)

@OneToMany 和 @ManyToOne :一个Group 包含多个 User;

Group.class

package com.XX.model;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.OneToMany;import javax.persistence.Table;import org.hibernate.annotations.GenericGenerator;@Entity@Table(name = "t_group")public class Group {    private String id;    private String name;    private String title;    private Set
     
       users;　　 //说明代码一    @Id    @GeneratedValue(generator = "sys-uuid")    @GenericGenerator(name = "sys-uuid", strategy = "uuid2")    public String getId() {        return id;    }    public String getName() {        return name;    }    public String getTitle() {        return title;    }　　//说明代码二    @OneToMany(cascade = {CascadeType.ALL}, fetch=FetchType.EAGER, mappedBy = "group")    public Set
      
        getUsers() {        return (users==null)? new HashSet
       
        ():users;    }    public void setId(String id) {        this.id = id;    }    public void setName(String name) {        this.name = name;    }    public void setTitle(String title) {        this.title = title;    }    public void setUsers(Set
        
          users) {        this.users = users;    }    　　//说明代码三/*    public void addUser(User user) {        if(this.users == null){            this.users = new HashSet
         
          ();        }        users.add(user);    }*/}

User.class

package com.XX.model;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToOne;import javax.persistence.Table;import org.hibernate.annotations.GenericGenerator;@Entity@Table(name = "t_user")public class User {    private String id;    private String name;    private int age;    private Group group;    @Id    @GeneratedValue(generator = "sys-uuid")    @GenericGenerator(name = "sys-uuid", strategy = "uuid2")    public String getId() {        return id;    }    public String getName() {        return name;    }        public int getAge() {        return age;    }　　　　//说明代码二    @ManyToOne(cascade = {CascadeType.ALL})    public Group getGroup() {        return group;    }    public void setId(String id) {        this.id = id;    }    public void setName(String name) {        this.name = name;    }    public void setAge(int age) {        this.age = age;    }    public void setGroup(Group group) {        this.group = group;    }    }

测试代码：

package com.XX.model;import java.util.Set;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.boot.Metadata;import org.hibernate.boot.MetadataSources;import org.hibernate.boot.registry.StandardServiceRegistry;import org.hibernate.boot.registry.StandardServiceRegistryBuilder;import org.junit.AfterClass;import org.junit.BeforeClass;import org.junit.Test;public class GroupTest {        private static SessionFactory sessionFactory;        @BeforeClass    public static void beforeClass(){　　　　　//hibernate 5.0.0.Final及之后的写法        StandardServiceRegistry registry = new StandardServiceRegistryBuilder().configure().build();                Metadata metadata = new MetadataSources(registry).getMetadataBuilder().build();        sessionFactory = metadata.getSessionFactoryBuilder().build();    }    @Test    public void test() {        Group group = new Group();        group.setName("group1");        group.setTitle("test");                User user = new User();        user.setName("yuc");        user.setAge(0);                Set
     
       users = group.getUsers();        users.add(user);        group.setUsers(users);　　　 　//说明代码三        //group.addUser(user);                Session session = sessionFactory.getCurrentSession();        session.beginTransaction();        session.save(group);        session.getTransaction().commit();        　　　　 //说明代码四        session = sessionFactory.getCurrentSession();        session.beginTransaction();        session.get(Group.class, "1");            }        @AfterClass    public static void afterClass(){        sessionFactory.close();    }    }

说明代码一：

　　ID用 String 存储，保存时自动生成，跨数据库(mysql、oracle均实用)；

说明代码二：

　　@manytoone & @onetomany 双向关联时，manytoone 一方用mappedBy 进行关联(否则数据库会出现两个关联关系，重复)；　　

代码说明三：

　　在多的一方辅助增加一个子元素。不足：破坏bean 的完整性(一般bean层级只有元素和对应的get/set方法)；

代码说明四：

　　用于说明@manytoone 和 @onetomany 的默认加载方式：

　　@manytoone 默认加载方式是FetchType.LAZY；

　　@onetomany 默认加载方式是FetchType.EAGER；

运行代码如下：　　

数据库结果如下：　　

@manyToMany双向关联 :一个Teacher有多个Student，一个Student有多个Teacher;

Teacher.class

package com.ajs.model;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToMany;import javax.persistence.Table;import org.hibernate.annotations.GenericGenerator;@Entity@Table(name = "t_teacher")public class Teacher {	private String id;	private String name;	private String title;	private Set
     
       students;	@Id	@GeneratedValue(generator = "sys-uuid")	@GenericGenerator(name = "sys-uuid", strategy = "uuid2")	public String getId() {		return id;	}	//说明代码五	@ManyToMany(targetEntity=Student.class, fetch = FetchType.EAGER, cascade={CascadeType.ALL}, mappedBy = "teachers")	public Set
      
        getStudents() {		return students;	}	public void setStudents(Set
       
         students) {		this.students = students;	}	public void setId(String id) {		this.id = id;	}	public String getName() {		return name;	}	public void setName(String name) {		this.name = name;	}	public String getTitle() {		return title;	}	public void setTitle(String title) {		this.title = title;	}}

Student.class　

测试代码　　

　　@Test    public void test() {        Teacher teacher = new Teacher();        teacher.setName("tea1");        teacher.setTitle("test");                Student student = new Student();        student.setName("yuc");        student.setAge(20);                Set
     
       students= new HashSet
      
       ();        students.add(student);        teacher.setStudents(students);        Set
       
         teachers= new HashSet
        
         ();        teachers.add(teacher);        student.setTeachers(teachers);                Session session = sessionFactory.getCurrentSession();        session.beginTransaction();        session.save(teacher);        session.getTransaction().commit();                session = sessionFactory.getCurrentSession();        session.beginTransaction();        session.get(Student.class, "1");    }

说明代码五：

package com.ajs.model;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.JoinTable;import javax.persistence.ManyToMany;import javax.persistence.Table;import org.hibernate.annotations.GenericGenerator;@Entity@Table(name = "t_student")public class Student {	private String id;	private String name;	private int age;	private Set
     
       teachers;	@Id	@GeneratedValue(generator = "sys-uuid")	@GenericGenerator(name = "sys-uuid", strategy = "uuid2")	public String getId() {		return id;	}	public String getName() {		return name;	}	public int getAge() {		return age;	}	//说明代码五	@ManyToMany(targetEntity=Teacher.class, fetch = FetchType.EAGER, cascade={CascadeType.ALL})	@JoinTable(name="t_teacher_student",    		joinColumns=@JoinColumn(name="student_id"),    		inverseJoinColumns=@JoinColumn(name="teacher_id"))	public Set
      
        getTeachers() {		return teachers;	}	public void setId(String id) {		this.id = id;	}	public void setName(String name) {		this.name = name;	}	public void setAge(int age) {		this.age = age;	}	public void setTeachers(Set
       
         teachers) {		this.teachers = teachers;	}}

　　双向一样用 mappedBy 对其中一方进行限制，fetch = FetchType.EAGER手动将加载方式改为急加载，需要用targetEntity指定对应的关联类是哪一个;

　　@JoinTable(name="t_teacher_student", joinColumns=@JoinColumn(name="student_id"),inverseJoinColumns=@JoinColumn(name="teacher_id"))

　　@JoinTable可以自定义对应的中间表名字，当前实体对象关联的中间表字段名称，已经中间表关联另一方的对应名称；

运行代码如下：

数据库结果如下：

转载于:https://www.cnblogs.com/xx0405/p/5378301.html

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