one2many &&many2many

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<User> 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<User> getUsers() {
        return (users==null)? new HashSet<User>():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<User> users) {
        this.users = users;
    }
    
　　//说明代码三
/*    public void addUser(User user) {
        if(this.users == null){
            this.users = new HashSet<User>();
        }
        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<User> 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<Student> 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<Student> getStudents() {
		return students;
	}

	public void setStudents(Set<Student> 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<Student> students= new HashSet<Student>();
        students.add(student);
        teacher.setStudents(students);
        Set<Teacher> teachers= new HashSet<Teacher>();
        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<Teacher> 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<Teacher> 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<Teacher> 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：