Hibernate之JPA的关联映射

JPA的关联映射：

　　1.导包，位于hibernate-lib-jpa里面的包；

　　　删除对Java EE 5 Libraries的使用。

　　2.书写实体类，书写注释。注意注释是导的javax.类;

　　@id

　　@GeneratedValue 为主键生成策略（默认为native）;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;


@Entity
@Table(name="a_user")
public class User {
    private int id;
    private String name;
    private int age;

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }

　　3.hibernate.cfg.xml中配置实体类，<mapping class="com.kebin.pojo.User" />

<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory name="foo">
    <property name="show_sql">true</property>
    <property name="myeclipse.connection.profile">MYSQL</property>
    <property name="connection.url">
        jdbc:mysql://127.0.0.1:3306/mysql
    </property>
    <property name="connection.username">root</property>
    <property name="connection.password">root</property>
    <property name="connection.driver_class">
        com.mysql.jdbc.Driver
    </property>
    <property name="dialect">
        org.hibernate.dialect.MySQL5Dialect
    </property>
    <property name="format_sql">true</property>
    <property name="hbm2ddl.auto">update</property>
    <mapping class="com.kebin.pojo.User" />

</session-factory>
</hibernate-configuration>

　　4.书写方法，创建表格；简单的配置就完成了，直接运行。

package com.kebin.util;

import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.cfg.Configuration;
import org.hibernate.tool.hbm2ddl.SchemaExport;

public class Userutil {

    public static void main(String[] args) {
        Configuration cfg = new AnnotationConfiguration().configure();
        SchemaExport se = new SchemaExport(cfg);
        se.create(true, true);

    }

}

一、多对一关联映射

　　项目一般都用双向配置，所以以下只给出双向的。

　　1.书写两个类，然后再hibernate.cfg.xml中配置实体类。

　　2. 多的一端维护:@ManyToOne；        添加一个外键。
 　　                     @JoinColumn(name="cid") ；     cid根据注释代码get方法的类取名。

　　3.少的一端：@OneToMany(mappedBy="classes");

　　　mappedBy方法只管加载数据，外键关系交给""处理。

　　4.hibernate.cfg.xml中配置：　<mapping class="com.kebin.pojo.Class" />
  　　　　　　　　　　　　　　　　<mapping class="com.kebin.pojo.Student" />

 

@Entity
@Table(name="t_student")
public class Student {
    private int id;
    private String name;
    private Class classes;


    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @ManyToOne
    @JoinColumn(name="cid")
    public Class getClasses() {
        return classes;
    }
    public void setClasses(Class classes) {
        this.classes = classes;
    }

 

@Entity
@Table(name="t_class")
public class Class {
    private int id;
    private String name;
    private Set<Student> students = new HashSet<Student>();
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @OneToMany(mappedBy="classes")
    public Set<Student> getStudents() {
        return students;
    }
    public void setStudents(Set<Student> students) {
        this.students = students;
    }

 一、一对一关联映射

 　　用得比较少，一般直接把多对一关系的外键设置唯一属性即可；

　　 1.一端：@ManyToOne；　　 

　　　       　@JoinColumn(name="cid",unique=true) ；

　　2.另一端：@OneToOne(mappedBy="wife")；代码如下

@Entity
@Table(name="t_husband")
public class Husband {
    private int id;
    private String name;
    private Wife wife;
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @ManyToOne
    @JoinColumn(name="wid",unique=true)
    public Wife getWife() {
        return wife;
    }
    public void setWife(Wife wife) {
        this.wife = wife;
    }
    

@Entity
@Table(name="t_wife")
public class Wife {
    private int id;
    private String name;
    private Husband husband;
    
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @OneToOne(mappedBy="wife")
    public Husband getHusband() {
        return husband;
    }
    public void setHusband(Husband husband) {
        this.husband = husband;
    }

 一、一对多关联映射

 　　这个就不介绍了，和多对一差不多。

 一、多对多关联映射

 　　1.一端：@ManyToMany

　　　　　　 @JoinTable(name="t_teacher_class",joinColumns={@JoinColumn(name="tid")}, inverseJoinColumns={@JoinColumn(name="cid")})

　　2.另一端：@ManyToMany(mappedBy="classes")

@Entity
@Table(name="t_teacher")
public class Teacher {
    private int id;
    private String name;
    private Set<Class> classes = new HashSet<Class>();
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @ManyToMany
    @JoinTable(name="t_teacher_class",joinColumns={@JoinColumn(name="tid")},
                inverseJoinColumns={@JoinColumn(name="cid")})
    public Set<Class> getClasses() {
        return classes;
    }
    public void setClasses(Set<Class> classes) {
        this.classes = classes;
    }

@Entity
@Table(name="t_class")
public class Class {
    private int id;
    private String name;
    private Set<Teacher> teachers = new HashSet<Teacher>();
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    @ManyToMany(mappedBy="classes")
    public Set<Teacher> getTeachers() {
        return teachers;
    }
    public void setTeachers(Set<Teacher> teachers) {
        this.teachers = teachers;
    }