一、综述

目前为止项目用到的实体类关系有3种：

单向一对多

单向多对一

双向一对多（双向多对一）

二、说明

@Entity              --注释声明该类为持久类。将一个Javabean类声明为一 个实体的数据库表映射类,最好实现序列化.此时,默认情况下,所有的类属性都为映射到数据表的持久性字段.若在类中,添加另外属性,而非映射来数据库的, 要用下面的Transient来注解.@Table(name="promotion_info")      --持久性映射的表(表名="promotion_info).@Table是类一级的注解,定义在@Entity下,为实体bean映射表,目录和schema的名字,默认为实体bean的类名,不带包名.

@Id--注释可以表明哪种属性是该类中的独特标识符(即相当于数据表的主键)。 @GeneratedValue   --定义自动增长的主键的生成策略. @Transient             --将忽略这些字段和属性,不用持久化到数据库.适用于,在当前的持久类中,某些属性不是用于映射到数据表,而是用于其它的业务逻辑需要,这时,须将这些属性进行transient的注解.否则系统会因映射不到数据表相应字段而出错. @Temporal(TemporalType.TIMESTAMP)--声明时间格式

@Enumerated         --声明枚举 @Version                --声明添加对乐观锁定的支持 @OneToOne            --可以建立实体bean之间的一对一的关联 @OneToMany          --可以建立实体bean之间的一对多的关联 @ManyToOne          --可以建立实体bean之间的多对一的关联 @ManyToMany        --可以建立实体bean之间的多对多的关联 @Formula               --一个SQL表达式，这种属性是只读的,不在数据库生成属性(可以使用sum、average、max等) @OrderBy               --Many端某个字段排序(List)

特别对于一对多的时候：

// mappedBy相当于过去的inverse="true".    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "myclass")    public Set
     
       getStudents() {        return this.students;    }

其中mappedBy="myclass"为关联表的外键列字段。

其类似在XML中的inverse=true。

三、双向一对多实体类配置

假设有班级和学生2个实体类。

对于一对多或者多对一，就删除另外一方的相关配置即可。

MyClasss

package com.mc.entity;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import static javax.persistence.GenerationType.IDENTITY;import javax.persistence.Id;import javax.persistence.OneToMany;import javax.persistence.Table;/** * Myclass entity. @author MyEclipse Persistence Tools */@Entity@Table(name = "myclass", catalog = "test")public class Myclass implements java.io.Serializable {    // Fields    private Integer id;    private String className;    private Set
     
       students = new HashSet
      
       (0);    // Constructors    /** default constructor */    public Myclass() {    }    /** full constructor */    public Myclass(String className, Set
       
         students) {        this.className = className;        this.students = students;    }    // Property accessors    @Id    @GeneratedValue(strategy = IDENTITY)    @Column(name = "ID", unique = true, nullable = false)    public Integer getId() {        return this.id;    }    public void setId(Integer id) {        this.id = id;    }    @Column(name = "CLASS_NAME")    public String getClassName() {        return this.className;    }    public void setClassName(String className) {        this.className = className;    }    // mappedBy相当于过去的inverse="true".    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "myclass")    public Set
        
          getStudents() {        return this.students;    }    public void setStudents(Set
         
           students) {        this.students = students;    }}

Student

package com.mc.entity;import java.sql.Timestamp;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.FetchType;import javax.persistence.GeneratedValue;import static javax.persistence.GenerationType.IDENTITY;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.ManyToOne;import javax.persistence.Table;/** * Student entity. @author MyEclipse Persistence Tools */@Entity@Table(name = "student", catalog = "test")public class Student implements java.io.Serializable {    // Fields    private Integer id;    private Myclass myclass;    private String stuName;    private Timestamp stuBirth;    // Constructors    /** default constructor */    public Student() {    }    /** full constructor */    public Student(Myclass myclass, String stuName, Timestamp stuBirth) {        this.myclass = myclass;        this.stuName = stuName;        this.stuBirth = stuBirth;    }    // Property accessors    @Id    @GeneratedValue(strategy = IDENTITY)    @Column(name = "ID", unique = true, nullable = false)    public Integer getId() {        return this.id;    }    public void setId(Integer id) {        this.id = id;    }    @ManyToOne(fetch = FetchType.LAZY)    @JoinColumn(name = "STUCLASS_ID")    public Myclass getMyclass() {        return this.myclass;    }    public void setMyclass(Myclass myclass) {        this.myclass = myclass;    }    @Column(name = "STU_NAME")    public String getStuName() {        return this.stuName;    }    public void setStuName(String stuName) {        this.stuName = stuName;    }    @Column(name = "STU_BIRTH", length = 19)    public Timestamp getStuBirth() {        return this.stuBirth;    }    public void setStuBirth(Timestamp stuBirth) {        this.stuBirth = stuBirth;    }}

四、保存

// Hibernate: insert into test.myclass (CLASS_NAME) values (?)    // Hibernate: insert into test.student (STUCLASS_ID, STU_BIRTH, STU_NAME) values (?, ?, ?)    // 有效    public static void test2() {        Session session = HibernateSessionFactory.getSession();        // Hibernate必须要有这个事物才行。        Transaction beginTransaction = session.beginTransaction();        Student student = new Student();        student.setStuName("LCF88");        Myclass myclass = new Myclass();        myclass.setClassName("JAVA88");        myclass.getStudents().add(student);        student.setMyclass(myclass);        session.save(myclass);        beginTransaction.commit();        session.close();    }    // 有效，3条语句（不推荐）    // Hibernate: insert into test.student (STUCLASS_ID, STU_BIRTH, STU_NAME) values (?, ?, ?)    // Hibernate: insert into test.myclass (CLASS_NAME) values (?)    // Hibernate: update test.student set STUCLASS_ID=?, STU_BIRTH=?, STU_NAME=? where ID=?    public static void test3() {        Session session = HibernateSessionFactory.getSession();        // Hibernate必须要有这个事物才行。        Transaction beginTransaction = session.beginTransaction();        Student student = new Student();        student.setStuName("LCF21");        Myclass myclass = new Myclass();        myclass.setClassName("JAVA21");        student.setMyclass(myclass);        session.save(student);        session.save(myclass);        beginTransaction.commit();        session.close();    }    // 有效，2条语句，推荐，理论上先插入class在插入student    // Hibernate: insert into test.myclass (CLASS_NAME) values (?)    // Hibernate: insert into test.student (STUCLASS_ID, STU_BIRTH, STU_NAME) values (?, ?, ?)    public static void test4() {        Session session = HibernateSessionFactory.getSession();        // Hibernate必须要有这个事物才行。        Transaction beginTransaction = session.beginTransaction();        Student student = new Student();        student.setStuName("LCF22");        Myclass myclass = new Myclass();        myclass.setClassName("JAVA22");        student.setMyclass(myclass);        session.save(myclass);        session.save(student);        beginTransaction.commit();        session.close();    }    // 无效    // Hibernate: insert into test.myclass (CLASS_NAME) values (?)    public static void test5() {        Session session = HibernateSessionFactory.getSession();        // Hibernate必须要有这个事物才行。        Transaction beginTransaction = session.beginTransaction();        Student student = new Student();        student.setStuName("LCF16");        Myclass myclass = new Myclass();        myclass.setClassName("JAVA16");        student.setMyclass(myclass);        session.save(myclass);        beginTransaction.commit();        session.close();    }

推荐第4种，最容易理解，在多的一方进行多的一方的数据的维护。

Demo下载地址如下：访问密码 963b

转载地址：http://pmkyo.baihongyu.com/

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