JPA 学习日志 5

五、关系

1. 1-m 一对多双向关联

  多的一方为关系维护端，关系维护端负责外键记录的维护，关系被维护端没有权力更新外键记录。

package cn.itcast.bean.order;

import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.FetchType;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.ManyToOne;

 

@Entity

public class OrderItem {

    private Integer id;

    private String productName;

    private Float sellPrice = 0f;

    private Order order;

    @Id @GeneratedValue

    public Integer getId() {

       return id;

    }

    public void setId(Integer id) {

       this.id = id;

    }

    @Column(length=40,nullable=false)

    public String getProductName() {

       return productName;

    }

    public void setProductName(String productName) {

       this.productName = productName;

    }

    @Column(nullable=false)

    public Float getSellPrice() {

       return sellPrice;

    }

    public void setSellPrice(Float sellPrice) {

       this.sellPrice = sellPrice;

    }

    @ManyToOne(cascade={CascadeType.REFRESH,CascadeType.MERGE},

           fetch=FetchType.EAGER,//XXXOne默认是EAGER，立即加载Order

           optional=false)//optional为true，就是允许Order这个字段为空，false不允许这个字段为null

    @JoinColumn(name="orderId")//设置外键名称

    public Order getOrder() {

       return order;

    }

    public void setOrder(Order order) {

       this.order = order;

    }

}

 

 

 

/********************************************************************/

package cn.itcast.bean.order;

 

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 javax.persistence.Id;

import javax.persistence.OneToMany;

import javax.persistence.Table;

 

@Entity

@Table(name="t_order")//order与数据库关系字冲突，改数据库名称

public class Order {

    private Integer orderId;

    private Float amount = 0f;

    private Set<OrderItem> items = new HashSet<OrderItem>();

    @Id @GeneratedValue

    public Integer getOrderId() {

       return orderId;

    }

    public void setOrderId(Integer orderId) {

       this.orderId = orderId;

    }

    @Column(nullable=false)

    public Float getAmount() {

       return amount;

    }

    public void setAmount(Float amount) {

       this.amount = amount;

    }

    @OneToMany(cascade={CascadeType.REFRESH, CascadeType.PERSIST, CascadeType.REMOVE, CascadeType.MERGE},

           fetch=FetchType.LAZY,

           mappedBy="order")//xxxMany后面是Many的话默认都是延迟加载，保证EntityManager未关闭，XXXOne是EAGER立即加载;

           //如果类中有mappedBy那就是关系被维护端，应该是少的一方。"order"指明关系维护由下面OrderItem类中的order这个属性来维护

    public Set<OrderItem> getItems() {

       return items;

    }

    public void setItems(Set<OrderItem> items) {

       this.items = items;

    }

    public void addOrderItem(OrderItem orderItem) {

       orderItem.setOrder(this);//OrderItem是外键维护端，要建立关系

       this.items.add(orderItem);

    }

   

}

 

 

2．m:n 双向多对多

       双方面是对等的关系，自行决定关系维护端。采用中间表（关联表）来存在它们之间的关系定义。

学生A，老师B

假如我们选定学生为关系维护端，那么如何在中间表中插入一条数据呢？

学生A.getTeachers().add(老师B)， 在数据库就会执行insert into ……

如果学生如果要删除这个老师

学生A.getTeachers().remove(老师B)，在数据库就会执行delete from ……

 

package cn.itcast.bean;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.JoinColumn;

import javax.persistence.JoinTable;

import javax.persistence.ManyToMany;

 

@Entity

public class Student {

    private Integer id;

    private String name;

    private Set<Teacher> teachers = new HashSet<Teacher>();

    @Id @GeneratedValue

    public Integer getId() {

       return id;

    }

    public void setId(Integer id) {

       this.id = id;

    }

    @Column(nullable=false,length=30)

    public String getName() {

       return name;

    }

    public void setName(String name) {

       this.name = name;

    }

    @ManyToMany(cascade=CascadeType.REFRESH)

    @JoinTable(name="student_teacher1",

           inverseJoinColumns=@JoinColumn(name="teacherId"),

           joinColumns=@JoinColumn(name="studentId"))

    //@JoinTable定义中间表，name表名称，inverseJoinColumns是被维护端老师，joinColumns是维护端学生，

    //在中间表中默认名称XXX_id，可以更改名称

    public Set<Teacher> getTeachers() {

       return teachers;

    }

    public void setTeachers(Set<Teacher> teachers) {

       this.teachers = teachers;

    }

}

 

 

/*****************************************************/

package cn.itcast.bean;

import java.util.HashSet;

import java.util.Set;

import javax.persistence.CascadeType;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.ManyToMany;

 

@Entity

public class Teacher {

    private Integer id;

    private String name;

    private Set<Student> students = new HashSet<Student>();

    @Id @GeneratedValue

    public Integer getId() {

       return id;

    }

    public void setId(Integer id) {

       this.id = id;

    }

    @Column(nullable=false, length=30)

    public String getName() {

       return name;

    }

    public void setName(String name) {

       this.name = name;

    }

    @ManyToMany(cascade=CascadeType.REFRESH,mappedBy="teachers")

    //老师是被维护端，由学生的teachers属性来维护，级联操作只能有更新，其它的出问题。

    //如果有级联删除，那么在删除老师的时候会删除他的学生

    public Set<Student> getStudents() {

       return students;

    }

    public void setStudents(Set<Student> students) {

       this.students = students;

    }

}

 

 

3．复合主键：

比如说航线，主键就是由出发点和终点定义的

复合主键我们一般要把它放到一个复合主键类中，这个类必须要遵守的JPA规范：

·要实现Serializable 序列化接口

·要重写hashCode()和equals()方法：用ctrl+shift+s  h 自动生成，选择包括的主键

·要保证有无参的构造方法

package cn.itcast.bean;

 

import java.io.Serializable;

 

import javax.persistence.Column;

import javax.persistence.Embeddable;

 

 

@Embeddable

public class AirLinePK implements Serializable {

    private String start;

    private String end;

   

    public AirLinePK() {}

    public AirLinePK(String start, String end) {

       this.start = start;

       this.end = end;

    }

    @Column(length=3)

    public String getStart() {

       return start;

    }

    public void setStart(String start) {

       this.start = start;

    }

    @Column(length=3)

    public String getEnd() {

       return end;

    }

    public void setEnd(String end) {

       this.end = end;

    }

    @Override

    public int hashCode() {

       final int prime = 31;

       int result = 1;

       result = prime * result + ((end == null) ? 0 : end.hashCode());

       result = prime * result + ((start == null) ? 0 : start.hashCode());

       return result;

    }

    @Override

    public boolean equals(Object obj) {

       if (this == obj)

           return true;

       if (obj == null)

           return false;

       if (getClass() != obj.getClass())

           return false;

       final AirLinePK other = (AirLinePK) obj;

       if (end == null) {

           if (other.end != null)

              return false;

       } else if (!end.equals(other.end))

           return false;

       if (start == null) {

           if (other.start != null)

              return false;

       } else if (!start.equals(other.start))

           return false;

       return true;

    }

}

 

 

 

package cn.itcast.bean;

 

import javax.persistence.Column;

import javax.persistence.EmbeddedId;

import javax.persistence.Entity;

 

@Entity

public class AireLine {

    private AirLinePK id;

    private String name;

    //要保证有一个无参数的构造方法

    public AireLine(){}

    //为了方便保存数据我们添加这个构造方法

    public AireLine(String start, String end, String name) {

       this.id = new AirLinePK(start, end);

       this.name = name;

    }

   

    @EmbeddedId

    public AirLinePK getId() {

       return id;

    }

    public void setId(AirLinePK id) {

       this.id = id;

    }

    @Column(length=20)

    public String getName() {

       return name;

    }

    public void setName(String name) {

       this.name = name;

    }

}