史上最简单的JPA关联教程

电玩女神 2022-06-01 10:35 223阅读 0赞

**JPA关联查询**

因为项目中我们用到的都是双向管理关系，所以这边单向的我就不多做介绍。

我们这边接着上一节的课程继续介绍，这边我新建了Goods,GoodsDetail,Classify,Address四个实体映射类。分别进行一对一，一对多，多对多的关联介绍。

**1.首先是一对一关系介绍，这边一对一的关系分别是Goods和GoodsDetail（商品表和商品详细表）**

**关联的注释为@OneToOne**

Goods实体类：

package com.lzq.jpa.entity;

import org.hibernate.annotations.GenericGenerator;

import javax.persistence.\*;

import java.io.Serializable;

import java.util.ArrayList;

import java.util.List;

/\*\*

\* Created by qiang on 2018/1/22.

\*/

@Entity

@Table(name="goods")

public class Goods implements Serializable \{

@Id

@GenericGenerator(name = "PKUUID", strategy = "uuid2")

@GeneratedValue(generator = "PKUUID")

@Column(length = 36)

protected String id;

/\*\*

\* 名字

\*/

@Column(name = "name", nullable = true, length = 30)

private String name;

@OneToOne(fetch = FetchType.LAZY,mappedBy="goods")

private GoodsDetail goodsDetail;

public Goods() \{

public String getId() \{

return id;

public void setId(String id) \{

this.id = id;

public String getName() \{

return name;

public void setName(String name) \{

this.name = name;

public GoodsDetail getGoodsDetail() \{

return goodsDetail;

public void setGoodsDetail(GoodsDetail goodsDetail) \{

this.goodsDetail = goodsDetail;

GoodsDetail实体类：

package com.lzq.jpa.entity;

import org.hibernate.annotations.GenericGenerator;

import javax.persistence.\*;

import java.io.Serializable;

/\*\*

\* Created by qiang on 2018/1/22.

\*/

@Entity

@Table(name="goods\_detail")

public class GoodsDetail implements Serializable \{

@Id

@GenericGenerator(name = "PKUUID", strategy = "uuid2")

@GeneratedValue(generator = "PKUUID")

@Column(length = 36)

protected String id;

/\*\*

\* 名字

\*/

@Column(name = "name", nullable = true, length = 30)

private String name;

@OneToOne(cascade=CascadeType.ALL)

@JoinColumn(name="goods\_id")

private Goods goods;

public GoodsDetail() \{

public String getId() \{

return id;

public void setId(String id) \{

this.id = id;

public String getName() \{

return name;

public void setName(String name) \{

this.name = name;

public Goods getGoods() \{

return goods;

public void setGoods(Goods goods) \{

this.goods = goods;

这边有一个问题是：如果采用这种双向的一对一关系就会产生【SSH异常系列】Cannot call sendError() after the response has been committed异常。因为goods会关联goodsDetail，然后goodsDetail会继续关联goods，这样就会产生死循环的问题。这边有三种解决办法，这边我就介绍一种解决办法，其它的大家可以参考下面这篇博客里介绍的方法：https://www.cnblogs.com/zr520/archive/2016/04/06/5357459.html

我这边介绍的方法是：在控制的一方添加：@JsonIgnore 注解，然后在实体类上面添加@JsonIgnoreProperties(\{"hibernateLazyInitializer", "handler"\}) 注解。如图所示：

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1][]

但是这种方法也会有问题，就是设置JsonIgnore 的一方，是不能将所关联的数据查询出来的。

就比如上面goods只能查询到商品本身的信息，但是goodsDetail是不会关联查询出来的。但是没有设置JsonIgnore 的一方就会全部关联查询出来。这是这个方法的缺陷，可以采用其他的方法，方法就在上面给出的博客里面。

**请求的结果如下所示：**

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 1][]

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 2][]

**2.接下来就是介绍双向一对多的关联查询了，这边我用用户实体类（user）和地址实体类（address）来做具体介绍，注解分别为@OneToMany和@ManyToOne（一对多和多对一）**

用户实体类：

package com.lzq.jpa.entity;

import com.fasterxml.jackson.annotation.JsonIgnore;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

import org.hibernate.annotations.GenericGenerator;

import javax.persistence.\*;

import java.io.Serializable;

import java.util.List;

/\*\*

\* Created by qiang on 2018/1/22.

\*/

@JsonIgnoreProperties(\{"hibernateLazyInitializer", "handler"\})

@Entity

@Table(name="user")

public class User implements Serializable \{

@Id

@GenericGenerator(name = "PKUUID", strategy = "uuid2")

@GeneratedValue(generator = "PKUUID")

@Column(length = 36)

protected String id;

/\*\*

\* 名字

\*/

@Column(name = "name", nullable = true, length = 30)

private String name;

/\*\*

\* 身高

\*/

@Column(name = "height", nullable = true, length = 10)

private Integer height;

/\*\*

\* 用户所创建的地址信息

\*/

@JsonIgnore

@OneToMany(cascade=\{CascadeType.ALL\},fetch = FetchType.LAZY,mappedBy = "user")

private List<Address> addresses;

public User() \{

public String getId() \{

return id;

public void setId(String id) \{

this.id = id;

public String getName() \{

return name;

public void setName(String name) \{

this.name = name;

public Integer getHeight() \{

return height;

public void setHeight(Integer height) \{

this.height = height;

public List<Address> getAddresses() \{

return addresses;

public void setAddresses(List<Address> addresses) \{

this.addresses = addresses;

地址实体类：

package com.lzq.jpa.entity;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

import org.hibernate.annotations.GenericGenerator;

import javax.persistence.\*;

import java.io.Serializable;

import java.util.List;

/\*\*

\* Created by qiang on 2018/1/22.

\*/

@Entity

@Table(name="address")

public class Address implements Serializable \{

@Id

@GenericGenerator(name = "PKUUID", strategy = "uuid2")

@GeneratedValue(generator = "PKUUID")

@Column(length = 36)

protected String id;

/\*\*

\* 名字

\*/

@Column(name = "name", nullable = true, length = 30)

private String name;

/\*\*

\* 地址详情信息

\*/

@Column(name = "detail", nullable = true, length = 100)

private String detail;

/\*\*

\* 地址所创建者的用户信息

\*/

@ManyToOne

@JoinColumn(name = "user\_id",foreignKey = @ForeignKey(name = "fk\_user\_id"))

private User user;

public Address() \{

public String getId() \{

return id;

public void setId(String id) \{

this.id = id;

public String getName() \{

return name;

public void setName(String name) \{

this.name = name;

public String getDetail() \{

return detail;

public void setDetail(String detail) \{

this.detail = detail;

public User getUser() \{

return user;

public void setUser(User user) \{

this.user = user;

这边也会存在循环关联的问题，我采用的方法也是通过JsonIgnore来解决的。**mappedBy**表示哪一方来主导，**fetch = FetchType.LAZY**表示进行懒加载，**cascade=\{CascadeType.ALL\}**表示进行相应的关联操作。这些参数我会在实体类参数介绍的时候，具体给大家讲解的，这节课先不多做介绍。

**请求的结果如下所示：**

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 3][]

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 4][]

**3.接下来最后的多对多查询，这边我用商品实体类（goods）和商品分类实体类（classify）给大家做细致的介绍。注解为：@ManyToMany**

商品实体类（goods）：

package com.lzq.jpa.entity;

import com.fasterxml.jackson.annotation.JsonIgnore;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

import org.hibernate.annotations.GenericGenerator;

import javax.persistence.\*;

import java.io.Serializable;

import java.util.ArrayList;

import java.util.List;

/\*\*

\* Created by qiang on 2018/1/22.

\*/

@JsonIgnoreProperties(\{"hibernateLazyInitializer", "handler"\})

@Entity

@Table(name="goods")

public class Goods implements Serializable \{

@Id

@GenericGenerator(name = "PKUUID", strategy = "uuid2")

@GeneratedValue(generator = "PKUUID")

@Column(length = 36)

protected String id;

/\*\*

\* 名字

\*/

@Column(name = "name", nullable = true, length = 30)

private String name;

@ManyToMany(fetch = FetchType.LAZY,cascade = CascadeType.MERGE)

@JoinTable(name = "goods\_classify\_links",

joinColumns= \{ @JoinColumn(name = "goods\_id", referencedColumnName = "id") \}

, inverseJoinColumns = \{ @JoinColumn(name = "classify\_id", referencedColumnName = "id") \}

,inverseForeignKey = @ForeignKey(name = "fk\_mr\_links\_goods\_classify")

,foreignKey = @ForeignKey(name = "fk\_mr\_links\_classify\_goods"))

private List<Classify> classifies= new ArrayList<>();

@JsonIgnore

@OneToOne(fetch = FetchType.LAZY,mappedBy="goods")

private GoodsDetail goodsDetail;

public Goods() \{

public String getId() \{

return id;

public void setId(String id) \{

this.id = id;

public String getName() \{

return name;

public void setName(String name) \{

this.name = name;

public List<Classify> getClassifies() \{

return classifies;

public void setClassifies(List<Classify> classifies) \{

this.classifies = classifies;

public GoodsDetail getGoodsDetail() \{

return goodsDetail;

public void setGoodsDetail(GoodsDetail goodsDetail) \{

this.goodsDetail = goodsDetail;

商品分类实体类（classify）：

package com.lzq.jpa.entity;

import com.fasterxml.jackson.annotation.JsonIgnore;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

import org.hibernate.annotations.GenericGenerator;

import javax.persistence.\*;

import java.io.Serializable;

import java.util.ArrayList;

import java.util.List;

/\*\*

\* Created by qiang on 2018/1/22.

\*/

@JsonIgnoreProperties(\{"hibernateLazyInitializer", "handler"\})

@Entity

@Table(name="classify")

public class Classify implements Serializable \{

@Id

@GenericGenerator(name = "PKUUID", strategy = "uuid2")

@GeneratedValue(generator = "PKUUID")

@Column(length = 36)

protected String id;

/\*\*

\* 名字

\*/

@Column(name = "name", nullable = true, length = 30)

private String name;

@JsonIgnore

@ManyToMany(fetch = FetchType.LAZY,cascade = CascadeType.MERGE)

@JoinTable(name = "goods\_classify\_links",

joinColumns= \{ @JoinColumn(name = "goods\_id", referencedColumnName = "id") \}

, inverseJoinColumns = \{ @JoinColumn(name = "classify\_id", referencedColumnName = "id") \}

,inverseForeignKey = @ForeignKey(name = "fk\_mr\_links\_goods\_classify")

,foreignKey = @ForeignKey(name = "fk\_mr\_links\_classify\_goods"))

private List<Goods> goodses= new ArrayList<>();

public Classify() \{

public String getId() \{

return id;

public void setId(String id) \{

this.id = id;

public String getName() \{

return name;

public void setName(String name) \{

this.name = name;

public List<Goods> getGoodses() \{

return goodses;

public void setGoodses(List<Goods> goodses) \{

this.goodses = goodses;

这里需要注意的是，虽然是两张表，但是在运行项目的时候会自动生成第三张关系映射表，表的名称和字段，就是@ManyToMany下面设置的字段和名称，还有表的外键也是在ForeignKey里面设置的。![SouthEast][]**请求的结果如下所示：**

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 5][]

![640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 6][]

这里有一个地方大家一定不要误解，我截图上面的只能查询本身，指的是我查询出本身这个实体类的时候，它所关联的实体类不会一起查询出来，但是如果用get实体类的方法，是可以查询所关联的数据的，这点大家不要误解了。

**到这里关于JPA关联查询就介绍完毕了，下一节将介绍jpa中原生sql查询**

**项目GitHub地址：****https://github.com/1913045515/JPA**

[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1]: /images/20220601/f30074c868ae4756b5968afa5f8f2224.png
[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 1]: /images/20220601/d19c348ced924d119cb7489d81bb1af5.png
[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 2]: /images/20220601/92a721c5bac64756819627739c1a8a61.png
[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 3]: /images/20220601/36f8a1a23733424292458e319a9a7b1b.png
[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 4]: /images/20220601/067789a377264ea0aea41d7e6a7d4547.png
[SouthEast]: /images/20220601/95538bf7524c400abceb32c4e319db08.png
[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 5]: /images/20220601/654584cc94834f778c607b92f414730d.png
[640_wx_fmt_png_tp_webp_wxfrom_5_wx_lazy_1 6]: /images/20220601/dafe5a3e977443ae884e080f4ba331b0.png