史上最简单的JPA关联教程

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：

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：

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：

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。

请求的结果如下所示：