详解Spring Data JPA系列之投影（Projection）的用法

我不是女神ヾ 2022-05-08 23:26 527阅读 0赞

本文介绍了Spring Data JPA系列之投影（Projection）的用法，分享给大家

在JPA的查询中，有一个不方便的地方，@Query注解，如果查询直接是

,这时候，查询的返回对象就是Customer这个完整的对象，包含所有字段，对于我们的示例并没有什么问题，但是对于比较庞大的domain类，这个查询时就比较要命，并不是所有的字段都能用到，比较头疼。另外，如果定义

这个查询结果，返回的对象是Object类型，而且无法直接转换成Customer对象，这样用起来就不是很方便。

对于这种情况，JPA提供了一种声明方式来解决，即声明一个接口类，然后直接使用这个接口类接受返回的数据即可。下面奉上代码：

如下代码 -> apply代码

/**
     * @author: 蔡长盟
     * @email: modules@163.com
     * @date: 201818-9-13
     * @module: 共享空间
     * @describe: 组申请详情是体类
     * @version: v1.0
     */
    package springdata.jpa.jpademo.test.bean;
    
    import org.hibernate.annotations.GenericGenerator;
    
    import javax.persistence.*;
    import javax.validation.constraints.Min;
    import javax.validation.constraints.NotBlank;
    import javax.validation.constraints.Pattern;
    import java.io.Serializable;
    
    @Table(name = "share_group_apply_info")
    @Entity
    public class Apply implements Serializable {
    
        /**
        * @title:    申请记录id
        * @remark:
        * @date:     18-9-13
        */
        @Id
        @GeneratedValue(generator="system_uuid")
        @GenericGenerator(name="system_uuid",strategy="uuid")
        @Column(name = "apply_id")
        private String applyId;
        /**
        * @title:    email
        * @remark:
        * @date:     18-9-15
        */
        private String email;
    
        /**
        * @title:    副标题
        * @remark:
        * @date:     18-9-19
        */
        @Column(name = "sub_title")
        private String subTitle;
    
    
        /**
        * @title:    活动编码
        * @remark:
        * @date:     18-9-13
        */
        @Column(name = "apply_code")
        private String applyCode;
        /**
        * @title:    活动主题
        * @remark:
        * @date:     18-9-13
        */
        // @NotBlank(message = "团建主题不得为空")
        private String title;
        /**
        * @title:    活动描述
        * @remark:
        * @date:     18-9-13
        */
        // @NotBlank(message = "团建描述不得为空")
        private String description;
        /**
        * @title:    活动人数
        * @remark:
        * @date:     18-9-13
        */
        // @Min(value = 1,message = "活动参与人数不得低于1人")
        @Column(name = "member_total")
        private Integer memberTotal;
        /**
        * @title:    开始时间
        * @remark:
        * @date:     18-9-13
        */
        // @NotBlank(message = "活动开始时间不得为空")
        @Pattern(regexp = "^(((20[0-3][0-9]-(0[13578]|1[02])-(0[1-9]|[12][0-9]|3[01]))|(20[0-3][0-9]-(0[2469]|11)-(0[1-9]|[12][0-9]|30))) (20|21|22|23|[0-1][0-9]):[0-5][0-9]:[0-5][0-9])$",
                message = "请输入正确的开始日期")
        @Column(name = "start_time")
        private String startTime;
        /**
        * @title:    结束时间
        * @remark:
        * @date:     18-9-13
        */
        // @NotBlank(message = "活动结束时间不得为空")
        @Pattern(regexp = "^(((20[0-3][0-9]-(0[13578]|1[02])-(0[1-9]|[12][0-9]|3[01]))|(20[0-3][0-9]-(0[2469]|11)-(0[1-9]|[12][0-9]|30))) (20|21|22|23|[0-1][0-9]):[0-5][0-9]:[0-5][0-9])$",
                message = "请输入正确的结束日期")
        @Column(name = "end_time")
        private String endTime;
        /**
        * @title:    创建人openid
        * @remark:
        * @date:     18-9-13
        */
        @Column(name = "owner_id")
        private String ownerId;
        /**
        * @title:    场地id
        * @remark:
        * @date:     18-9-13
        */
        // @NotBlank(message = "请选择所需共享场地")
        @Column(name = "space_id")
        private String spaceId;
        /**
        * @title:    组状态
        * @remark:   团队状态 0: 未开始; 1: 进行中; 2:已结束
        * @date:     18-9-13F
        */
        @Column(name = "group_status")
        private Integer groupStatus = 0;
        /**
         * @title:    审批结果
         * @remark:   0:未审批; 1:审批通过; 2:审批未通过 3:审批中 4: 用户未关注,创建未成功.
         * @date:     18-9-13
         */
        @Column(name = "ratify_result")
        private Integer ratifyResult =0;
        /**
         * @title:    团长名称
         * @remark:
         * @date:     18-9-13F
         */
        // @NotBlank(message = "用户名称不得为空")
        @Column(name = "owner_name")
        private String ownerName;
        /**
         * @title:    团长电话
         * @remark:
         * @date:     18-9-13
         */
        // @NotBlank(message = "用户电话不得为空")
        @Pattern(regexp = "^[1][3|4|5|7|8][0-9]{9}$",
            message = "请输入正确的电话")
        @Column(name = "owner_phone")
        private String ownerPhone;
    
        @OneToOne()
        @JoinColumn(name = "space_id",insertable = false,updatable = false , referencedColumnName = "space_id")
        private Space space;
    
        public Space getSpace() {
            return space;
        }
    
        public void setSpace(Space space) {
            this.space = space;
        }
    
        public String getEmail() {
            return email;
        }
    
        public void setEmail(String email) {
            this.email = email;
        }
    
        public String getOwnerName() {
            return ownerName;
        }
    
        public void setOwnerName(String ownerName) {
            this.ownerName = ownerName;
        }
    
        public String getOwnerPhone() {
            return ownerPhone;
        }
    
        public void setOwnerPhone(String ownerPhone) {
            this.ownerPhone = ownerPhone;
        }
    
        public String getTitle() {
            return title;
        }
    
        public void setTitle(String title) {
            this.title = title;
        }
    
        public String getDescription() {
            return description;
        }
    
        public void setDescription(String description) {
            this.description = description;
        }
    
        public String getApplyCode() {
            return applyCode;
        }
    
        public void setApplyCode(String applyCode) {
            this.applyCode = applyCode;
        }
    
        public Integer getMemberTotal() {
            return memberTotal;
        }
    
        public void setMemberTotal(Integer memberTotal) {
            this.memberTotal = memberTotal;
        }
    
        public String getStartTime() {
            return startTime;
        }
    
        public void setStartTime(String startTime) {
            this.startTime = startTime;
        }
    
        public String getEndTime() {
            return endTime;
        }
    
        public void setEndTime(String endTime) {
            this.endTime = endTime;
        }
    
        public String getOwnerId() {
            return ownerId;
        }
    
        public void setOwnerId(String ownerId) {
            this.ownerId = ownerId;
        }
    
        public Integer getGroupStatus() {
            return groupStatus;
        }
    
        public void setGroupStatus(Integer groupStatus) {
            this.groupStatus = groupStatus;
        }
    
        public Integer getRatifyResult() {
            return ratifyResult;
        }
    
        public void setRatifyResult(Integer ratifyResult) {
            this.ratifyResult = ratifyResult;
        }
    
        public String getApplyId() {
            return applyId;
        }
    
        public void setApplyId(String applyId) {
            this.applyId = applyId;
        }
    
        public String getSpaceId() {
            return spaceId;
        }
    
        public void setSpaceId(String spaceId) {
            this.spaceId = spaceId;
        }
    
        public String getSubTitle() {
            return subTitle;
        }
    
        public void setSubTitle(String subTitle) {
            this.subTitle = subTitle;
        }
    
        @Override
        public String toString() {
            return "ShareGroupApplyInfoEntity{" +
                    "applyId=" + applyId +
                    ", code='" + applyCode + '\'' +
                    ", title='" + title + '\'' +
                    ", describe='" + description + '\'' +
                    ", memberTotal=" + memberTotal +
                    ", startDate=" + startTime +
                    ", endDate=" + endTime +
                    ", openId='" + ownerId + '\'' +
                    ", spaceId=" + spaceId +
                    ", groupStatus=" + groupStatus +
                    '}';
        }
    }

**1、增加applyProjection接口类**

package springdata.jpa.jpademo.test.projections;
    
    /**
     * @author: 蔡长盟
     * @email: modules@163.com
     * @date: 20182018/9/28
     * @module: 类所属模块
     * @describe:
     * @version: v1.0
     */
    public interface ApplyPro {
        public String getEmail();
        public String getOwnerName();
        public String getOwnerPhone();
        public String getTitle();
        public String getDescription();
        public String getApplyCode();
        public Integer getMemberTotal();
        public String getStartTime();
        public String getEndTime();
        public String getOwnerId();
        public Integer getGroupStatus();
        public Integer getRatifyResult();
        public String getApplyId();
        public String getSpaceId();
        public String getSubTitle();
    }

这里声明的方式是可以直接通过get+属性名，这是普通的，另外也可以通过@Value注解来实现指定字段，除了指定字段也可以做聚合展示，比如有些地方需要展示客户的全名，这里定义的getFullName()方法及注解@Value即完成这一操作。需要注意这里的@Value中的target表达式写法及拼接方法。

**2、增加applyRepository方法**

package springdata.jpa.jpademo.test.repository;
    
    import org.springframework.data.domain.Pageable;
    import org.springframework.data.domain.Slice;
    import org.springframework.data.jpa.domain.Specification;
    import org.springframework.data.jpa.repository.JpaRepository;
    import org.springframework.data.jpa.repository.JpaSpecificationExecutor;
    import org.springframework.data.jpa.repository.Query;
    import springdata.jpa.jpademo.test.bean.Apply;
    import springdata.jpa.jpademo.test.projections.ApplyPro;
    
    import java.util.List;
    
    /**
     * @author: 蔡长盟
     * @email: modules@163.com
     * @date: 2018/9/27
     * @module: 类所属模块
     * @describe:
     * @version: v1.0
     */
    public interface ApplyRepository extends JpaRepository<Apply,String>, JpaSpecificationExecutor<Apply> {
        @Query(value = "SELECT\n" +
                "\tapply.apply_id AS applyId,\n" +
                "\tapply.apply_code AS applyCode,\n" +
                "\tapply.title\n" +
                "FROM\n" +
                "\tshare_group_apply_info AS apply",nativeQuery = true)
        List<ApplyPro> findhaha();
    }

**3、增加CustomerController方法**

package springdata.jpa.jpademo.test.controller;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.web.bind.annotation.GetMapping;
    import org.springframework.web.bind.annotation.RequestMapping;
    import org.springframework.web.bind.annotation.RequestParam;
    import org.springframework.web.bind.annotation.RestController;
    import springdata.jpa.jpademo.test.projections.ApplyPro;
    import springdata.jpa.jpademo.test.repository.ApplyRepository;
    import java.util.List;
    /**
     * @author: 蔡长盟
     * @email: modules@163.com
     * @date: 20182018/9/28
     * @module: 类所属模块
     * @describe:
     * @version: v1.0
     */
    @RestController
    @RequestMapping("pro")
    public class ProjectionController {
    
        @Autowired
        private ApplyRepository applyRepository;
    
        @GetMapping("1")
        public List<ApplyPro> find() {
            return applyRepository.findhaha();
        }
    }

返回值:

[
        {
            "description": null,
            "endTime": null,
            "startTime": null,
            "spaceId": null,
            "groupStatus": null,
            "ownerPhone": null,
            "title": "title",
            "ratifyResult": null,
            "ownerName": null,
            "subTitle": null,
            "applyCode": "2",
            "applyId": "4028648166199b4a0166199befc90002",
            "email": null,
            "memberTotal": null,
            "ownerId": null
        },
        {
            "description": null,
            "endTime": null,
            "startTime": null,
            "spaceId": null,
            "groupStatus": null,
            "ownerPhone": null,
            "title": "title",
            "ratifyResult": null,
            "ownerName": null,
            "subTitle": null,
            "applyCode": "3",
            "applyId": "4028648166199b4a066199befc90002",
            "email": null,
            "memberTotal": null,
            "ownerId": null
        },
        {
            "description": null,
            "endTime": null,
            "startTime": null,
            "spaceId": null,
            "groupStatus": null,
            "ownerPhone": null,
            "title": "title",
            "ratifyResult": null,
            "ownerName": null,
            "subTitle": null,
            "applyCode": "1",
            "applyId": "4028648166194a066199befc90002",
            "email": null,
            "memberTotal": null,
            "ownerId": null
        }
    ]

#### 查询关联的子对象. ####

以上图中的apply为例.  
下为applyPro1

下为 ApplyRepository代码

package springdata.jpa.jpademo.test.repository;
    
    import org.springframework.data.domain.Pageable;
    import org.springframework.data.domain.Slice;
    import org.springframework.data.jpa.domain.Specification;
    import org.springframework.data.jpa.repository.JpaRepository;
    import org.springframework.data.jpa.repository.JpaSpecificationExecutor;
    import org.springframework.data.jpa.repository.Query;
    import springdata.jpa.jpademo.test.bean.Apply;
    import springdata.jpa.jpademo.test.projections.ApplyPro;
    import springdata.jpa.jpademo.test.projections.ApplyPro1;
    
    import java.util.List;
    
    /**
     * @author: 蔡长盟
     * @email: modules@163.com
     * @date: 20182018/9/27
     * @module: 类所属模块
     * @describe:
     * @version: v1.0
     */
    public interface ApplyRepository extends JpaRepository<Apply,String>, JpaSpecificationExecutor<Apply> {
        List<ApplyPro1> findBySubTitle(String title);
    }

##### 重点, pro中的space的pro名字要与该space类的类名一致(类名可以扩展该类的属性, 有兴趣可以测试) #####

返回值

[
    	{
    		"space": {
    			"title": "space1"
    		},
    		"title": "title"
    	},
    	{
    		"space": {
    			"title": "space3"
    		},
    		"title": "title"
    	},
    	{
    	"space": {
    			"title": "space2"
    		},
    		"title": "title"
    	}
    ]

这里只是做了简单示意，深入的内容需要自己去挖掘探索。不过关于Projection的资料比较少，我也是扒了不少资料才理解的差不多了，还需要多多实践。

另外spring-data-examples项目中有一些JPA的例子，可以用来学习，梳理思路。[https://github.com/spring-projects/spring-data-examples/tree/master/jpa][https_github.com_spring-projects_spring-data-examples_tree_master_jpa]

参考：

官方文档，[http://docs.spring.io/spring-data/jpa/docs/current/reference/html/][http_docs.spring.io_spring-data_jpa_docs_current_reference_html]

DEMO，[https://github.com/icnws/spring-data-jpa-demo][https_github.com_icnws_spring-data-jpa-demo]

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。

[https_github.com_spring-projects_spring-data-examples_tree_master_jpa]: https://github.com/spring-projects/spring-data-examples/tree/master/jpa
[http_docs.spring.io_spring-data_jpa_docs_current_reference_html]: http://docs.spring.io/spring-data/jpa/docs/current/reference/html/
[https_github.com_icnws_spring-data-jpa-demo]: https://github.com/icnws/spring-data-jpa-demo