Spring Data JPA中踩過的坑

說老實話，Spring Data JPA很好用，上次使用還是2013年，當時只是完成Java Bean和數據庫中表的映射。

最近想起來用Spring Data JPA的起因是手頭有一個項目，源代碼是用原生SQL+JDBC實現的，在第一次部署時要初始化數據庫，還hardcode了很多數據庫配置參數。正好最近有空，就打算用Spring Boot Data JPA（spring-boot-starter-data-jpa）改造一下，仔細看了一下源代碼發現和幾年前已經天差地別，如果你的業務邏輯不是特別復雜或者表結構設計合理，一行真正SQL邏輯實現都不要寫，良心啊，真是為我這樣的懶人着想啊。

首先開始寫Repository，常見的用法有兩種，一個是寫一個接口繼承JpaRepository，代碼如下：

package com.company.inventory.repository;

import java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;

import comcompany.inventory.model.Device;

public interface DeviceRepository extends JpaRepository<Device, Long> {

    List<Device> findBySnOrderByGmtCreatedDesc(String sn);

}

另外一個是繼承CrudRepository，代碼如下：

package com.company.inventory.repository;

import java.util.List;

import org.springframework.data.repository.CrudRepository;

import comcompany.inventory.model.Device;

public interface DeviceRepository extends CrudRepository<Device, Long> {

    List<Device> findBySnOrderByGmtCreatedDesc(String sn);

}

JpaRepository和CrudRepository關系如下：

 

public interface JpaRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> 

public interface PagingAndSortingRepository<T, ID> extends CrudRepository<T, ID> 

public interface CrudRepository<T, ID> extends Repository<T, ID>

從上面的關系就能看出JpaRepository除了能做CrudRepository能做的所有事外，還多了分頁和排序功能以及QueryByExampleExecutor提供的QueryByExample功能。但是JpaRepository也和JPA持久化技術進行了綁定。http://jtuts.com/2014/08/26/difference-between-crudrepository-and-jparepository-in-spring-data-jpa/

所以建議盡量使用CrudRepository或者PagingAndSortingRepository

Supported keywords inside method names

Keyword	Sample	JPQL snippet
And	findByLastnameAndFirstname	… where x.lastname = ?1 and x.firstname = ?2
Or	findByLastnameOrFirstname	… where x.lastname = ?1 or x.firstname = ?2
Is,Equals	findByFirstname, findByFirstnameIs, findByFirstnameEquals	… where x.firstname = ?1
Between	findByStartDateBetween	… where x.startDate between ?1 and ?2
LessThan	findByAgeLessThan	… where x.age < ?1
LessThanEqual	findByAgeLessThanEqual	… where x.age <= ?1
GreaterThan	findByAgeGreaterThan	… where x.age > ?1
GreaterThanEqual	findByAgeGreaterThanEqual	… where x.age >= ?1
After	findByStartDateAfter	… where x.startDate > ?1
Before	findByStartDateBefore	… where x.startDate < ?1
IsNull	findByAgeIsNull	… where x.age is null
IsNotNull,NotNull	findByAge(Is)NotNull	… where x.age not null
Like	findByFirstnameLike	… where x.firstname like ?1
NotLike	findByFirstnameNotLike	… where x.firstname not like ?1
StartingWith	findByFirstnameStartingWith	… where x.firstname like ?1(parameter bound with appended %)
EndingWith	findByFirstnameEndingWith	… where x.firstname like ?1(parameter bound with prepended %)
Containing	findByFirstnameContaining	… where x.firstname like ?1(parameter bound wrapped in %)
OrderBy	findByAgeOrderByLastnameDesc	… where x.age = ?1 order by x.lastname desc
Not	findByLastnameNot	… where x.lastname <> ?1
In	findByAgeIn(Collection<Age> ages)	… where x.age in ?1
NotIn	findByAgeNotIn(Collection<Age> ages)	… where x.age not in ?1
True	findByActiveTrue()	… where x.active = true
False	findByActiveFalse()	… where x.active = false
IgnoreCase	findByFirstnameIgnoreCase	… where UPPER(x.firstame) = UPPER(?1)

 接下來是定義Entity，遇到了第一個坑，代碼如下：

package com.company.inventory.model;

import java.io.Serializable;

import java.util.Date;

import javax.persistence.Column;

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

import javax.persistence.Table;

@Entity

@Table( name = "device" )

public class Device implements Serializable {

    @Id

    @GeneratedValue(strategy=GenerationType.IDENTITY)

    private Long id;

    @Column(unique = true, nullable = false)

    private String sn;

    @Column(name = "gmtCreated", nullable = false)

    private Date gmtCreated = new Date();

    public Long getId() {

        return id;

    }

    public void setId(Long id) {

        this.id = id;

    }

    public Date getGmtCreated() {

        return gmtCreated;

    }

    public void setGmtCreated(Date gmtCreated) {

        this.gmtCreated = gmtCreated;

    } 

 }

第一個坑，該類必須是實體類，不能繼承於任何基類，除非其中的參數不需要映射到數據庫中。 

例如有一個基類如下：

public class BaseModel{

    protected String name;

    //set, get方法忽略 

} 

public class Person extends BaseModel  implements Serializable{

    @Id

    @GeneratedValue(strategy=GenerationType.IDENTITY)

   private Long id; 

} 

name將不能映射到數據庫中。

第二個坑，如果Field類似gmtCreated，有大寫字母的，默認映射到數據庫字段名會變為gmt_created。如果你的設計不是這樣，而是直接映射不做修改。需要在application.properties中指定Naming strategy，否則默認是org.hibernate.boot.orm.jpa.hibernate.SpringPhysicalNamingStrategy，在進行領域映射時，首字母小寫，大寫字母變為下划線加小寫。添加代碼如下：

spring.jpa.hibernate.naming.physical-strategy = org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl 

第三個坑，如果使用like，代碼如下：

public Page<User> findUserByKey(String key, int page, int size){

    Page<User> result = userRepository.findAll(new Specification<User> () {  

            public Predicate toPredicate(Root<User> root, CriteriaQuery<?> query, CriteriaBuilder cb) {  

                Join<User, Role> join = root.join("role", JoinType.INNER);

                Path<String> namePath = join.get("name");  

                Path<String> statusPath = root.get("status");  

                Path<String> usernamePath = root.get("username"); 

                Path<String> nicknamePath = root.get("nickname");

                Predicate or = cb.or(cb.like(namePath, key), cb.like(usernamePath, key), cb.like(nicknamePath, key));

                return cb.and(or, cb.equal(statusPath, 0));   

            }  

        },  PageRequest.of(page - 1, size));

        return result;

    }

其中的key，必須是類似%abc%，而不是'%abc%'。
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