SpringBoot之JPA

前言：用Spring全家桶，大家常用的应该是jpa，上次我看过一次调查统计，歪果人使用持久化框架jpa比较多，目前国内已知互联网公司mybatis会比较多，可能大家都觉得mybatis的查询效率更高。 SpringData JPA只是SpringData中的一个子模块，JPA是一套标准接口，而Hibernate是JPA的实现，SpringData JPA 底层默认实现是使用Hibernate，SpringDataJPA 的首个接口就是Repository，它是一个标记接口。只要我们的接口实现这个接口，那么我们就相当于在使用SpringDataJPA了。

Spring Data存储库抽象中的中央接口是Repository。它将域类以及域类的ID类型作为类型参数进行管理。此接口主要用作标记接口，用于捕获要使用的类型，并帮助您发现扩展此接口的接口。该CrudRepository规定对于正在管理的实体类复杂的CRUD功能。

1public interface CrudRepository<T, ID extends Serializable>
2  extends Repository<T, ID> {
3
4  //保存给定的实体
5  <S extends T> S save(S entity);      
6
7  //返回指定ID的实体
8  Optional<T> findById(ID primaryKey); 
9
10  //返回所有的实体
11  Iterable<T> findAll();  
12
13  //返回实体数量             
14  long count();   
15
16                     
17  //删除
18  void delete(T entity);   
19
20             
21  //查询是否存在某一个数据
22  boolean existsById(ID primaryKey);   
23
24  // … more functionality omitted.
25}

最重要的是CrudRepository，有一个PagingAndSortingRepository抽象添加了额外的方法来简化对实体的分页访问

1public interface PagingAndSortingRepository<T, ID> extends CrudRepository<T, ID> {
2
3	
4    //排序
5	Iterable<T> findAll(Sort sort);
6
7    //分页加查询
8	Page<T> findAll(Pageable pageable);
9}

一般我们自己的jpa接口会继承 jpa，因为jpa既继承了page分页和 query查询的接口。

1public interface JpaRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> {
2    List<T> findAll();
3
4    List<T> findAll(Sort var1);
5
6    List<T> findAllById(Iterable<ID> var1);
7
8    <S extends T> List<S> saveAll(Iterable<S> var1);
9
10    void flush();
11
12    <S extends T> S saveAndFlush(S var1);
13
14    void deleteInBatch(Iterable<T> var1);
15
16    void deleteAllInBatch();
17
18    T getOne(ID var1);
19
20    <S extends T> List<S> findAll(Example<S> var1);
21
22    <S extends T> List<S> findAll(Example<S> var1, Sort var2);
23}

jpa常用的关键词

关键词

样品

JPQL代码段

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（附加参数绑定%）

EndingWith

findByFirstnameEndingWith

… where x.firstname like ?1（与前置绑定的参数%）

Containing

findByFirstnameContaining

… where x.firstname like ?1（包含参数绑定%）

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)

我们自定义hql语句

1public interface UserRepository extends JpaRepository<User, Long> {
2
3  @Query("select u from User u where u.emailAddress = ?1")
4  User findByEmailAddress(String emailAddress);
5}

我们自定义的原生sql语句

1public interface UserRepository extends JpaRepository<User, Long> {
2
3  @Query(value = "SELECT * FROM USERS WHERE EMAIL_ADDRESS = ?1", nativeQuery = true)
4  User findByEmailAddress(String emailAddress);
5}

占位符？用下标来

占位符 :name 用参数名来占位用@Param(“name”) 来替换

增删改语句必须用@Modifying注解

1@Modifying
2@Query("update User u set u.firstname = ?1 where u.lastname = ?2")
3int setFixedFirstnameFor(String firstname, String lastname);

CrudRepository里有个findById该方法的返回值是一个Optional<T>，在Optional类中有个get()方法，返回的是当前对象

1/**
2 * Retrieves an entity by its id.
3 *
4 * @param id must not be {@literal null}.
5 * @return the entity with the given id or {@literal Optional#empty()} if none found
6 * @throws IllegalArgumentException if {@code id} is {@literal null}.
7 */
8Optional<T> findById(ID id);

JpaRepository里有个getOne()方法返回的是实体对象的代理对象（a reference）

如果不开启事务会抛出org.hibernate.LazyInitializationException: could not initialize proxy - no Session的异常

1/**
2 * Returns a reference to the entity with the given identifier. Depending on how the JPA persistence provider is
3 * implemented this is very likely to always return an instance and throw an
4 * {@link javax.persistence.EntityNotFoundException} on first access. Some of them will reject invalid identifiers
5 * immediately.
6 *
7 * @param id must not be {@literal null}.
8 * @return a reference to the entity with the given identifier.
9 * @see EntityManager#getReference(Class, Object) for details on when an exception is thrown.
10 */
11T getOne(ID id);

QueryByExampleExecutor里有个findOne 查询不到数据会 throw new NoSuchElementException("No value present");

1/**
2	 * Returns a single entity matching the given {@link Example} or {@literal null} if none was found.
3	 *
4	 * @param example must not be {@literal null}.
5	 * @return a single entity matching the given {@link Example} or {@link Optional#empty()} if none was found.
6	 * @throws org.springframework.dao.IncorrectResultSizeDataAccessException if the Example yields more than one result.
7	 */
8<S extends T> Optional<S> findOne(Example<S> example);

持续更新中。。。。

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