SpringBoot之JPA

撸了今年阿里、网易和美团的面试，我有一个重要发现.......>>>

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

CrudRepository

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

//保存给定的实体
<S extends T> S save(S entity);

//返回指定ID的实体
Optional<T> findById(ID primaryKey);

//返回所有的实体
Iterable<T> findAll();

//返回实体数量
long count();

//删除
void delete(T entity);

//查询是否存在某一个数据
boolean existsById(ID primaryKey);

// … more functionality omitted.
}

最重要的是

CrudRepository

PagingAndSortingRepository

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

//排序
Iterable<T> findAll(Sort sort);

//分页加查询
Page<T> findAll(Pageable pageable);
}

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。

public interface JpaRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> {
List<T> findAll();

List<T> findAll(Sort var1);

List<T> findAllById(Iterable<ID> var1);

<S extends T> List<S> saveAll(Iterable<S> var1);

void flush();

<S extends T> S saveAndFlush(S var1);

void deleteInBatch(Iterable<T> var1);

void deleteAllInBatch();

T getOne(ID var1);

<S extends T> List<S> findAll(Example<S> var1);

<S extends T> List<S> findAll(Example<S> var1, Sort var2);
}

jpa常用的关键词

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

%

EndingWith

findByFirstnameEndingWith

… where x.firstname like ?1

%

Containing

findByFirstnameContaining

… where x.firstname like ?1

%

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

public interface UserRepository extends JpaRepository<User, Long> {

@Query("select u from User u where u.emailAddress = ?1")
User findByEmailAddress(String emailAddress);
}

 我们自定义的原生sql语句

public interface UserRepository extends JpaRepository<User, Long> {

@Query(value = "SELECT * FROM USERS WHERE EMAIL_ADDRESS = ?1", nativeQuery = true)
User findByEmailAddress(String emailAddress);
}

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

@Modifying
@Query("update User u set u.firstname = ?1 where u.lastname = ?2")
int 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

/**
* Retrieves an entity by its id.
*
* @param id must not be {@literal null}.
* @return the entity with the given id or {@literal Optional#empty()} if none found
* @throws IllegalArgumentException if {@code id} is {@literal null}.
*/
Optional<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

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

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

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

持续更新中。。。。

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