What is implementation class of org.springframework.data.jpa.repository.JpaRepository interface?

Notice

I wrote this article and was originally published on Qiita on 17 September 2019.

From my notice board example application (Spring Data JPA version). You may notice that only 2 interface is defined only on repository layer

no implementation class on these interface is defined. So what actual instance is injected into info.saladlam.example.spring.noticeboard.service.MessageServiceImpl?

That instance is a proxy, with target class instance org.springframework.data.jpa.repository.support.SimpleJpaRepository and many advice/advisor/interceptor is attached. One important interceptor is org.springframework.data.repository.core.support.RepositoryFactorySupport.QueryExecutorMethodInterceptor.

Definition of info.saladlam.example.spring.noticeboard.repository.MessageRepository

public interface MessageRepository extends JpaRepository<Message, Long> { @Query("SELECT m FROM Message m WHERE m.approvedBy IS NOT NULL AND m.publishDate <= :at AND (m.removeDate IS NULL OR m.removeDate > :at) ORDER BY m.publishDate DESC") List<Message> findPublished(@Param("at") LocalDateTime at); @Query("SELECT m FROM Message m WHERE m.approvedBy IS NULL ORDER BY m.publishDate DESC") List<Message> findWaitingApprove(); @Query("SELECT m FROM Message m WHERE m.owner = :owner ORDER BY m.publishDate DESC") List<Message> findByOwner(@Param("owner") String owner); } 

and its parent interface org.springframework.data.jpa.repository.JpaRepository

@NoRepositoryBean public interface JpaRepository<T, ID> extends PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> { List<T> findAll(); List<T> findAll(Sort sort); List<T> findAllById(Iterable<ID> ids); <S extends T> List<S> saveAll(Iterable<S> entities); void flush(); <S extends T> S saveAndFlush(S entity); void deleteInBatch(Iterable<T> entities); void deleteAllInBatch(); T getOne(ID id); @Override <S extends T> List<S> findAll(Example<S> example); @Override <S extends T> List<S> findAll(Example<S> example, Sort sort); } 

For example, when a method findAll() defined in JpaRepository is called, as result method findAll() of SimpleJpaRepository is called. And when a method findWaitingApprove() defined in MessageRepository is called, interceptor org.springframework.data.repository.core.support.RepositoryFactorySupport.QueryExecutorMethodInterceptor is involved. In method org.springframework.data.repository.core.support.RepositoryFactorySupport.QueryExecutorMethodInterceptor.doInvoke()

 @Nullable private Object doInvoke(MethodInvocation invocation) throws Throwable { Method method = invocation.getMethod(); Object[] arguments = invocation.getArguments(); if (hasQueryFor(method)) { return queries.get(method).execute(arguments); } return invocation.proceed(); } 

queries is a map, which store org.springframework.data.jpa.repository.query.JpaQueryMethod instance with key method defined in MessageRepository.