Spring repository method which are returning Java 8 stream doesn't close JDBC connection

Question

I have a Spring data repository:

@Repository
interface SomeRepository extends CrudRepository<Entity, Long> {
    Stream<Entity> streamBySmth(String userId);
}

I am calling that method in some Spring bean:

@Scheduled(fixedRate = 10000)
private void someMethod(){
    someRepository.streamBySmth("smth").forEach(this::callSomeMethod);
}

I am using MySQL database. And when I am running application after some successful method invocations it throws an exception:

o.h.engine.jdbc.spi.SqlExceptionHelper   : SQL Error: 0, SQLState: 08001
o.h.engine.jdbc.spi.SqlExceptionHelper   : Could not create connection to database server.
o.s.s.s.TaskUtils$LoggingErrorHandler    : Unexpected error occurred in scheduled task.

org.springframework.dao.DataAccessResourceFailureException: Unable to acquire JDBC Connection; nested exception is org.hibernate.exception.JDBCConnectionException: Unable to acquire JDBC Connection

It seems, that connection was not closed properly by Spring. If I have changed method return value to List from Stream it works correctly.

UPDATE: Spring Boot version is 1.4.1.RELEASE

Answer 1

As the reference documentation clearly states, Streams need to be used with a try-with-resources block.

Also, make sure you keep a (read-only) transaction open for the time of the consumption of the stream by annotating the surrounding method with @Transactional. Otherwise the default settings apply and the resources are attempted to be freed on repository method return.

@Transactional
public void someMethod() {

  try (Stream<User> stream = repository.findAllByCustomQueryAndStream()) {
    stream.forEach(…);
  } 
}