Our environment is Spring Boot with Spring Data JPA. I’m dealing with significant legacy code, and we have a generic problem I’m trying to fix.

We have a number of places where a REST call spawns a background thread:

org.hibernate.LazyInitializationException: could not initialize proxy [org.showpage.threadingtransactions.dbmodel.Author#1] - no Session
    at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:165) ~[hibernate-core-6.5.2.Final.jar:6.5.2.Final]
    at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:314) ~[hibernate-core-6.5.2.Final.jar:6.5.2.Final]
    at org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor.intercept(ByteBuddyInterceptor.java:44) ~[hibernate-core-6.5.2.Final.jar:6.5.2.Final]
    at org.hibernate.proxy.ProxyConfiguration$InterceptorDispatcher.intercept(ProxyConfiguration.java:102) ~[hibernate-core-6.5.2.Final.jar:6.5.2.Final]
    at org.showpage.threadingtransactions.dbmodel.Author$HibernateProxy$n6D0Du2E.getName(Unknown Source) ~[classes/:na]
    at org.showpage.threadingtransactions.service.SubService.populate(SubService.java:44) ~[classes/:na]
    at org.showpage.threadingtransactions.service.BookInfoService.lambda$spawnWillPass$1(BookInfoService.java:115) ~[classes/:na]
    at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]

This is from a sample project I created to try to work on a general solution, which can be seen at https://github.com/jplflyer/SpringDataThreading.git.

My sample application is very small. I have a WebController.java:

@RestController
@RequiredArgsConstructor
@Slf4j
public class WebController {
    private final AuthorRepository authorRepository;
    private final BookRepository bookRepository;
    private final BookInfoService bookInfoService;

    @GetMapping("/request")
    public ResponseEntity<String> makeRequest(
            @RequestParam(required = false, defaultValue = "false") boolean pass,
            @RequestParam(name = "no_spawn", required = false, defaultValue = "false") boolean noSpawn
    ) {
        log.info("makeRequest({}, {})", pass, noSpawn);
        BookInfoService.Mode mode = noSpawn
                ? BookInfoService.Mode.NoSpawn
                : ( pass ? BookInfoService.Mode.SpawnWillPass : BookInfoService.Mode.SpawnWillFail);
        int requestId = bookInfoService.makeRequest(mode);
        return ResponseEntity.ok(String.format("%dn", requestId));
    }


}

BookInfoService.java is longer only because I’ve tried a bunch of ways to fix this:

@Service
@RequiredArgsConstructor
@Slf4j
public class BookInfoService {
    public enum Mode {
        SpawnWillFail,
        SpawnWillPass,
        NoSpawn
    }

    private final BookRepository bookRepository;
    private final EntityManager entityManager;
    private SessionFactory sessionFactory;

    private static int requestId = 0;

    /**
     * Spawn the request.
     */
    public int makeRequest(Mode mode) {
        BookInfo info = BookInfo
                .builder()
                .requestId(++requestId)
                .build();

        switch (mode) {
            case SpawnWillFail: spawnWillFail(info); break;
            case SpawnWillPass: spawnWillPass(info); break;
            case NoSpawn: runNoSpawn(info); break;
        }
        return info.getRequestId();
    }

    /**
     * This version to prove it works if we don't run worker threads.
     */
    private void runNoSpawn(BookInfo info) {
        SubService subService = new SubService(entityManager, bookRepository);

        try {
            subService.populate(info);
        }
        catch (Exception e) {
            log.error("Exception", e);
        }
    }

    /**
     * This version will fail with a lazy initialization exception.
     */
    private void spawnWillFail(BookInfo info) {
        List<Book> allBooks = bookRepository.findAll();

        new Thread(() -> {
            log.info("Bad Thread start");
            populate(info, allBooks);
            log.info("Bad Thread done");
        }).start();
    }

    /**
     * This version will pass once I get it to work but currently fails.
     */
    private void spawnWillPass(BookInfo info) {
        EntityManager em = entityManager
                .getEntityManagerFactory()
                .createEntityManager();

        SessionFactory sessionFactory = em.getEntityManagerFactory().unwrap(SessionFactory.class);

        SubService subService = new SubService(em, bookRepository);

        new Thread(() -> {
            log.info("Good Thread start");

            try {
                Session session = sessionFactory.openSession();
                em.getTransaction().begin();
                subService.populate(info);
                em.getTransaction() .commit();
            }
            catch (Exception e) {
                log.error("Exception", e);
                em.getTransaction().rollback();
            }
            finally {
                em.close();
                sessionFactory.close();
            }

            log.info("Good Thread done");
        }).start();
    }
}

SubService.java is boring:

@RequiredArgsConstructor
@Slf4j
@Service
public class SubService {

    @PersistenceContext
    private final EntityManager em;

    private final BookRepository bookRepository;

    /**
     * Worker for the above.
     */
    @Transactional
    public void populate(BookInfo info) {
        List<Book> allBooks = bookRepository.findAll();

        try {
            Thread.sleep(1000);
        } catch (InterruptedException ignored) {
        }

        StringBuilder sb = new StringBuilder();

        try {
            for (Book book : allBooks) {
                sb.append(
                        String.format("Name: %s Author: %sn",
                                book.getName(),
                                book.getAuthor().getName())
                );
            }
        }
        catch (Exception e) {
            log.error("Exception", e);
        }

        info.setData(sb.toString());
        info.setDone(true);
    }
}

I have absolutely no beans configured, and my application.yml file is only enough to define the connection to the database.

I test this with:

curl -s "http://localhost:8080/request?no_spawn=false&pass=true"

and then I watch the logs in IntelliJ.

Set no_spawn to true, and you get the don’t thread version. But this call is the one I’m trying to fix — spawning a thread, but it’s the one that actually tries to work.

I know that the @async flag might be a solution, but as I said — significant legacy code I’m trying to fix. I know I could Fetch-Eager, but that’s a particularly bad solution. I could prefetch All The Data, but that’s a lot of places where I’d have to handle prefetching before spawning.

Currently, we’re fixing these one at a time by using the repository directly instead of traversing the ORM. That is, instead of book.getAuthor().getName(), I would fetch the author from the AuthorRepository using book.getAuthorId(). But again, that involves finding them one by one, and there’s a lot of these.

I’ve read a couple of dozen “solutions”, which is where I found references to trying to use EntityManager as you see here as well as SessionFactory. But nothing I’ve done has actually solved the problem.

So… Is there some way I can solve the No Session failure?