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How to call Spring MongoRepository from an ExecutorService

The problem

Recently I encountered an issue in code when I tried to call the Spring MongoRepository from a non-Spring-managed thread in a Java Spring Boot application. Here is a brief description of the issue.

The Repository interface

public interface ResponseRepository extends MongoRepository<Response, String> {

}

It’s a standard Spring Repository interface without any custom methods.

The calling class

@Component //A Spring Component
public class AnImaginaryComponent {

    @Autowired
    private ResponseRepository responseRepo;

    public void saveResponseInANewThread(Response r) {
        //Create an Executor service
        ExecutorService executor = Executors.newSingleThreadExecutor();

        //Submit a Runnable instance
        executor.submit(() -> responseRepo.save(r));
    }
}

When anImaginaryComponent.saveResponseInANewThread(r) is invoked, it never saves the r Response into the database. And the worst part is that it doesn’t throw any exceptions or print any log messages for the failure.

The solution

Rewrite the ResponseRepository interface as below:

import org.springframework.scheduling.annotation.Async;

public interface ResponseRepository extends MongoRepository<Response, String> {

    @Async
    <S extends Response> S save(S entity);
}

And enable sync.

import org.springframework.scheduling.annotation.EnableAsync;

@SpringBootApplication
@EnableAsync
public class MySuperImportantSpringBootApplication {

Apparently the @Async annotation tells Spring to execute the annotated methods asynchronously and not to wait for the main thread.

It was not easy to find the workaround as there were very few resources on the internet on this issue (which is why I decided to post it on my blog) but this StackOverflow post did help.


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