Fix: LazyInitializationException In Email Event

by SLV Team 48 views
Bug Fix: Resolving LazyInitializationException in Email Event Publishing

Hey everyone,

We've encountered an interesting bug related to email event publishing that I wanted to share and discuss the fix. It's all about a LazyInitializationException that pops up during email sending, specifically when triggered by an event. Let's dive into the details!

What's the Bug? 🐛

So, the main issue is that our system throws a LazyInitializationException when trying to send emails. This sneaky exception happens within an asynchronous thread, making it a bit tricky to catch without proper logging. Basically, the email sending process gets interrupted, and nobody likes undelivered emails, right? This issue can lead to a frustrating user experience, where users believe their actions have been completed successfully (like submitting an application), but crucial notifications (like confirmation emails) never reach them.

Why is LazyInitializationException a Problem?

The LazyInitializationException typically occurs in the context of Object-Relational Mapping (ORM) frameworks like Hibernate. In essence, it means that the application is trying to access data from an entity that hasn't been fully loaded from the database within the current session. This often happens when you have lazy-loaded relationships between entities. For example, an email object might be associated with a user object, but the user details are only loaded if and when they are accessed. If the database session is closed before the user details are accessed, a LazyInitializationException is thrown.

Impact on User Experience

This bug can severely impact user experience. Imagine a scenario where a user submits a critical form or application, expecting an immediate confirmation email. If the email fails to send due to the LazyInitializationException, the user might be left in the dark, unsure if their submission was successful. This uncertainty can lead to frustration and a negative perception of the application's reliability.

How to Reproduce the Issue 🧐

Here’s how you can reproduce this bug:

  1. Click the “Submit Application” button.
  2. You'll see a confirmation message, but the email won't actually send.
  3. Check the logs, and you'll find the infamous LazyInitializationException staring back at you.

Step-by-Step Reproduction

To better understand how this bug manifests, let's break down the reproduction steps:

  1. User Action: A user interacts with the application by filling out a form (e.g., an application form) and clicking the "Submit" button. This action triggers the application's backend processes.

  2. Event Trigger: Upon submission, an event is triggered within the application to initiate the email sending process. This event is typically handled asynchronously to avoid blocking the main application thread and ensure a responsive user interface.

  3. Asynchronous Processing: The event listener picks up the event and starts processing it in a separate thread. This is where the email sending logic resides.

  4. Lazy Loading: Within the email sending logic, there might be a need to access related entities (e.g., user details, application data) that are lazily loaded. Lazy loading is a performance optimization technique where related entities are not loaded from the database until they are explicitly accessed.

  5. Session Closure: The database session, which was active during the initial form submission, might be closed before the email sending thread attempts to access the lazily loaded entities. This is a critical point because Hibernate, or any ORM, requires an active session to load related entities.

  6. Exception Thrown: When the email sending thread tries to access a lazily loaded entity without an active session, Hibernate throws a LazyInitializationException. This exception indicates that the requested data cannot be loaded because the session has already been closed.

  7. Email Failure: As a result of the exception, the email sending process fails, and the user does not receive the expected confirmation email.

Expected Outcome 🤔

Ideally, the email should be sent successfully without any hiccups. Users should receive timely notifications to confirm their actions.

Why Email Delivery Matters

Email notifications are a critical component of many applications. They serve various purposes, such as:

  • Confirmation: Notifying users that their actions (e.g., form submission, registration) have been successfully processed.
  • Updates: Providing users with updates on the status of their requests or applications.
  • Reminders: Sending reminders for upcoming events or deadlines.
  • Security: Alerting users about unusual activity or security-related issues.

Failing to deliver these emails can lead to confusion, frustration, and even distrust in the application. Therefore, ensuring reliable email delivery is paramount.

Log Snippet 📸

Here’s a snippet from the logs that clearly shows the LazyInitializationException:

2025-10-16T18:29:45.128+09:00 ERROR 1 --- [Team18_BE] [ task-2] .a.i.SimpleAsyncUncaughtExceptionHandler : Unexpected exception occurred invoking async method: public void com.kakaotech.team18.backend_server.domain.email.eventListener.ApplicationNotificationListener.onSubmitted(com.kakaotech.team18.backend_server.domain.email.dto.ApplicationSubmittedEvent) org.hibernate.LazyInitializationException: Could not initialize proxy [com.kakaotech.team18.backend_server.domain.clubApplyForm.entity.ClubApplyForm#1] - no session at org.hibernate.proxy.AbstractLazyInitializer.initialize(AbstractLazyInitializer.java:174) ~[hibernate-core-6.6.26.Final.jar!/:6.6.26.Final] at org.hibernate.proxy.AbstractLazyInitializer.getImplementation(AbstractLazyInitializer.java:328) ~[hibernate-core-6.6.26.Final.jar!/:6.6.26.Final] at org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor.intercept(ByteBuddyInterceptor.java:44) ~[hibernate-core-6.6.26.Final.jar!/:6.6.26.Final] at org.hibernate.proxy.ProxyConfiguration$InterceptorDispatcher.intercept(ProxyConfiguration.java:102) ~[hibernate-core-6.6.26.Final.jar!/:6.6.26.Final] at com.kakaotech.team18.backend_server.domain.clubApplyForm.entity.ClubApplyForm$HibernateProxy.getClub(Unknown Source) ~[!/:0.0.1-SNAPSHOT] at com.kakaotech.team18.backend_server.domain.email.service.EmailService.sendToApplicant(EmailService.java:43) ~[!/:0.0.1-SNAPSHOT] at com.kakaotech.team18.backend_server.domain.email.eventListener.ApplicationNotificationListener.onSubmitted(ApplicationNotificationListener.java:34) ~[!/:0.0.1-SNAPSHOT] at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103) ~[na:na]

Analyzing the Log

Let's break down the log snippet to understand what's happening:

  • Timestamp: The log entry starts with a timestamp (2025-10-16T18:29:45.128+09:00), indicating when the error occurred. This is crucial for correlating the error with specific user actions or system events.

  • Error Level: The ERROR keyword indicates the severity of the issue. Errors are critical and need immediate attention.

  • Thread Information: The [ task-2] part shows that the error occurred in an asynchronous task thread. This confirms that the email sending process is running in a separate thread, as expected.

  • Exception Type: The main part of the log entry is the org.hibernate.LazyInitializationException. As we discussed earlier, this exception is a clear indicator of a lazy loading issue.

  • Error Message: The message Could not initialize proxy [com.kakaotech.team18.backend_server.domain.clubApplyForm.entity.ClubApplyForm#1] - no session provides more context. It tells us that the application tried to access a lazily loaded ClubApplyForm entity with ID 1, but there was no active Hibernate session.

  • Stack Trace: The stack trace shows the sequence of method calls that led to the exception. It starts from the AbstractLazyInitializer.initialize method and goes up to the ApplicationNotificationListener.onSubmitted method, which is the event listener responsible for sending emails. This helps us pinpoint the exact location in the code where the error occurred.

  • EmailService Call: The stack trace includes a call to EmailService.sendToApplicant, which confirms that the exception occurred during the email sending process.

Solution and Fix

Understanding the Root Cause

Before diving into the fix, it's crucial to understand the root cause of the LazyInitializationException. As we've discussed, this exception occurs when the application tries to access a lazily loaded entity outside the scope of a Hibernate session. In our case, the email sending process runs in a separate thread, and the database session that was active during the initial form submission might be closed by the time the email sending thread tries to access the lazily loaded ClubApplyForm entity.

Strategies to Resolve the Issue

There are several strategies we can employ to resolve this issue. Let's explore some of the most common and effective approaches:

  1. Eager Loading: One way to avoid the LazyInitializationException is to switch from lazy loading to eager loading. With eager loading, the related entities are loaded from the database at the same time as the main entity. This ensures that all necessary data is available within the same session. However, eager loading can have performance implications if not used judiciously, as it might lead to loading more data than necessary.

  2. Open Session in View Pattern: The "Open Session in View" pattern involves keeping the Hibernate session open until the view (or the email sending process in our case) is rendered. This ensures that the lazily loaded entities can be accessed within the same session. However, this pattern can have its own challenges, such as increased database connection usage and potential for long-running transactions.

  3. Data Transfer Objects (DTOs): A more robust and recommended approach is to use Data Transfer Objects (DTOs). DTOs are simple objects that carry data between processes. In our case, we can load all the necessary data from the database within the initial transaction and then populate a DTO with this data. The DTO can then be passed to the email sending thread, which can access the data without needing an active Hibernate session. This approach decouples the email sending process from the database session and avoids the LazyInitializationException.

  4. Hibernate.initialize(): This method allows you to explicitly initialize a lazy-loaded collection or entity within the current session. By calling Hibernate.initialize() before passing the entity to the asynchronous thread, you ensure that the data is loaded before the session is closed.

Implementing the DTO Approach

For our scenario, the DTO approach is the most suitable. Here’s how we can implement it:

  1. Create a DTO: Define a DTO that contains all the necessary data for the email sending process. This might include user details, application details, and any other relevant information.

    public class ApplicationSubmittedDTO {
    private String applicantName;
    private String clubName;
    private String applicationDetails;
    // Add other necessary fields

    // Constructors, getters, and setters
}

  2. Populate the DTO: In the service layer, before triggering the email sending event, load all the required data from the database and populate the DTO.

    @Service
public class ApplicationService {
    @Autowired
    private ApplicationRepository applicationRepository;
    @Autowired
    private ApplicationEventPublisher eventPublisher;

    @Transactional
    public void submitApplication(ApplicationForm applicationForm) {
        // Save the application form
        Application savedApplication = applicationRepository.save(applicationForm);

        // Create and populate the DTO
        ApplicationSubmittedDTO dto = new ApplicationSubmittedDTO();
        dto.setApplicantName(savedApplication.getApplicant().getName());
        dto.setClubName(savedApplication.getClub().getName());
        dto.setApplicationDetails(savedApplication.getDetails());

        // Publish the event with the DTO
        eventPublisher.publishEvent(new ApplicationSubmittedEvent(dto));
    }
}

  3. Modify the Event Listener: Update the event listener to receive the DTO instead of the entity. This ensures that the email sending process receives all the necessary data without needing to access the database.

    @Component
public class ApplicationNotificationListener {
    @Autowired
    private EmailService emailService;

    @EventListener
    public void onSubmitted(ApplicationSubmittedEvent event) {
        ApplicationSubmittedDTO dto = event.getDto();
        emailService.sendToApplicant(dto);
    }
}

  4. Update the Email Service: Modify the email service to use the data from the DTO to send the email.

    @Service
public class EmailService {
    public void sendToApplicant(ApplicationSubmittedDTO dto) {
        // Use the data from the DTO to construct and send the email
        String recipient = dto.getApplicantEmail();
        String subject = "Application Submitted";
        String body = "Dear " + dto.getApplicantName() + ",\n\nYour application to " + dto.getClubName() + " has been submitted successfully.\n\nDetails: " + dto.getApplicationDetails();

        // Send the email
        sendEmail(recipient, subject, body);
    }
}

Advantages of the DTO Approach

Using DTOs offers several advantages:

  • Decoupling: DTOs decouple the email sending process from the database session, preventing LazyInitializationException.
  • Performance: DTOs allow you to load only the necessary data, improving performance and reducing database load.
  • Testability: DTOs make it easier to test the email sending process in isolation.
  • Maintainability: DTOs provide a clear contract between different parts of the application, making the code more maintainable.

Conclusion

Dealing with LazyInitializationException can be tricky, especially in asynchronous environments. By understanding the root cause and implementing the right solution, such as using DTOs, we can ensure reliable email delivery and a better user experience. Remember, a well-handled exception is a step towards a more robust and user-friendly application!

By using DTOs, we ensure that all the necessary data is loaded within the initial transaction and passed to the asynchronous thread, thus preventing the LazyInitializationException. This approach not only fixes the bug but also improves the overall architecture of the application.

I hope this detailed explanation helps you guys understand the issue and the solution better. Keep coding and keep those emails flowing! 🚀