Understanding SSE and SseEmitter

Oct 13, 2025 by leolavanderia.com.br 137 views

Introduction

Server-Sent Events (SSE) is a technology that allows a server to push updates to a client over HTTP. It's a great way to implement real-time updates in web applications. However, when working with SSE in Spring Boot, you might encounter issues where events are not being received until the SseEmitter is complete. In this article, we'll explore this issue and provide a solution.

Understanding SSE and SseEmitter

Before we dive into the issue, let's quickly understand how SSE and SseEmitter work.

What is Server-Sent Events (SSE)?

Server-Sent Events (SSE) is a unidirectional communication channel between a server and a client. The server sends events to the client, and the client can receive these events in real-time. SSE is based on HTTP and uses a special type of HTTP request called an EventSource.

What is SseEmitter?

SseEmitter is a class in Spring Framework that helps you send events to clients using SSE. It's a simple way to create an SSE event source and send events to clients.

The Issue: SSE Events Not Being Received Until SseEmitter is Complete

Now, let's get to the issue at hand. When using SseEmitter in Spring Boot, you might notice that events are not being received by the client until the SseEmitter is complete. This can be frustrating, especially if you're trying to implement real-time updates in your application.

Why is this happening?

The reason for this issue is that SseEmitter is designed to be a non-blocking API. When you call SseEmitter.send() to send an event, it doesn't block the execution of the thread. Instead, it schedules the event to be sent in the background. However, this means that the event might not be sent immediately, and the client might not receive it until the SseEmitter is complete.

Solution: Using SseEmitter with a Thread Pool

To fix this issue, you can use a thread pool to send events using SseEmitter. This will ensure that events are sent in the background, without blocking the execution of the thread.

Example Code

Here's an example code snippet that demonstrates how to use a thread pool with SseEmitter:

import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
public class SseEventSender {
private final ThreadPoolTaskExecutor executor;
public SseEventSender(ThreadPoolTaskExecutor executor) {
    this.executor = executor;
}

public void sendEvent(SseEmitter emitter, String event) {
    executor.execute(() -&gt; {
        try {
            emitter.send(event);
        } catch (IOException e) {
            // Handle exception
        }
    });
}

}

In this example, we create a SseEventSender class that takes a ThreadPoolTaskExecutor instance in its constructor. We then use this executor to send events using SseEmitter.

Using the SseEventSender Class

To use the SseEventSender class, you can create a ThreadPoolTaskExecutor instance and pass it to the SseEventSender constructor. You can then use the sendEvent method to send events using SseEmitter.

Example Use Case

Here's an example use case that demonstrates how to use the SseEventSender class:

import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
import org.springframework.web.servlet.mvc.method.annotation.SseEmitter;
public class SseEventController {
private final SseEventSender sender;
public SseEventController(SseEventSender sender) {
    this.sender = sender;
}

@GetMapping(&quot;/events&quot;)
public SseEmitter getEvents() {
    SseEmitter emitter = new SseEmitter();
    sender.sendEvent(emitter, &quot;Hello, World!&quot;);
    return emitter;
}

}

In this example, we create a SseEventController class that takes a SseEventSender instance in its constructor. We then use the sendEvent method to send an event using SseEmitter.

Conclusion

In this article, we explored the issue of SSE events not being received until SseEmitter is complete. We discussed the reasons behind this issue and provided a solution using a thread pool with SseEmitter. We also provided example code snippets to demonstrate how to use the SseEventSender class.

Best Practices

When working with SSE and SseEmitter in Spring Boot, keep the following best practices in mind:

Use a thread pool to send events using SseEmitter.
Avoid blocking the execution of the thread when sending events.
Use a SseEventSender class to encapsulate the logic of sending events using SseEmitter.

Introduction

In our previous article, we explored the issue of SSE events not being received until SseEmitter is complete. We discussed the reasons behind this issue and provided a solution using a thread pool with SseEmitter. In this article, we'll answer some frequently asked questions (FAQs) related to this issue.

Q: What is the difference between SseEmitter and EventSource?

A: SseEmitter is a class in Spring Framework that helps you send events to clients using SSE. It's a simple way to create an SSE event source and send events to clients. On the other hand, EventSource is a JavaScript API that allows you to receive SSE events from a server.

Q: Why do I need to use a thread pool with SseEmitter?

A: When you call SseEmitter.send() to send an event, it doesn't block the execution of the thread. Instead, it schedules the event to be sent in the background. However, this means that the event might not be sent immediately, and the client might not receive it until the SseEmitter is complete. Using a thread pool ensures that events are sent in the background, without blocking the execution of the thread.

Q: Can I use SseEmitter with a single-threaded executor?

A: While it's technically possible to use SseEmitter with a single-threaded executor, it's not recommended. This is because SseEmitter is designed to be a non-blocking API, and using a single-threaded executor can lead to performance issues and blocking.

Q: How do I handle exceptions when sending events using SseEmitter?

A: When sending events using SseEmitter, you can catch exceptions using a try-catch block. If an exception occurs, you can handle it by logging the error or sending an error event to the client.

Q: Can I use SseEmitter with other Spring Framework features, such as WebSockets?

A: Yes, you can use SseEmitter with other Spring Framework features, such as WebSockets. However, keep in mind that SseEmitter is designed to be a non-blocking API, and using it with other features that block the execution of the thread can lead to performance issues.

Q: How do I configure the thread pool for SseEmitter?

A: To configure the thread pool for SseEmitter, you can use the ThreadPoolTaskExecutor class. You can create a ThreadPoolTaskExecutor instance and pass it to the SseEventSender constructor.

Q: Can I use SseEmitter with a custom thread pool?

A: Yes, you can use SseEmitter with a custom thread pool. However, keep in mind that the thread pool should be designed to handle non-blocking tasks, such as sending events using SseEmitter.

Q: How do I debug issues with SseEmitter?

A: To debug issues with SseEmitter, you can use the Spring Framework's built-in debugging features, such as logging and tracing. You can also use a debugger to step through the code and identify the source of the issue.

Conclusion

In this article, we answered some frequently asked questions (FAQs) related to the issue of SSE events not being received until SseEmitter is complete. We discussed the reasons behind this issue and provided a solution using a thread pool with SseEmitter. By following the best practices and using the SseEventSender class, you can ensure that your SSE events are sent in real-time and received by the client without any issues.

Best Practices

When working with SSE and SseEmitter in Spring Boot, keep the following best practices in mind:

Use a thread pool to send events using SseEmitter.
Avoid blocking the execution of the thread when sending events.
Use a SseEventSender class to encapsulate the logic of sending events using SseEmitter.
Configure the thread pool for SseEmitter using the ThreadPoolTaskExecutor class.
Use a custom thread pool designed to handle non-blocking tasks.
Debug issues with SseEmitter using Spring Framework's built-in debugging features.