Vaadin and receiving email asynchronouosly - spring-integration

Question

My Vaadin 14 application should receive emails in the background. If emails with a certain subject have been received, the user should be informed about this via PUSH message on the UI.
For the entire email handling I implemented the email / message handling from Spring integration and that works too. Two beans (IntegrationFlow and a ServiceActivator) are generated via #Configuration and #Bean annotation in the Spring Application Context like so:
#Configuration
public class EmailReceiver {
#Bean
public HeaderMapper<MimeMessage> mailHeaderMapper() {
return new DefaultMailHeaderMapper();
}
#Bean
public IntegrationFlow imapMailFlow() {
IntegrationFlow flow = IntegrationFlows
.from(Mail.imapInboundAdapter("imaps://user:pass#imap.ionos.de/INBOX")
.userFlag("testSIUserFlag")
.javaMailProperties(new Properties()),
e -> e.autoStartup(true)
.poller(p -> p.fixedDelay(5000)))
.transform(Mail.toStringTransformer())
.channel(MessageChannels.queue("imapChannel"))
.get();
return flow;
}
#Bean(name = PollerMetadata.DEFAULT_POLLER)
public PollerMetadata defaultPoller() {
PollerMetadata pollerMetadata = new PollerMetadata();
pollerMetadata.setTrigger(new PeriodicTrigger(1000));
return pollerMetadata;
}
#Bean
#ServiceActivator(inputChannel = "imapChannel")
public MessageHandler processNewEmail() {
MessageHandler messageHandler = new MessageHandler() {
#Override
public void handleMessage(org.springframework.messaging.Message<?> message) throws MessagingException {
System.out.println("new email received");
}
};
return messageHandler;
}
}
See also here: https://docs.spring.io/spring-integration/docs/current/reference/html/mail.html#mail-java-dsl-configuration
With such a #Configuration annotated class, the emails are received in the background of the Vaadin app. Check.
But how can I integrate a callback into a Vaadin view in the method EmailReceiver.processNewEmail?
#Bean
#ServiceActivator(inputChannel = "imapChannel")
public MessageHandler processNewEmail(UI ui) {
This always throws an error at application start: Scope vaadin-ui is not active for the current thread; consider defining a scoped proxy for this bean.
There is the example for asynchronous updates with Vaadin https://vaadin.com/docs/v14/flow/advanced/tutorial-push-access.
In contrast to this, I have to create a #Bean for #ServiceActivator handling. As soon as that is the case, there is always the error There is no UI available. The UI scope is not active.
If I move the method processNewEmail() into a separate class I still cannot reference a Vaadin UI:
#MessageEndpoint
class EmailMessageHandler {
private UI ui;
public EmailMessageHandler(UI ui) {
this.ui = ui;
}
#Bean
#ServiceActivator(inputChannel = "imapChannel")
public MessageHandler processNewEmail() {
MessageHandler messageHandler = new MessageHandler() {
#Override
public void handleMessage(org.springframework.messaging.Message<?> message) throws MessagingException {
System.out.println("new email received" + message);
}
};
return messageHandler;
}
}
How can I combine Vaadin asynchronous handling and Spring-Integration Email/ServiceActivator processing?

Answer 1

The point is that your mail receiving functionality is singleton per your application. On the other hand you are going to have as many UIs as many users do HTTP requests to your application. So, you need to think about some intermediary to dump email and get them from there when UI request happens.
You already have that imapChannel as a QueueChannel. So, you can take it from your UI scoped code and call its receive() API to pull the next message. Only the problem that it is a queue: as long as one call receive(), the other won't see the same message. Probably this is OK for your so far, but better to think about something what could be treated as topic in messaging terms. A good candidate easy to use is a Reactor's Sinks.Many: https://projectreactor.io/docs/core/release/reference/#sinks