The documentation for StreamBridge states, that the send method returns false or throws an exception if data wasn't sent successfully.
I am wondering for which cases sending is returning false and for which it is throwing an exception. I hope somebody can give me some clarification regarding this open question.
StreamBridge#send internally uses Spring's MessageChannel to send the message. Therefore, the boolean that StreamBridge returns is completely based on the MessageChannel#send implementation. See this for more details on that: https://github.com/spring-projects/spring-framework/blob/main/spring-messaging/src/main/java/org/springframework/messaging/MessageChannel.java#L44
More specifically:
/**
* Send a {@link Message} to this channel. If the message is sent successfully,
* the method returns {@code true}. If the message cannot be sent due to a
* non-fatal reason, the method returns {@code false}. The method may also
* throw a RuntimeException in case of non-recoverable errors.
* <p>This method may block indefinitely, depending on the implementation.
* To provide a maximum wait time, use {@link #send(Message, long)}.