SseEmitter is not thread safe [SPR-13224]

[spring-projects-issues]

Tomasz Nurkiewicz opened SPR-13224 and commented

SseEmitter misbehaves when multiple threads call send() at the same time. Sometimes container fails, but most of the time different messages get interleaved, e.g.:

emitter.send("foo");  //thread A
emitter.send("bar");  //thread B

sometimes results in:

data:data:AB

It happens because emitter.send("A") actually sends three independent messages: data:, A and \n\n. These three messages can be interleaved with messages from different threads. Please either synchronize properly or document clearly that SseEmitter is not thread safe, so that if clients want to use from multiple threads (I think it's quite useful), it must be synchronized manually:

synchronized(sseEmitter) {
  sseEmitter.send("foo");
}

---

Affects: 4.2 RC2

Issue Links:

• ResponseBodyEmitter skips same messages during initialization [SPR-13223] #17814 ResponseBodyEmitter skips same messages during initialization

Referenced from: commits bdb6348

[spring-projects-issues]

Juergen Hoeller commented

Implemented together with #17814, using a common struct for a value with media type both in ResponseBodyEmitter and in SseEventBuilder, and using common synchronization for all affected operations - including the option to manually synchronize on the emitter, for aggregating multiple values just like SseEmitter does by default now.

Juergen

[martinandersson]

Can you please re-open this issue? The thread-safety of relevant types are still not documented, leading to 1) time waste for people concerned about thread-safety having to google this - and that should be most of us, 2) developers programming against a contract must manually synchronize regardless of implementation details they might have come across online - and that should be most of us.

[snicoll]

@martinandersson we're not going to reopen an issue that was closed in a released version of the framework. Please create a separate issue with additional details against the current GA version.