Spring Websocket CORS issue

I have a web application served by an Angular frontend and Spring Boot backend. They are deployed with a multi-stage Docker build on the same server. I had an issue of the websocket on Angular not connecting to the Spring websocket until I allowed CORS on the Spring websocket.

Does anyone know why this is the case? I thought that since they are on the same server, they should work even without CORS.

Here's my code:

Angular:

onConnect(): Promise<any> {
return new Promise(resolve => {
console.log("Connecting...")
this.ws = new WebSocket(`wss://${window.location.host}/websocket`)
this.ws.onopen = this.onWebSocketOpen.bind(this)
this.ws.onmessage = this.onWebSocketMessage.bind(this)
this.ws.onclose = this.onWebSocketclose.bind(this)
this.ws.onerror = (error) => console.error(error)
resolve(this.ws)
})
}


Spring Boot:

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

@Value("${webapp.host.url}")
private String hostUrl;

@Override
public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
registry.addHandler(new WebSocketHandler(), "/websocket");
.setAllowedOrigins(hostUrl);
}
}


When CORS was not set, the Angular websocket connection failed with an close stats of 1006, reason ''. However, Postman is able to connect.

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