21. RxJava 与 Spring MVC

Spring Cloud Netflix 包含RxJava。

RxJava 是Reactive Extensions的 Java VM implementation：一个 library，用于通过使用 observable sequences 来编写异步和 event-based 程序。

Spring Cloud Netflix 支持从 Spring MVC 控制器返回rx.Single objects。它还支持将rx.Observable objects 用于Server-sent events(SSE)。如果您的内部 API 已经使用 RxJava 构建(例如，请参见第 17.4 节，“ Feign Hystrix 支持”)，这可能非常方便。

以下是使用rx.Single的一些示例：

@RequestMapping(method = RequestMethod.GET, value = "/single")
public Single<String> single() {
	return Single.just("single value");
}

@RequestMapping(method = RequestMethod.GET, value = "/singleWithResponse")
public ResponseEntity<Single<String>> singleWithResponse() {
	return new ResponseEntity<>(Single.just("single value"),
			HttpStatus.NOT_FOUND);
}

@RequestMapping(method = RequestMethod.GET, value = "/singleCreatedWithResponse")
public Single<ResponseEntity<String>> singleOuterWithResponse() {
	return Single.just(new ResponseEntity<>("single value", HttpStatus.CREATED));
}

@RequestMapping(method = RequestMethod.GET, value = "/throw")
public Single<Object> error() {
	return Single.error(new RuntimeException("Unexpected"));
}

如果您有Observable而不是单个，则可以使用.toSingle()或.toList().toSingle()。这里有些例子：

@RequestMapping(method = RequestMethod.GET, value = "/single")
public Single<String> single() {
	return Observable.just("single value").toSingle();
}

@RequestMapping(method = RequestMethod.GET, value = "/multiple")
public Single<List<String>> multiple() {
	return Observable.just("multiple", "values").toList().toSingle();
}

@RequestMapping(method = RequestMethod.GET, value = "/responseWithObservable")
public ResponseEntity<Single<String>> responseWithObservable() {

	Observable<String> observable = Observable.just("single value");
	HttpHeaders headers = new HttpHeaders();
	headers.setContentType(APPLICATION_JSON_UTF8);
	return new ResponseEntity<>(observable.toSingle(), headers, HttpStatus.CREATED);
}

@RequestMapping(method = RequestMethod.GET, value = "/timeout")
public Observable<String> timeout() {
	return Observable.timer(1, TimeUnit.MINUTES).map(new Func1<Long, String>() {
		@Override
		public String call(Long aLong) {
			return "single value";
		}
	});
}

如果您有流端点和 client，则 SSE 可以是一个选项。要将rx.Observable转换为 Spring SseEmitter，请使用RxResponse.sse()。这里有些例子：

@RequestMapping(method = RequestMethod.GET, value = "/sse")
public SseEmitter single() {
	return RxResponse.sse(Observable.just("single value"));
}

@RequestMapping(method = RequestMethod.GET, value = "/messages")
public SseEmitter messages() {
	return RxResponse.sse(Observable.just("message 1", "message 2", "message 3"));
}

@RequestMapping(method = RequestMethod.GET, value = "/events")
public SseEmitter event() {
	return RxResponse.sse(APPLICATION_JSON_UTF8,
			Observable.just(new EventDto("Spring io", getDate(2016, 5, 19)),
					new EventDto("SpringOnePlatform", getDate(2016, 8, 1))));
}