Java Reactive Programming

响应式编程

在 Spring Boot 中，支持了响应式编程，带来了性能和内存使用方面的优化。

详见：

困难

但是不同于 async/await 模式，响应式编程也给编码带来了一些困难，主要如下：

  • 一个代码块只能最多调用一个响应式 API。
  • 对 null 处理不友好，甚至是灾难性的。

响应式编程的规则

  • 控制层，返回响应式对象，大多数情况下使用 Mono<T>。

  • 服务层，使用@Transactional 的 API 必须返回响应式对象。

  • 数据访问层(R2DBC)返回响应式对象: Mono<T>, Flux<T>

  • 使用响应式方法的 API 尽量返回响应式对象。

  • 不要使用任何 block(), blockFirst(), share().block() 等 API，会引起严重的性能问题。

  • 在重载传统接口的情况下使用 subscribe()。

  • 对于计数的 API，使用 Mono<Long> 作为返回对象。

    这是因为 Flux.count() 返回的是一个Mono<Long>。因此在其它的计数 API 中使用 Mono<Long> 作为返回对象，让我们可以保持一致。

  • Mono<Void> 和 null

    • 响应式 API 不能返回 null 或者 Mono.just(null) 或者其等价方式。

      会引起下面的错误：

      Caused by: java.lang.NullPointerException: The mapper returned a null value.

    • Mono.empty() 不能调用随后的映射方法 map()、flatMap()、transform()等。
    • Mono.empty() 在 doOnSuccess() 等函数中获取值是 null。
    • Flux 中使用元素 Mono<Void> 可以调用随后的映射方法 colllectList()等方法。

      Mono<Void> 不会被记入 count() 和 colllectList()
    • 建议: 避免定义返回 Mono<Void>的方法。

      这种返回不能调用随后的映射方法 map()、flatMap()、transform()等方法。

      一个例外是，这个方法在控制层的最后被调用。

响应式编程模式

响应式编程是一种流编程，我把编程模式分为: 启动模式、映射模式、返回模式、异常模式。

空模式

  • 响应式 API 不能返回 null 或者 Mono.just(null) 或者其等价方式。

    会引起下面的错误：

    Caused by: java.lang.NullPointerException: The mapper returned a null value.

	public static Mono<Void> monoNullTest() {

		return Mono.just(null);

	}

	monoNullTest().log().subscribe();

/*

Exception in thread "main" java.lang.NullPointerException: value

        at java.base/java.util.Objects.requireNonNull(Objects.java:246)

        at reactor.core.publisher.MonoJust.<init>(MonoJust.java:35)

        at reactor.core.publisher.Mono.just(Mono.java:719)

        at demo.ReactiveVoidTest.monoNullTest(ReactiveVoidTest.java:22)

        at demo.ReactiveVoidTest.main(ReactiveVoidTest.java:16)

*/
  • Mono.empty() 不能调用随后的映射方法 map()、flatMap()、transform()等。
	public static Mono<Integer> monoVoidTest() {

		logger.info("case: mono void test");

		return Mono.empty().map(o -> {

			logger.info(MessageFormat.format("map: {0}", o));

			return o;

		}).doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: {0}", o));

		}).thenReturn(1);

	}

	monoVoidTest().log().subscribe();

/*

10:52:06.993 [main] DEBUG reactor.util.Loggers - Using Slf4j logging framework

10:52:07.023 [main] INFO reactor.Mono.IgnoreThen.1 - onSubscribe(MonoIgnoreThen.ThenIgnoreMain)

10:52:07.030 [main] INFO reactor.Mono.IgnoreThen.1 - request(unbounded)

10:52:07.036 [main] INFO demo.ReactiveVoidTest - doOnSuccess: null

10:52:07.038 [main] INFO reactor.Mono.IgnoreThen.1 - onNext(1)

10:52:07.042 [main] INFO reactor.Mono.IgnoreThen.1 - onComplete()

*/
  • Flux 中使用元素 Mono<Void> 可以调用随后的映射方法 colllectList()等方法。

    Mono<Void> 不会被记入 count() 和 colllectList()
	public static Mono<String> fluxVoidTest() {

		logger.info("case: flux void test");

		return Flux.fromIterable(Arrays.asList(0, 1, 2)).flatMap(o -> {

			logger.info(MessageFormat.format("emit an empty: {0}", o));

			return Mono.empty();

		}).map(o -> {

			logger.info(MessageFormat.format("map: {0}", o));

			return o;

		}).count().doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: count is {0}", o));

		}).map(o -> {

			return "abc";

		});

	}

	fluxVoidTest().log().subscribe();

/*

11:20:35.788 [main] INFO demo.ReactiveVoidTest - case: flux void test

11:20:35.986 [main] DEBUG reactor.util.Loggers - Using Slf4j logging framework

11:20:36.121 [main] INFO reactor.Mono.MapFuseable.1 - | onSubscribe([Fuseable] FluxMapFuseable.MapFuseableSubscriber)

11:20:36.127 [main] INFO reactor.Mono.MapFuseable.1 - | request(unbounded)

11:20:36.129 [main] INFO demo.ReactiveVoidTest - emit an empty: 0

11:20:36.130 [main] INFO demo.ReactiveVoidTest - emit an empty: 1

11:20:36.131 [main] INFO demo.ReactiveVoidTest - emit an empty: 2

11:20:36.133 [main] INFO demo.ReactiveVoidTest - doOnSuccess: count is 0

11:20:36.134 [main] INFO reactor.Mono.MapFuseable.1 - | onNext(abc)

11:20:36.138 [main] INFO reactor.Mono.MapFuseable.1 - | onComplete()

*/

异常模式

响应式编程对于异常处理，建议使用下面的方法:

  • 抛出 RuntimeException。
  • 要小心使用 Mono.error(t) 方法。
  • 在 Mono API 中返回 Mono.error(t)

    会被当成一个 MonoError 值被处理，

    可以在map, doOnNext, doOnSuccess处理。

    不会被 doOnError处理。
	public static Mono<Integer> monoErrorTest() {

		logger.info("case: mono error test");

		return Mono.just(0).onErrorStop().map(o -> {

			if (o < 2) {

				return Mono.error(new RuntimeException("test"));

			}

			return o;

		}).doOnError(e -> {

			logger.info(MessageFormat.format("doOnError: {0}", e.getMessage()));

			throw new RuntimeException(e);

		}).doOnNext(o -> {

			logger.info(MessageFormat.format("doOnNext: {0}", o));

		}).doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: {0}", o));

		}).thenReturn(1);

	}

	monoErrorTest().log().subscribe();

/*

00:08:22.338 [main] INFO demo.ReactiveErrorDemo - case: mono error test

00:08:22.460 [main] DEBUG reactor.util.Loggers - Using Slf4j logging framework

00:08:22.484 [main] INFO reactor.Mono.IgnoreThen.1 - onSubscribe(MonoIgnoreThen.ThenIgnoreMain)

00:08:22.488 [main] INFO reactor.Mono.IgnoreThen.1 - request(unbounded)

00:08:22.495 [main] INFO demo.ReactiveErrorDemo - doOnNext: MonoError

00:08:22.496 [main] INFO demo.ReactiveErrorDemo - doOnSuccess: MonoError

00:08:22.497 [main] INFO reactor.Mono.IgnoreThen.1 - onNext(1)

00:08:22.499 [main] INFO reactor.Mono.IgnoreThen.1 - onComplete()

*/
  • 在 Mono API 中抛出异常，会被doOnError截获，并且跳过 map, doOnSuccess
	public static Mono<Integer> monoExceptionTest() {

		logger.info("case: mono error test");

		return Mono.just(0).map(o -> {

			if (o < 2) {

				throw new RuntimeException("test");

			}

			return o;

		}).map(o -> {

			return 2;

		}).doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: {0}", o));

		}).doOnError(e -> {

			logger.info(MessageFormat.format("doOnError: {0}", e.getMessage()));

		});

	}

	monoExceptionTest().log().subscribe();

/*

00:08:22.499 [main] INFO demo.ReactiveErrorDemo - case: mono exception test

00:08:22.502 [main] INFO reactor.Mono.PeekTerminal.2 - | onSubscribe([Fuseable] MonoPeekTerminal.MonoTerminalPeekSubscriber)

00:08:22.502 [main] INFO reactor.Mono.PeekTerminal.2 - | request(unbounded)

00:08:22.508 [main] INFO demo.ReactiveErrorDemo - doOnError: test

00:08:22.510 [main] ERROR reactor.Mono.PeekTerminal.2 - | onError(java.lang.RuntimeException: test)

00:08:22.515 [main] ERROR reactor.Mono.PeekTerminal.2 -

java.lang.RuntimeException: test

        at demo.ReactiveErrorDemo.lambda$0(ReactiveErrorDemo.java:26)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:281)

        at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2398)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:354)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:354)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.request(MonoPeekTerminal.java:139)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.request(MonoPeekTerminal.java:139)

        at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)

        at reactor.core.publisher.LambdaMonoSubscriber.onSubscribe(LambdaMonoSubscriber.java:121)

        at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onSubscribe(MonoPeekTerminal.java:152)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onSubscribe(MonoPeekTerminal.java:152)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:263)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:263)

        at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:55)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4400)

        at reactor.core.publisher.Mono.subscribeWith(Mono.java:4515)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4232)

        at demo.ReactiveErrorDemo.main(ReactiveErrorDemo.java:18)

00:08:22.520 [main] ERROR reactor.core.publisher.Operators - Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.RuntimeException: test

Caused by: java.lang.RuntimeException: test

        at demo.ReactiveErrorDemo.lambda$0(ReactiveErrorDemo.java:26)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onNext(FluxMapFuseable.java:281)

        at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:2398)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:354)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.request(FluxMapFuseable.java:354)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.request(MonoPeekTerminal.java:139)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.request(MonoPeekTerminal.java:139)

        at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.request(FluxPeekFuseable.java:144)

        at reactor.core.publisher.LambdaMonoSubscriber.onSubscribe(LambdaMonoSubscriber.java:121)

        at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onSubscribe(FluxPeekFuseable.java:178)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onSubscribe(MonoPeekTerminal.java:152)

        at reactor.core.publisher.MonoPeekTerminal$MonoTerminalPeekSubscriber.onSubscribe(MonoPeekTerminal.java:152)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:263)

        at reactor.core.publisher.FluxMapFuseable$MapFuseableConditionalSubscriber.onSubscribe(FluxMapFuseable.java:263)

        at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:55)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4400)

        at reactor.core.publisher.Mono.subscribeWith(Mono.java:4515)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4232)

        at demo.ReactiveErrorDemo.main(ReactiveErrorDemo.java:18)

*/
  • 在 Flux API 中返回 Mono.error(t)

    会被当成一个异常被处理，

    不会在map, doOnNext, doOnSuccess处理。

    会被 doOnError处理。
	public static Mono<String> fluxErrorTest() {

		logger.info("case: flux error test");

		return Flux.fromIterable(Arrays.asList(0, 1, 2)).flatMap(o -> {

			logger.info(MessageFormat.format("flatMap: {0}", o));

			if (o == 1) {

				return Mono.error(new RuntimeException("test"));

			}

			return Mono.just(o);

		}).map(o -> {

			logger.info(MessageFormat.format("map: {0}", o));

			return o;

		}).count().doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: count is {0}", o));

		}).doOnError(e -> {

			logger.info(MessageFormat.format("doOnError: {0}", e.getMessage()));

		}).map(o -> {

			return "abc";

		});

	}

fluxErrorTest().log().subscribe();

/*

00:18:12.204 [main] INFO demo.ReactiveErrorDemo - case: flux error test

00:18:12.367 [main] DEBUG reactor.util.Loggers - Using Slf4j logging framework

00:18:12.472 [main] INFO reactor.Mono.MapFuseable.1 - | onSubscribe([Fuseable] FluxMapFuseable.MapFuseableSubscriber)

00:18:12.476 [main] INFO reactor.Mono.MapFuseable.1 - | request(unbounded)

00:18:12.478 [main] INFO demo.ReactiveErrorDemo - flatMap: 0

00:18:12.478 [main] INFO demo.ReactiveErrorDemo - map: 0

00:18:12.478 [main] INFO demo.ReactiveErrorDemo - flatMap: 1

00:18:12.484 [main] INFO demo.ReactiveErrorDemo - doOnError: test

00:18:12.486 [main] ERROR reactor.Mono.MapFuseable.1 - | onError(java.lang.RuntimeException: test)

00:18:12.491 [main] ERROR reactor.Mono.MapFuseable.1 -

java.lang.RuntimeException: test

        at demo.ReactiveErrorDemo.lambda$13(ReactiveErrorDemo.java:82)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:386)

        at reactor.core.publisher.FluxIterable$IterableSubscription.slowPath(FluxIterable.java:272)

        at reactor.core.publisher.FluxIterable$IterableSubscription.request(FluxIterable.java:230)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onSubscribe(FluxFlatMap.java:371)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:165)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:87)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4400)

        at reactor.core.publisher.Mono.subscribeWith(Mono.java:4515)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4232)

        at demo.ReactiveErrorDemo.main(ReactiveErrorDemo.java:19)

00:18:12.495 [main] ERROR reactor.core.publisher.Operators - Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.RuntimeException: test

Caused by: java.lang.RuntimeException: test

        at demo.ReactiveErrorDemo.lambda$13(ReactiveErrorDemo.java:82)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:386)

        at reactor.core.publisher.FluxIterable$IterableSubscription.slowPath(FluxIterable.java:272)

        at reactor.core.publisher.FluxIterable$IterableSubscription.request(FluxIterable.java:230)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onSubscribe(FluxFlatMap.java:371)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:165)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:87)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4400)

        at reactor.core.publisher.Mono.subscribeWith(Mono.java:4515)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4232)

        at demo.ReactiveErrorDemo.main(ReactiveErrorDemo.java:19)

*/
  • 在 Flux API 中抛出异常，和返回 Mono.error() 一样

    会被当成一个异常被处理，

    不会在map, doOnNext, doOnSuccess处理。

    会被 doOnError处理。
	public static Mono<String> fluxExceptionTest() {

		logger.info("case: flux error test");

		return Flux.fromIterable(Arrays.asList(0, 1, 2)).flatMap(o -> {

			logger.info(MessageFormat.format("flatMap: {0}", o));

			if (o == 1) {

				throw new RuntimeException("test");

			}

			return Mono.just(o);

		}).map(o -> {

			logger.info(MessageFormat.format("map: {0}", o));

			return o;

		}).count(

		).doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: count is {0}", o));

		}).doOnError(e -> {

			logger.info(MessageFormat.format("doOnError: {0}", e.getMessage()));

		}).map(o -> {

			return "abc";

		});

	}

	fluxExceptionTest().log().subscribe();

/*

00:20:38.104 [main] INFO demo.ReactiveErrorDemo - case: flux error test

00:20:38.265 [main] DEBUG reactor.util.Loggers - Using Slf4j logging framework

00:20:38.358 [main] INFO reactor.Mono.MapFuseable.1 - | onSubscribe([Fuseable] FluxMapFuseable.MapFuseableSubscriber)

00:20:38.364 [main] INFO reactor.Mono.MapFuseable.1 - | request(unbounded)

00:20:38.365 [main] INFO demo.ReactiveErrorDemo - flatMap: 0

00:20:38.366 [main] INFO demo.ReactiveErrorDemo - map: 0

00:20:38.366 [main] INFO demo.ReactiveErrorDemo - flatMap: 1

00:20:38.373 [main] INFO demo.ReactiveErrorDemo - doOnError: test

00:20:38.376 [main] ERROR reactor.Mono.MapFuseable.1 - | onError(java.lang.RuntimeException: test)

00:20:38.381 [main] ERROR reactor.Mono.MapFuseable.1 -

java.lang.RuntimeException: test

        at demo.ReactiveErrorDemo.lambda$8(ReactiveErrorDemo.java:61)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:386)

        at reactor.core.publisher.FluxIterable$IterableSubscription.slowPath(FluxIterable.java:272)

        at reactor.core.publisher.FluxIterable$IterableSubscription.request(FluxIterable.java:230)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onSubscribe(FluxFlatMap.java:371)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:165)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:87)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4400)

        at reactor.core.publisher.Mono.subscribeWith(Mono.java:4515)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4232)

        at demo.ReactiveErrorDemo.main(ReactiveErrorDemo.java:20)

00:20:38.385 [main] ERROR reactor.core.publisher.Operators - Operator called default onErrorDropped

reactor.core.Exceptions$ErrorCallbackNotImplemented: java.lang.RuntimeException: test

Caused by: java.lang.RuntimeException: test

        at demo.ReactiveErrorDemo.lambda$8(ReactiveErrorDemo.java:61)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onNext(FluxFlatMap.java:386)

        at reactor.core.publisher.FluxIterable$IterableSubscription.slowPath(FluxIterable.java:272)

        at reactor.core.publisher.FluxIterable$IterableSubscription.request(FluxIterable.java:230)

        at reactor.core.publisher.FluxFlatMap$FlatMapMain.onSubscribe(FluxFlatMap.java:371)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:165)

        at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:87)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4400)

        at reactor.core.publisher.Mono.subscribeWith(Mono.java:4515)

        at reactor.core.publisher.Mono.subscribe(Mono.java:4232)

        at demo.ReactiveErrorDemo.main(ReactiveErrorDemo.java:20)

*/

启动模式

  • 常用的启动模式
Mono.just(data);

Mono.fromXXX(xxx);

Flux.from(data);
  • 冷响应式
Mono.defer(() -> supplier);

冷响应式是指，在启动时，不会立即执行，而是在被订阅时才执行。

下面 IllegalArgumentException 会在 subscribe 后才会被调用。

// Sample code

	private Mono<Integer> monoAdd(Integer a, Integer b) {

		return Mono.defer(() -> {

			if (a == null) {

				throw new IllegalArgumentException("a is null");

			}

			if (b == null) {

				throw new IllegalArgumentException("b is null");

			}

			return Mono.just(a + b);

		});

	}

映射模式

这里讨论的映射模式，大都是关于多个响应式 API 之间的协作。

平行模式(flat pattern)

主要是用 flatMap() 方法。代码成 flatMap().flatMap().flatMap() 形状。

用于后面的 API 只使用前面 API 输出结果的情况。

	public static Mono<Integer> monoFlat(Integer a) {

		return Mono.defer(() -> {

			if (a == null) {

				throw new IllegalArgumentException("a is null");

			}

			return Mono.just(a);

		}).flatMap(data -> Mono.just(data * 2))

		.flatMap(data -> Mono.just(data + 100));

	}

	monoFlat(1).log().subscribe();

/*

00:15:17.005 [main] DEBUG reactor.util.Loggers - Using Slf4j logging framework

00:15:17.164 [main] INFO reactor.Mono.FlatMap.1 - | onSubscribe([Fuseable] MonoFlatMap.FlatMapMain)

00:15:17.168 [main] INFO reactor.Mono.FlatMap.1 - | request(unbounded)

00:15:17.171 [main] INFO reactor.Mono.FlatMap.1 - | onNext(102)

00:15:17.173 [main] INFO reactor.Mono.FlatMap.1 - | onComplete()

*/

嵌套模式(nested pattern)

对于后面的 API 需要使用多个前面 API 输出结果的情况，可以使用嵌套模式。

在嵌套模式中，后面的 API 可以直接使用前面 API 的结果。

	public static Mono<Integer> monoNested(Integer a, Integer b) {

		// return a * 100 + b * 100

		return Mono.defer(() -> {

			if (a == null) {

				throw new IllegalArgumentException("a is null");

			}

			return Mono.just(a * 100).flatMap(o1 -> {

				if (b == null) {

					throw new IllegalArgumentException("a is null");

				}

				return Mono.just(b * 100).map(

						// 在这里可以同时使用 o1 和 o2

						o2 -> o1 + o2);

			});

		});

	}

monoNested(1, 2).log().subscribe();

/*

00:22:43.816 [main] INFO reactor.Mono.Defer.2 - onSubscribe([Fuseable] FluxMapFuseable.MapFuseableSubscriber)

00:22:43.817 [main] INFO reactor.Mono.Defer.2 - request(unbounded)

00:22:43.817 [main] INFO reactor.Mono.Defer.2 - onNext(300)

00:22:43.818 [main] INFO reactor.Mono.Defer.2 - onComplete()

*/

拉链模式(zip pattern)

对于后面的 API 需要使用多个前面 API 输出结果的情况，可以使用拉链模式。

在拉链模式中，后面的 API 可以通过参数获取前面 API 的结果。

	public static Mono<Integer> monoZip(Integer a, Integer b) {

		// return a * 100 + b * 100

		return Mono.zip(

				Mono.defer(() -> {

					if (a == null) {

						throw new IllegalArgumentException("a is null");

					}

					return Mono.just(a * 100);

				}),

				Mono.defer(() -> {

					if (b == null) {

						throw new IllegalArgumentException("b is null");

					}

					return Mono.just(b * 100);

				}), (o1, o2) -> o1 + o2);

	}

	monoZip(1, 2).log().subscribe();

/*

00:32:22.326 [main] INFO reactor.Mono.Zip.3 - onSubscribe([Fuseable] MonoZip.ZipCoordinator)

00:32:22.326 [main] INFO reactor.Mono.Zip.3 - request(unbounded)

00:32:22.327 [main] INFO reactor.Mono.Zip.3 - onNext(300)

00:32:22.328 [main] INFO reactor.Mono.Zip.3 - onComplete()

*/

原子模式(atomic pattern)

拉链模式和嵌套模式都不能处理 null 值，原子模式可以。

注意下面示例中的 return Mono.just(0) 可以确保不会忽略 null 值的情况。

	public static Mono<Integer> monoAtomic(Integer a, Integer b) {

		AtomicReference<Integer> a100Ref = new AtomicReference<>(0);

		AtomicReference<Integer> b100Ref = new AtomicReference<>(0);

		// return a * 100 + b * 100

		return Mono.defer(() -> {

			if (a == null) {

				a100Ref.set(null);

			} else {

				a100Ref.set(a * 100);

			}

			return Mono.just(0);

		}).flatMap(o -> {

			if (b == null) {

				b100Ref.set(null);

			} else {

				b100Ref.set(b * 100);

			}

			return Mono.just(0);

		}).map(o -> {

			if (a100Ref.get() == null || b100Ref.get() == null) {

				return 0;

			}

			return a100Ref.get() + b100Ref.get();

		});

	}

monoAtomic(1, 2).log().subscribe();

/*

11:03:46.162 [main] INFO reactor.Mono.Map.4 - onSubscribe(FluxMap.MapSubscriber)

11:03:46.163 [main] INFO reactor.Mono.Map.4 - request(unbounded)

11:03:46.163 [main] INFO reactor.Mono.Map.4 - onNext(0)

11:03:46.164 [main] INFO reactor.Mono.Map.4 - onComplete()

*/

null 对象模式(null pattern)

我们还可以使用默认值来处理 null 值的情况。

在处理 null 值时，一个常见的需求是:

在一个 lambda 闭包中:

  • 可以知道这个值是 null 还是非 null。
  • 可以获取这个值。
  • 可以调用并返回一个新的响应式对象（发布者）

一个技巧是使用 .defaultIfEmpty() 方法来处理 null 值。

这个技巧对于数值或者 String 类型的值可能有效的，但是对于类实例就不好用了。

在这种情况下，可以考虑定义一个接口。

public interface Nullable {

	boolean isNone();

}

package demo.reactive;

public class Employee implements Nullable {

    private static final Employee none = new Employee(true);

    public static Employee none() {

        return none;

    }

    private Employee(boolean isNone) {

        this.isNone = isNone;

    }

    private Employee() {

    }

    private int id;

    private String name;

    private boolean isNone;

    public int getId() {

        return id;

    }

    public void setId(int id) {

        this.id = id;

    }

    public String getName() {

        return name;

    }

    public void setName(String name) {

        this.name = name;

    }

    @Override

    public boolean isNone() {

        return isNone;

    }

    public void setNull(boolean isNone) {

        this.isNone = isNone;

    }

}

	public static Mono<Employee> monoNullable() {

		// return nullable object

		return Mono.defer(() -> {

			return Mono.<Employee>empty();

		}).defaultIfEmpty(Employee.none());

	}

monoNullable().map(o -> {

			logger.info(MessageFormat.format("map.isNone: {0}", o.isNone()));

			return o;

		}).doOnSuccess(o -> {

			logger.info(MessageFormat.format("doOnSuccess: {0}", o));

		}).log().subscribe();

/*

18:28:06.789 [main] INFO reactor.Mono.PeekTerminal.1 - | onSubscribe([Fuseable] MonoPeekTerminal.MonoTerminalPeekSubscriber)

18:28:06.794 [main] INFO reactor.Mono.PeekTerminal.1 - | request(unbounded)

18:28:06.796 [main] INFO demo.ReactiveDemo - map.isNone: true

18:28:06.796 [main] INFO demo.ReactiveDemo - doOnSuccess: demo.reactive.Employee@120d6fe6

18:28:06.797 [main] INFO reactor.Mono.PeekTerminal.1 - | onNext(demo.reactive.Employee@120d6fe6)

18:28:06.799 [main] INFO reactor.Mono.PeekTerminal.1 - | onComplete()

*/

返回模式

下面是常见的返回模式。

Mono.empty();

Mono.then();

Mono.then(mono);

Mono.thenReturn(data);

参照

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