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README.md

ตัวอย่างการเขียน Reactor

https://projectreactor.io

Reactor

API

https://projectreactor.io/docs/core/release/api/

Reactor เป็น library สำหรับเขียน Reactive เหมือนกันกับ Rx (Reactive Extension) ที่ทาง Microsoft สร้างขึ้นมา http://reactivex.io/
ซึ่ง implements ตาม Spec ของ Reactive Streams https://www.reactive-streams.org/
ใช้เป็น Core ในการเขียน Spring-boot WebFlux ซึ่งเป็นการเขียน Spring-boot แบบ Non-Blocking I/O
การใช้ Reactor จะมี 2 ส่วนหลัก ๆ ที่ต้องทำความเข้าใจ คือ

  • Mono เป็น Publisher สำหรับปล่อย (publish) ข้อมูลตั้งแต่ 0 ถึง 1 element และ
  • Flux เป็น Publisher สำหรับปล่อย (publish) ข้อมูลตั้งแต่ 0 ถึง N elements

พื้นฐานการใช้ Reactor จะเหมือนกันกับ RxJava ฉะนั้นสามารถอ่าน Concept ต่าง ๆ แทนกันได้ โดยอ่านได้จากบทความนี้ RxJava series - part 1 - ตอน อะไรเอ่ย ReactiveX? ขอบคุณเจ้าของบทความครับ

บทความอื่น ๆ

Flow

Reactor Mono และ Flux จะมี Flow การทำงานตามลำดับเป็นดังนี้

Mono Flow

@Slf4j
public class MonoFlowExample {

    public static void main(String[] args) {
        Mono.just("A")
                .doFirst(() -> log.info("doFirst..."))
                .doOnRequest(value -> log.info("doOnRequest... {}", value))
                .doOnEach(signal -> log.info("doOnEach... {} : value => {}", signal.getType().name(), signal.get()))
                .doOnNext(value -> log.info("doOnNext... {}", value))
                .doOnCancel(() -> log.info("doOnCacel..."))
                .doOnError(e -> log.info("doOnError... {}", e.getMessage()))
                .doOnSuccess(value -> log.info("doOnSuccess... {}", value))
                .doOnSuccessOrError((value, e) -> log.info("doOnSuccessOrError... {} or {}", value, (e == null ? null : e.getMessage())))
                .doAfterSuccessOrError((value, e) -> log.info("doAfterSuccessOrError... {} or {}", value, (e == null ? null : e.getMessage())))
                .doAfterTerminate(() -> log.info("doAfterTerminate..."))
                .doOnTerminate(() -> log.info("doOnTerminate..."))
                .doOnSubscribe(subscription -> {
                    long id = 1234567890;
                    subscription.request(id);
                    log.info("doOnSubscribe... {}", id);
                })
                .doFinally(signalType -> log.info("doFinally... {}", signalType.toString()))
                .doOnDiscard(Object.class, value -> log.info("doOnDiscard... {}", value))
                .subscribe();
    }

}

output

- doFirst...  
- doOnRequest... 1234567890  
- doOnEach... ON_NEXT : value => A  
- doOnEach... ON_COMPLETE : value => null  
- doOnNext... A  
- doOnSuccess... A  
- doOnSuccessOrError... A or null  
- doAfterTerminate...  
- doAfterSuccessOrError... A or null  
- doOnTerminate...  
- doFinally... onComplete  
- doOnSubscribe... 1234567890  
- doOnCacel...

Flux Flow

@Slf4j
public class FluxFlowExample {

    public static void main(String[] args) {
        Flux.just("A", "B", "C")
                .doFirst(() -> log.info("doFirst..."))
                .doOnRequest(value -> log.info("doOnRequest... {}", value))
                .doOnEach(signal -> log.info("doOnEach... {} : value => {}", signal.getType().name(), signal.get()))
                .doOnNext(value -> log.info("doOnNext... {}", value))
                .doOnCancel(() -> log.info("doOnCacel..."))
                .doOnError(e -> log.info("doOnError... {}", e.getMessage()))
                .doOnComplete(() -> log.info("doOnComplete..."))
                .doAfterTerminate(() -> log.info("doAfterTerminate..."))
                .doOnTerminate(() -> log.info("doOnTerminate..."))
                .doOnSubscribe(subscription -> {
                    long id = 123456890;
                    subscription.request(id);
                    log.info("doOnSubscribe... {}", id);
                })
                .doFinally(signalType -> log.info("doFinally... {}", signalType.toString()))
                .doOnDiscard(Object.class, value -> log.info("doOnDiscard... {}", value))
                .subscribe();
    }

}

output

- doFirst...  
- doOnRequest... 123456890  
- doOnNext... A  
- doOnEach... ON_NEXT : value => A  
- doOnNext... B  
- doOnEach... ON_NEXT : value => B  
- doOnNext... C  
- doOnEach... ON_NEXT : value => C  
- doOnEach... ON_COMPLETE : value => null  
- doOnComplete...  
- doOnTerminate...  
- doFinally... onComplete  
- doAfterTerminate...  
- doOnSubscribe... 123456890    
- doOnCacel...

Table of Content

Mono

ตัวอย่างการใช้ Mono

Mono.never

Return a Mono that will never signal any data, error or completion signal, essentially running indefinitely.

จะไม่มีการปล่อยข้อมูลออกมา

@Slf4j
public class MonoNeverExample {

    public static void main(String[] args) {
        Mono.never()
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

  • ไม่มีข้อมูล

กลับไปข้างบน ↑

Mono.empty

Create a Mono that completes without emitting any item.

เป็นการสร้าง empty mono ซึ่งจะไม่มีข้อมูลปล่อยออกมา

@Slf4j
public class MonoEmptyExample {

    public static void main(String[] args) {
        Mono.empty()
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

  • ไม่มีข้อมูล

กลับไปข้างบน ↑

Mono.just

Create a new Mono that emits the specified item, which is captured at instantiation time.

การสร้าง Mono จากข้อมูลที่มีอยู่แล้ว (ข้อมูลต้องพร้อมแล้ว)

  • ข้อมูลห้ามเป็น null **** เพราะจะเกิด java.lang.NullPointerException: value
  • จะ captured ข้อมูล ณ ตอนสร้าง Mono
@Slf4j
public class MonoJustExample {

    public static void main(String[] args) {
        Mono.just("Hello at " + LocalDateTime.now())
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => Hello at 2019-07-22T16:07:03.309

กลับไปข้างบน ↑

Mono.log

Observe all Reactive Streams signals and trace them using Logger support.
Default will use Level#INFO and java.util.logging.
If SLF4J is available, it will be used instead.

การ log ข้อมูลแต่ละ step ออกมาดู

@Slf4j
public class MonoLogExample {

    public static void main(String[] args) {
        Mono.just("Hello at " + LocalDateTime.now())
                .log()
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- | onSubscribe([Synchronous Fuseable] Operators.ScalarSubscription)  
- | request(unbounded)  
- | onNext(Hello at 2019-07-22T21:12:06.912)  
- message => Hello at 2019-07-22T21:12:06.912  
- | onComplete()

กลับไปข้างบน ↑

Mono.justOrEmpty

Create a new Mono that emits the specified item if Optional#isPresent() otherwise only emits onComplete.

การสร้าง Mono จากข้อมูลที่มีอยู่แล้ว (ข้อมูลต้องพร้อมแล้ว)

  • ข้อมูลเป็น null ได้
@Slf4j
public class MonoJustOrEmptyExample {

    public static void main(String[] args) {
        Mono.justOrEmpty(null)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

  • จะไม่มีการเรียก doOnNext เนื่องจากไม่มีข้อมูลปล่อยออกมา

กลับไปข้างบน ↑

Mono.defaultIfEmpty

Provide a default single value if this mono is completed without any data

คืนค่า default กรณีที่ไม่มีข้อมูลปล่อยออกมา

@Slf4j
public class MonoDefaultIfEmptyExample {

    public static void main(String[] args) {
        Mono.justOrEmpty(null)
                .defaultIfEmpty("Hello World")
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => Hello World

กลับไปข้างบน ↑

Mono.switchIfEmpty

Fallback to an alternative Mono if this mono is completed without data

ทำการเปลี่ยน (switch) Mono ถ้าไม่มีข้อมูลถูกปล่อยออกมาจาก source

@Slf4j
public class MonoSwithIfEmptyExample {

    public static void main(String[] args) {
        Mono.justOrEmpty(null)
                .switchIfEmpty(Mono.just("Hello at " + LocalDateTime.now()))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => Hello at 2019-07-22T16:35:16.656

กลับไปข้างบน ↑

Mono.error

Create a Mono that terminates with the specified error immediately after

สำหรับปล่อยข้อมูล error หรือ Exception ออกมา

@Slf4j
public class MonoErrorExample {

    public static void main(String[] args) {
        Mono.justOrEmpty(null)
                .switchIfEmpty(Mono.error(new RuntimeException("Not found data")))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .doOnError(e -> {
                    log.info("errror => {}", e.getMessage());
                })
                .subscribe();
    }

}

output

- errror => Not found data

กลับไปข้างบน ↑

Mono.map

Transform the item emitted by this Mono by applying a synchronous function to it.

ทำการแปลง (Transform) ข้อมูลก่อนส่งออกมา

@Slf4j
public class MonoMapExample {

    public static void main(String[] args) {
        Mono.just(1000)
                .map(number -> number * 2)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 2000

กลับไปข้างบน ↑

Mono.filter

If this Mono is valued, test the result and replay it if predicate returns true.
Otherwise complete without value.

ทำการกรอง (filter) ข้อมูลตามเงื่อนไขที่กำหนด

  • example 1
@Slf4j
public class MonoFilterExample1 {

    public static void main(String[] args) {
        Mono.just(2)
                .filter(number -> (number % 2 == 0))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 2
  • example 2
@Slf4j
public class MonoFilterExample1 {

    public static void main(String[] args) {
        Mono.just(3)
                .filter(number -> (number % 2 == 0))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

  • ไม่มีข้อมูล

กลับไปข้างบน ↑

Mono.fromCallable

Create a Mono producing its value using the provided Callable.
If the Callable resolves to null, the resulting Mono completes empty.

การสร้าง Mono แบบ Lazy Load

@Slf4j
public class MonoFromCallableExample {
    
    public static void main(String[] args) {
        Mono.fromCallable(() -> "Hello at " + LocalDateTime.now())
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => Hello at 2019-07-23T22:47:50.758

กลับไปข้างบน ↑

Mono.defer

Create a Mono provider that will Supplier#get supply a target Mono to subscribe to for each Subscriber downstream.

การสร้าง Mono แบบ Lazy Load

@Slf4j
public class MonoDeferExample {

    public static void main(String[] args) {
        Mono.defer(() -> Mono.just("Hello at " + LocalDateTime.now()))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => Hello at 2019-07-22T16:09:45.886

กลับไปข้างบน ↑

Mono.create

Creates a deferred emitter that can be used with callback-based APIs to signal at most one value, a complete or an error signal.

การสร้าง Mono แบบ Asynchronous

@Slf4j
public class MonoCreateExample {

    public static void main(String[] args) {
        Mono.create(callback -> {
            try {
                log.info("wait 3 seconds... at " + LocalDateTime.now());
                Thread.sleep(3000L);
            } catch (InterruptedException ex) {
                //
            }
            callback.success("Hello at " + LocalDateTime.now());
        })
        .doOnNext(message -> {
            log.info("message => {}", message);
        })
        .subscribe();
    }

}

output

- wait 3 seconds... at 2019-07-22T16:16:55.602  
- message => Hello at 2019-07-22T16:16:58.603

กลับไปข้างบน ↑

Mono.flatMap

Transform the item emitted by this Mono asynchronously, returning the value emitted by another Mono (possibly changing the value type).

คล้าย ๆ map คือ ทำการแปลง (Transform) ข้อมูลก่อนส่งออกมา แต่เป็นแบบ Asyncronous

@Slf4j
public class MonoFlatMapExample {

    public static void main(String[] args) {
        Mono.just(100)
                .flatMap(number -> {
                    return Mono.create(callback -> {
                        try {
                            log.info("wait 3 seconds... at " + LocalDateTime.now());
                            Thread.sleep(3000);
                        } catch (InterruptedException ex) {
                            
                        }
                        callback.success(number * 5);
                    });
                })
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- wait 3 seconds... at 2019-07-22T16:54:23.923
- message => 500

กลับไปข้างบน ↑

Mono.zip

Merge given monos into a new Mono that will be fulfilled when all of the given Monos have produced an item, aggregating their values into a Tuple2. An error or empty completion of any source will cause other sources to be cancelled and the resulting Mono to immediately error or complete, respectively.

เป็นการรวม response จาก Mono ต่าง ๆ

  • Sequencial
@Slf4j
public class MonoZipExample1 {

    private static void delay(String name, int seconds) {
        try {
            log.info("{} wait {} seconds", name, seconds);
            Thread.sleep(seconds * 1000L);
        } catch (InterruptedException ex) {

        }
    }

    public static void main(String[] args) {
        
        Mono<String> task1 = Mono.create(callback -> {
            delay("task 1", 3);
            callback.success("Hello from Task 1");
        });
        
        Mono<String> task2 = Mono.create(callback -> {
            delay("task 2", 1);
            callback.success("Hello from Task 2");
        });
 
        log.info("start at {}", LocalDateTime.now());
        
        Mono.zip(task1, task2)
                .doOnNext(response -> {
                    log.info("task 1-> {}", response.getT1());
                    log.info("task 2-> {}", response.getT2());
                })
                .doOnSuccess(response -> {
                    log.info("end at {}", LocalDateTime.now());
                })
                .subscribe();
    }

}

output (ใช้เวลาทำงาน 3 + 1 = 4 วินาที)

- start at 2019-07-22T18:42:30.950
- task 1 wait 3 seconds  
- task 2 wait 1 second  
- task 1-> Hello from Task 1  
- task 2-> Hello from Task 2   
- end at 2019-07-22T18:42:35.045
  • Parallel (ใช้ .subscribeOn(Schedulers.newElastic(name, ttlSeconds)))
@Slf4j
public class MonoZipExample2 {

    private static void delay(String name, int seconds) {
        try {
            log.info("{} wait {} seconds", name, seconds);
            Thread.sleep(seconds * 1000L);
        } catch (InterruptedException ex) {

        }
    }

    public static void main(String[] args) {
        
        Mono<String> task1 = Mono.create((MonoSink<String> callback) -> {
            delay("task 1", 3);
            callback.success("Hello from Task 1");
        }).subscribeOn(Schedulers.newElastic("scheduler 1", 1));
        
        Mono<String> task2 = Mono.create((MonoSink<String> callback) -> {
            delay("task 2", 1);
            callback.success("Hello from Task 2");
        }).subscribeOn(Schedulers.newElastic("scheduler 2", 1));
        
        Mono<String> task3 = Mono.create((MonoSink<String> callback) -> {
            delay("task 3", 5);
            callback.success("Hello from Task 3");
        }).subscribeOn(Schedulers.newElastic("scheduler 3", 1));
        
        log.info("start at {}", LocalDateTime.now());
        
        Mono.zip(task1, task2, task3)
                .doOnNext(response -> {
                    log.info("task 1-> {}", response.getT1());
                    log.info("task 2-> {}", response.getT2());
                    log.info("task 3-> {}", response.getT3());
                })
                .doOnSuccess(response -> {
                    log.info("end at {}", LocalDateTime.now());
                })
                .subscribe();
    }

}

output (ใช้เวลาทำงานมากที่สุดคือ 5 วินาที)

- start at 2019-07-22T18:38:45.892
- task 1 wait 3 seconds  
- task 2 wait 1 seconds  
- task 3 wait 5 seconds  
- task 1-> Hello from Task 1  
- task 2-> Hello from Task 2  
- task 3-> Hello from Task 3  
- end at 2019-07-22T18:38:50.920

กลับไปข้างบน ↑

Mono.block

Subscribe to this Mono and block indefinitely until a next signal is received. Returns that value, or null if the Mono completes empty. In case the Mono errors, the original exception is thrown (wrapped in a RuntimeException if it was a checked exception).

การทำงานแบบ Blocking I/O หรือ Synchronous

@Slf4j
public class MonoBlockExample {

    public static void main(String[] args) {
        String message = Mono.just("Hello World").block();
        log.info("message => {}", message);
    }

}

output

- message => Hello World

กลับไปข้างบน ↑

Mono.cache

Turn this Mono into a hot source and cache last emitted signals for further Subscriber, with an expiry timeout.

สำหรับ Cache ข้อมูล ตามเวลาที่กำหนด

  • กรณีไม่ cache
@Slf4j
public class MonoCacheExample1 {

    public static void main(String[] args) {
        Mono<String> defer = Mono.defer(() -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ex) {

            }
            return Mono.just("Hello at " + LocalDateTime.now());
        });
        
        log.info("message => {}", defer.block());
        log.info("message => {}", defer.block());
        log.info("message => {}", defer.block());
    }

}

output

- message => Hello at 2019-07-22T21:06:21.169  
- message => Hello at 2019-07-22T21:06:22.186  
- message => Hello at 2019-07-22T21:06:23.186

สังเกตว่า ผลลัพธ์ (เวลาของแต่ละ message) ไม่เหมือนกัน

  • กรณี cache
@Slf4j
public class MonoCacheExample2 {

    public static void main(String[] args) {
        Mono<String> defer = Mono.defer(() -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ex) {

            }
            return Mono.just("Hello at " + LocalDateTime.now());
        }).cache(Duration.ofMinutes(5));
        
        log.info("message => {}", defer.block());
        log.info("message => {}", defer.block());
        log.info("message => {}", defer.block());
    }

}

output

- message => Hello at 2019-07-22T21:08:31.852  
- message => Hello at 2019-07-22T21:08:31.852  
- message => Hello at 2019-07-22T21:08:31.852

ข้อมูล message ที่ 2 และ 3 เหมือน message ที่ 1 เนื่องจากมีการ cache result ไว้ 5 นาที

กลับไปข้างบน ↑

Mono.flux

Convert this Mono to a Flux

การแปลงจาก Mono -> Flux

@Slf4j
public class MonoFluxExample {

    public static void main(String[] args) {
        Flux<String> flux = Mono.just("Hello at " + LocalDateTime.now())
                .flux();

        flux.doOnNext(message -> {
            log.info("message => {}", message);
        })
                .subscribe();
    }

}

output

- message => Hello at 2019-07-22T21:17:33.880

กลับไปข้างบน ↑

Mono.then

Let this Mono complete then play another Mono.

In other words ignore element from this Mono and transform its completion signal into the emission and completion signal of a provided Mono<V>. Error signal is replayed in the resulting Mono<V>.

การทำงานตามลำดับด้วย then

@Slf4j
public class MonoThenExample {

    private static Mono<String> task(final String message, int delay) {
        return Mono.defer(() -> {
            try {
                Thread.sleep(delay * 1000);
            } catch (InterruptedException ex) {

            }
            return Mono.just(message + " " + LocalDateTime.now());
        });
    }

    public static void main(String[] args) {

        Mono<String> task1 = task("Hello 1 at", 3);
        Mono<String> task2 = task("Hello 2 at", 1);
        Mono<String> task3 = task("Hello 3 at", 2);
        
        task1
                .doOnNext(message -> {
                    log.info("message 1 => {}", message);
                })
                .then(task2)
                .doOnNext(message -> {
                    log.info("message 2 => {}", message);
                })
                .then(task3)
                .doOnNext(message -> {
                    log.info("message 3 => {}", message);
                })
                .subscribe();
    }

}

output

- message 1 => Hello 1 at 2019-07-22T21:28:56.144  
- message 2 => Hello 2 at 2019-07-22T21:28:57.146  
- message 3 => Hello 3 at 2019-07-22T21:28:59.147

กลับไปข้างบน ↑

Mono.concatWith

Concatenate emissions of this Mono with the provided Publisher (no interleave).

เป็นการเชื่อม Mono 2 อันเข้าด้วยกัน กลายเป็น Flux

@Slf4j
public class MonoConcatWithExample {

    private static Mono<String> task(final String message, int delay) {
        return Mono.defer(() -> {
            try {
                Thread.sleep(delay * 1000);
            } catch (InterruptedException ex) {

            }
            return Mono.just(message + " " + LocalDateTime.now());
        });
    }

    public static void main(String[] args) {

        Mono<String> task1 = task("Hello 1 at", 3);
        Mono<String> task2 = task("Hello 2 at", 1);

        Flux<String> flux = task1.concatWith(task2);

        flux
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => Hello 1 at 2019-07-22T21:41:58.132  
- message => Hello 2 at 2019-07-22T21:41:59.161

กลับไปข้างบน ↑

Mono.timeout

Propagate a TimeoutException in case no item arrives within the given Duration.

ใช้สำหรับจำกัดเวลาในการตอบสนอง เช่น ถ้าไม่ตอบสนองภายใน 3 วินาที จะเกิด java.util.concurrent.TimeoutException

@Slf4j
public class MonoTimeoutExample {

    public static void main(String[] args) {
        Mono.create(callback -> {
            try {
                log.info("wait 5 seconds... at " + LocalDateTime.now());
                Thread.sleep(5000L);
            } catch (InterruptedException ex) {
                //
            }
            callback.success("Hello at " + LocalDateTime.now());
        })
                .timeout(Duration.ofSeconds(3))
                .onErrorResume(TimeoutException.class, e -> Mono.just("Hello from timeout"))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- wait 5 seconds... at 2019-07-23T14:53:27.468  
- message => Hello from timeout

กลับไปข้างบน ↑

Mono.filterWhen

If this Mono is valued, test the value asynchronously using a generated Publisher<Boolean> test. The value from the Mono is replayed if the first item emitted by the test is true. It is dropped if the test is either empty or its first emitted value is false.

Note that only the first value of the test publisher is considered, and unless it is a Mono, test will be cancelled after receiving that first value.

ทำการกรอง (filter) ข้อมูลตามเงื่อนไขที่กำหนด เหมือน Mono.filter แต่ทำงานแบบ Asynchronous

@Slf4j
public class MonoFilterWhenExample {

    public static void main(String[] args) {
        int randomNumber = (int) (Math.random() * 100); //0 to 100
        log.info("random number => {}", randomNumber);
        Mono.just(randomNumber)
                .filterWhen(number -> {
                    return Mono.create(callback -> {
                        try {
                            log.info("wait 3 seconds... at " + LocalDateTime.now());
                            Thread.sleep(3000L);
                        } catch (InterruptedException ex) {
                            //
                        }
                        callback.success(number % 2 == 0);
                    });
                })
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .doOnSuccess(message -> {
                    log.info("success at " + LocalDateTime.now());
                })
                .subscribe();
    }

}

output

  • result 1
- random number => 65  
- wait 3 seconds... at 2019-07-23T15:34:27.101  
- success at 2019-07-23T15:34:30.101
  • result 2
- random number => 46  
- wait 3 seconds... at 2019-07-23T15:35:41.460  
- message => 46  
- success at 2019-07-23T15:35:44.462

กลับไปข้างบน ↑

Mono.onErrorResume

Subscribe to a fallback publisher when an error matching the given type occurs, using a function to choose the fallback depending on the error.

สำหรับดักจับ error หรือ exception ที่เกิดขึ้น (อารมณ์เหมือน try/catch)

@Slf4j
public class MonoOnErrorResumeExample {

    private static class NotFoundException extends RuntimeException {

        public NotFoundException(String message) {
            super(message);
        }

    }

    @Getter
    @Setter
    @Builder
    @NoArgsConstructor
    @AllArgsConstructor
    private static class User {

        private String username;
    }

    private static Mono<User> getUser() {
        return Mono.create((MonoSink<User> callbback) -> {
            final int randomNumber = (int) (Math.random() * 100);
            if (randomNumber % 2 == 0) {
                callbback.success(User.builder().username("jittagornp").build());
            } else {
                callbback.error(new NotFoundException("Not found user"));
            }
        });
    }

    public static void main(String[] args) {
        getUser()
                .onErrorResume(NotFoundException.class, e -> {
                    log.info("error => {}", e.getMessage());
                    return Mono.just(User.builder().username("anonymous").build());
                })
                .doOnNext(user -> {
                    log.info("username => {}", user.getUsername());
                })
                .subscribe();
    }

}

output

  • result 1
- username => jittagornp
  • result 2 (เข้า onErrorResume)
- error => Not found user
- username => anonymous

กลับไปข้างบน ↑

Flux

ตัวอย่างการใช้ Flux

Flux.just

Create a Flux that emits the provided elements and then completes.

การสร้าง Flux จากข้อมูลที่มีอยู่แล้ว (ข้อมูลต้องพร้อมแล้ว)

  • ข้อมูลห้ามเป็น null เพราะจะเกิด java.lang.NullPointerException
@Slf4j
public class FluxJustExample {

    public static void main(String[] args) {
        Flux.just("1", "2", "3", "4", "5")
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 1  
- message => 2  
- message => 3  
- message => 4  
- message => 5

กลับไปข้างบน ↑

Flux.fromIterable

Create a Flux that emits the items contained in the provided Iterable.
A new iterator will be created for each subscriber.

การสร้าง Flux จาก Java Collections (Iterable)

  • ข้อมูลห้ามเป็น null เพราะจะเกิด java.lang.NullPointerException: iterable
@Slf4j
public class FluxFromIterableExample {

    public static void main(String[] args) {
        List<String> list = Arrays.asList("1", "2", "3", "4", "5");
        Flux.fromIterable(list)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 1  
- message => 2  
- message => 3  
- message => 4  
- message => 5

กลับไปข้างบน ↑

Flux.fromStream

Create a Flux that emits the items contained in the provided Stream.
Keep in mind that a Stream cannot be re-used, which can be problematic in case of multiple subscriptions or re-subscription (like with #repeat() or #retry()). The Stream is Stream#close() closed automatically by the operator on cancellation, error or completion.

การสร้าง Flux จาก Java 8 Stream

  • ข้อมูลห้ามเป็น null เพราะจะเกิด java.lang.NullPointerException: Stream s must be provided
@Slf4j
public class FluxFromStreamExample {

    public static void main(String[] args) {
        Stream<String> stream = Stream.of("1", "2", "3", "4", "5");
        Flux.fromStream(stream)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 1  
- message => 2  
- message => 3  
- message => 4  
- message => 5

กลับไปข้างบน ↑

Flux.range

Build a Flux that will only emit a sequence of count incrementing integers, starting from start. That is, emit integers between start (included) and start + count (excluded) then complete.

การสร้าง Flux จากช่วงที่กำหนด (start จาก 3 ไป 5 จำนวน)

@Slf4j
public class FluxRangeExample {

    public static void main(String[] args) {
        int start = 3;
        int count = 5;
        Flux.range(start, count)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 3  
- message => 4  
- message => 5  
- message => 6  
- message => 7

กลับไปข้างบน ↑

Flux.defaultIfEmpty

Provide a default unique value if this sequence is completed without any data

คืนค่า default กรณีที่ไม่มีข้อมูลปล่อยออกมา ทำงานเหมือน Mono.defaultIfEmpty

@Slf4j
public class FluxDefaultIfEmptyExample {
    
    public static void main(String[] args) {
        Flux.fromIterable(Collections.emptyList())
                .defaultIfEmpty("0")
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 0

กลับไปข้างบน ↑

Flux.switchIfEmpty

Switch to an alternative Publisher if this sequence is completed without any data.

ทำการเปลี่ยน (switch) Flux ถ้าไม่มีข้อมูลถูกปล่อยออกมา ทำงานเหมือน Mono.switchIfEmpty

@Slf4j
public class FluxSwitchIfEmptyExample {
    
    public static void main(String[] args) {
        Flux.fromIterable(Collections.emptyList())
                .switchIfEmpty(Flux.just("1", "2", "3"))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 1  
- message => 2  
- message => 3

กลับไปข้างบน ↑

Flux.error

Create a Flux that terminates with the specified error immediately after being subscribed to.

สำหรับปล่อยข้อมูล error หรือ Exception ออกมา เหมือน Mono.error

@Slf4j
public class FluxErrorExample {
    
    public static void main(String[] args) {
        Flux.empty()
                .switchIfEmpty(Flux.error(new RuntimeException("Not found data")))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .doOnError(e -> {
                    log.info("error => {}", e.getMessage());
                })
                .subscribe();
    }
    
}

output

- error => Not found data

กลับไปข้างบน ↑

Flux.concat

Concatenate all sources provided as a vararg, forwarding elements emitted by the sources downstream.
Concatenation is achieved by sequentially subscribing to the first source then waiting for it to complete before subscribing to the next, and so on until the last source completes. Any error interrupts the sequence immediately and is forwarded downstream.

สำหรับ concat หรือ ต่อ Publisher ต่าง ๆ ให้เป็น Flux เดียว

  • example 1
@Slf4j
public class FluxConcatExample1 {

    public static void main(String[] args) {
        Flux.concat(
                Mono.just("task 1"),
                Mono.just("task 2"),
                Mono.just("task 3"),
                Mono.just("task 4"),
                Mono.just("task 5")
        )
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => task 1  
- message => task 2  
- message => task 3  
- message => task 4  
- message => task 5
  • example 2
@Slf4j
public class FluxConcatExample2 {

    public static void main(String[] args) {
        Flux.concat(
                Mono.just("1"),
                Flux.just("2", "3"),
                Mono.just("4"),
                Flux.just("5", "6", "7"),
                Mono.just("8")
        )
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 1  
- message => 2  
- message => 3  
- message => 4  
- message => 5  
- message => 6  
- message => 7  
- message => 8

กลับไปข้างบน ↑

Flux.create

Programmatically create a Flux with the capability of emitting multiple elements in a synchronous or asynchronous manner through the FluxSink API.
This includes emitting elements from multiple threads.

การสร้าง Flux แบบ Asynchronous เหมือน Mono.create

@Slf4j
public class FluxCreateExample {

    public static void main(String[] args) {
        Flux.create(callback -> {
            for (int i = 1; i <= 5; i++) {
                try {
                    Thread.sleep(1000L);
                } catch (InterruptedException ex) {
                   
                }
                callback.next("task " + i + " at " + LocalDateTime.now());
            }
            callback.complete();
            
        })
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}
  • หากหยุดปล่อยข้อมูลแล้ว อย่าลืม call .complete() ให้เป็นนิสัย เพื่อไม่ให้เกิด Memory Leak
    output
- message => task 1 at 2019-07-22T23:47:54.078  
- message => task 2 at 2019-07-22T23:47:55.079  
- message => task 3 at 2019-07-22T23:47:56.093  
- message => task 4 at 2019-07-22T23:47:57.106  
- message => task 5 at 2019-07-22T23:47:58.106

กลับไปข้างบน ↑

Flux.generate

Programmatically create a Flux by generating signals one-by-one via a consumer callback and some state. The stateSupplier may return null.

สำหรับปล่อยข้อมูลออกมาเรื่อย ๆ จนกว่าจะสั่งหยุด หรือเรียก complete

@Slf4j
public class FluxGenerateExample {

    public static void main(String[] args) {
        Flux.generate(
                () -> 0, //initial value or state 
                (value, sink) -> {

                    if (value >= 5) {
                        sink.complete();
                    }

                    sink.next(value + " at " + LocalDateTime.now());

                    try {
                        Thread.sleep(1000L);
                    } catch (InterruptedException ex) {

                    }

                    return value + 1;
                })
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 0 at 2019-07-24T15:50:25.223  
- message => 1 at 2019-07-24T15:50:26.225  
- message => 2 at 2019-07-24T15:50:27.226  
- message => 3 at 2019-07-24T15:50:28.227  
- message => 4 at 2019-07-24T15:50:29.227

กลับไปข้างบน ↑

Flux.count

Counts the number of values in this Flux.
The count will be emitted when onComplete is observed.

การนับจำนวน elements

@Slf4j
public class FluxCountExample {

    public static void main(String[] args) {
        Flux.just("1", "2", "3", "4", "5")
                .count()
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 5

กลับไปข้างบน ↑

Flux.repeat

Repeatedly subscribe to the source numRepeat times. This results in numRepeat + 1 total subscriptions to the original source. As a consequence, using 0 plays the original sequence once.

การทำซ้ำ

@Slf4j
public class FluxRepeatExample {

    public static void main(String[] args) {
        Flux.just("1", "2", "3")
                .repeat(1)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 1  
- message => 2  
- message => 3  
- message => 1  
- message => 2  
- message => 3

กลับไปข้างบน ↑

Flux.collectList

Collect all elements emitted by this Flux into a List that is emitted by the resulting Mono when this sequence completes.

การแปลงจาก Flux<?> ไปเป็น Mono<List<?>>

@Slf4j
public class FluxCollectListExample {

    public static void main(String[] args) {
        Mono<List<String>> list = Flux.just("1", "2", "3", "4", "5")
                .collectList();
        
                list.doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => [1, 2, 3, 4, 5]

กลับไปข้างบน ↑

Flux.collectMap

Collect all elements emitted by this Flux into a hashed Map that is emitted by the resulting Mono when this sequence completes. The key is extracted from each element by applying the keyExtractor Function. In case several elements map to the same key, the associated value will be the most recently emitted element.

การเก็บลง map

@Slf4j
public class FluxTodoExample {

    public static void main(String[] args) {
        Flux.just(
                "Sunday",
                "Monday",
                "Tuesday",
                "Wednesday",
                "Thursday",
                "Friday",
                "Saturday"
        )
                .collectMap(value -> value.substring(0, 3))
                .doOnNext(map -> {
                    log.info("size => {}", map.size());
                    log.info("map => {}", map);
                    log.info("day => {}", map.get("Sun"));
                    log.info("day => {}", map.get("Mon"));
                    log.info("day => {}", map.get("Tue"));
                    log.info("day => {}", map.get("Wed"));
                    log.info("day => {}", map.get("Thu"));
                    log.info("day => {}", map.get("Fri"));
                    log.info("day => {}", map.get("Sat"));
                })
                .subscribe();
    }

}

output

- size => 7  
- map => {Thu=Thursday, Tue=Tuesday, Wed=Wednesday, Sat=Saturday, Fri=Friday, Sun=Sunday, Mon=Monday}  
- day => Sunday  
- day => Monday  
- day => Tuesday  
- day => Wednesday  
- day => Thursday  
- day => Friday  
- day => Saturday

กลับไปข้างบน ↑

Flux.skip

Skip the specified number of elements from the beginning of this Flux then emit the remaining elements.

สำหรับกระโดดข้ามข้อมูลตามจำนวนที่กำหนด

@Slf4j
public class FluxSkipExample {
    
    public static void main(String[] args) {
        Flux.just("A", "B", "C", "D", "E", "F", "G", "H")
                .skip(2)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => C  
- message => D  
- message => E   
- message => F   
- message => G   
- message => H

กลับไปข้างบน ↑

Flux.take

Take only the first N values from this Flux, if available.

If N is zero, the resulting Flux completes as soon as this Flux signals its first value (which is not not relayed, though).

สำหรับเลือกข้อมูลตามจำนวนที่กำหนด

@Slf4j
public class FluxTakeExample {

    public static void main(String[] args) {
        Flux.just("A", "B", "C", "D", "E", "F", "G", "H")
                .skip(2)
                .take(3)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => C  
- message => D  
- message => E

กลับไปข้างบน ↑

Flux.all

Emit a single boolean true if all values of this sequence match the Predicate.

The implementation uses short-circuit logic and completes with false if the predicate doesn't match a value.

สำหรับเช็คว่าข้อมูล ทั้งหมด match กับเงื่อนไขที่ตั้งไว้หรือไม่

@Slf4j
public class FluxAllExample {

    public static void main(String[] args) {
        Flux.just(
                "Sunday",
                "Monday",
                "Tuesday",
                "Wednesday",
                "Thursday",
                "Friday",
                "Saturday"
        )
                .all(day -> day.endsWith("day"))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => true

กลับไปข้างบน ↑

Flux.any

Emit a single boolean true if any of the values of this Flux sequence match the predicate.

The implementation uses short-circuit logic and completes with false if any value doesn't match the predicate.

สำหรับเช็คว่าข้อมูล บางตัว match กับเงื่อนไขที่ตั้งไว้หรือไม่

@Slf4j
public class FluxAnyExample {

    public static void main(String[] args) {
        Flux.just(
                "Sunday",
                "Monday",
                "Tuesday",
                "Wednesday",
                "Thursday",
                "Friday",
                "Saturday"
        )
                .any(day -> day.startsWith("Mon"))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => true

กลับไปข้างบน ↑

Flux.filter

Evaluate each source value against the given Predicate. If the predicate test succeeds, the value is emitted. If the predicate test fails, the value is ignored and a request of 1 is made upstream.

ทำการกรอง (filter) ข้อมูลตามเงื่อนไขที่กำหนด เหมือน Mono.filter

@Slf4j
public class FluxFilterExample {
    
    public static void main(String[] args) {
        Flux.just(1, 2, 3, 4, 5)
                .filter(number -> (number % 2 == 0))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 2  
- message => 4

กลับไปข้างบน ↑

Flux.map

Transform the items emitted by this Flux by applying a synchronous function to each item.

ทำการแปลง (Transform) ข้อมูลก่อนส่งออกมา เหมือน Mono.map

@Slf4j
public class FluxMapExample {
    
    public static void main(String[] args) {
        Flux.just(1, 2, 3, 4, 5)
                .map(number -> number * 2)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 2  
- message => 4 
- message => 6  
- message => 8 
- message => 10

กลับไปข้างบน ↑

Flux.buffer

Collect incoming values into multiple List buffers that will be emitted by the returned Flux each time the given max size is reached or once this Flux completes.

เก็บข้อมูลลง buffer ตามจำนวนที่กำหนด แล้วค่อยปล่อยออกมาเป็นชุด

@Slf4j
public class FluxBufferExample {

    public static void main(String[] args) {
        Flux.create(callback -> {
            for (int i = 0; i < 15; i++) {
                try {
                    Thread.sleep(1000L);
                } catch (InterruptedException ex) {

                }
                callback.next(i);
            }
            callback.complete();

        })
                .buffer(5)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}
  • การใช้ Flux.create อย่าลืม call .complete() ด้วยเสมอ เพื่อป้องกัน Memory Leak
    output
- message => [0, 1, 2, 3, 4]  
- message => [5, 6, 7, 8, 9]  
- message => [10, 11, 12, 13, 14]

กลับไปข้างบน ↑

Flux.sample

Sample this Flux by periodically emitting an item corresponding to that Flux latest emitted value within the periodical time window. Note that if some elements are emitted quicker than the timespan just before source completion, the last of these elements will be emitted along with the onComplete signal.

สำหรับชะลอการรับข้อมูลตามเวลาที่กำหนด (บางครั้งข้อมูลปล่อยออกมาเร็วเกินไป)

  • เช่น การพิมพ์ keyboard
@Slf4j
public class FluxSampleExample {

    public static void main(String[] args) {
        Flux.create(callback -> {
            for (int i = 0; i < 15; i++) {
                try {
                    Thread.sleep(100L);
                } catch (InterruptedException ex) {

                }
                callback.next(i);
            }
            callback.complete();

        })
                .sample(Duration.ofMillis(300))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => 1  
- message => 4  
- message => 7  
- message => 10  
- message => 12  
- message => 14
  • การใช้ Flux.create อย่าลืม call .complete() ด้วยเสมอ เพื่อป้องกัน Memory Leak

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Flux.distinct

For each Subscriber, track elements from this Flux that have been seen and filter out duplicates.

The values themselves are recorded into a HashSet for distinct detection. Use distinct(Object::hashcode) if you want a more lightweight approach that doesn't retain all the objects, but is more susceptible to falsely considering two elements as distinct due to a hashcode collision.

การจำแนกข้อมูลที่แตกต่างกัน

@Slf4j
public class FluxDistinctExample {
    
    public static void main(String[] args) {
        Flux.just("A", "B", "C", "A", "A", "B", "D")
                .distinct()
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => A  
- message => B  
- message => C  
- message => D

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Flux.sort

Sort elements from this Flux using a Comparator function, by collecting and sorting elements in the background then emitting the sorted sequence once this sequence completes.

Note that calling sort with long, non-terminating or infinite sources might cause OutOfMemoryError

สำหรับจัดเรียงข้อมูล

@Slf4j
public class FluxTodoExample {

    public static void main(String[] args) {
        Flux.just(2, 3, 5, 4, 1, 9, 7, 6, 8, 0)
                .doOnNext(message -> {
                    log.info("before sort => {}", message);
                })
                .sort((numb1, numb2) -> numb1 - numb2)
                .doOnNext(message -> {
                    log.info("sorted => {}", message);
                })
                .subscribe();
    }

}

output

- before sort => 2  
- before sort => 3  
- before sort => 5  
- before sort => 4  
- before sort => 1  
- before sort => 9  
- before sort => 7  
- before sort => 6  
- before sort => 8  
- before sort => 0  
- sorted => 0  
- sorted => 1  
- sorted => 2  
- sorted => 3  
- sorted => 4  
- sorted => 5  
- sorted => 6  
- sorted => 7   
- sorted => 8   
- sorted => 9

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Flux.zip

Zip two sources together, that is to say wait for all the sources to emit one element and combine these elements once into a Tuple2.
The operator will continue doing so until any of the sources completes.
Errors will immediately be forwarded.
This "Step-Merge" processing is especially useful in Scatter-Gather scenarios.

เป็นการผสาน/รวม ข้อมูลแต่ละกลุ่มหรือคู่ flux เข้าด้วยกัน

  • example 1
@Slf4j
public class FluxZipExample1 {
    
    public static void main(String[] args) {
        
        Flux<String> flux1 = Flux.just("1", "2", "3", "4", "5");
        Flux<String> flux2 = Flux.just("6", "7", "8");
        
        Flux.zip(flux1, flux2)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => [1,6]  
- message => [2,7]  
- message => [3,8]
  • example 2
@Slf4j
public class FluxZipExample2 {
    
    public static void main(String[] args) {
        
        Flux<String> flux1 = Flux.just("1", "2", "3", "4", "5");
        Flux<String> flux2 = Flux.just("6", "7", "8");
        Flux<String> flux3 = Flux.just("9", "10", "11", "12");
        
        Flux.zip(flux1, flux2, flux3)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => [1,6,9]  
- message => [2,7,10]  
- message => [3,8,11]
  • example 3 (ทดลองหน่วงเวลาแต่ละ Flux ไม่เท่ากัน)
@Slf4j
public class FluxZipExample3 {

    private static Flux<String> create(int start, int to, long delayMillsec) {
        return Flux.create(callback -> {
            for (int i = start; i <= to; i++) {
                try {
                    Thread.sleep(delayMillsec);
                } catch (InterruptedException ex) {
                   
                }
                callback.next("" + i);
            }
            callback.complete();

        });
    }

    public static void main(String[] args) {

        Flux<String> flux1 = create(1, 5, 1000L);
        Flux<String> flux2 = create(6, 8, 3000L);
        Flux<String> flux3 = create(9, 12, 2000L);

        Flux.zip(flux1, flux2, flux3)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => [1,6,9]  
- message => [2,7,10]  
- message => [3,8,11]
  • แสดงว่า .zip() ไม่ได้ขึ้นอยู่กับเวลา

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Flux.zipWith

Zip this Flux with another Publisher source, that is to say wait for both to emit one element and combine these elements once into a Tuple2.
The operator will continue doing so until any of the sources completes.
Errors will immediately be forwarded.
This "Step-Merge" processing is especially useful in Scatter-Gather scenarios.

เป็นการผสาน/รวม ข้อมูลแต่ละคู่ flux เข้าด้วยกัน คล้าย ๆ Flux.zip

@Slf4j
public class FluxZipWithExample {
    
    public static void main(String[] args) {
        Flux.just("1", "2", "3")
                .zipWith(Flux.just("4", "5", "6", "7", "8"))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => [1,4]  
- message => [2,5]  
- message => [3,6]

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Flux.concatWithValues

Concatenates the values to the end of the Flux

สำหรับเชื่อมต่อข้อมูลต่าง ๆ เข้าไปใน Flux

@Slf4j
public class FluxConcatWithValuesExample {
    
    public static void main(String[] args) {
        Flux.just("1", "2", "3")
                .concatWithValues("4", "5", "6")
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 1  
- message => 2  
- message => 3  
- message => 4  
- message => 5  
- message => 6

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Flux.groupBy

Divide this sequence into dynamically created Flux (or groups) for each unique key, as produced by the provided keyMapper Function. Note that groupBy works best with a low cardinality of groups, so chose your keyMapper function accordingly.

สำหรับจัดกลุ่มข้อมูลเข้าด้วยกัน

@Slf4j
public class FluxGroupByExample {

    public static void main(String[] args) {
        Flux.just("A", "A", "B", "C", "D", "B", "F", "C", "A")
                .groupBy(value -> value)
                .flatMap(flux -> flux.count().map(count -> flux.key() + ":" + count))
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }

}

output

- message => A:3  
- message => B:2  
- message => C:2  
- message => D:1  
- message => F:1

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Flux.hasElement

Emit a single boolean true if any of the elements of this Flux sequence is equal to the provided value.

The implementation uses short-circuit logic and completes with true if an element matches the value.

เช็คว่ามีข้อมูลนี้อยู่หรือไม่

  • example 1
@Slf4j
public class FluxHasElementExample1 {
    
    public static void main(String[] args) {
        Flux.just("A", "B", "C", "D", "F")
                .hasElement("A")
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => true
  • example 2
@Slf4j
public class FluxHasElementExample2 {
    
    public static void main(String[] args) {
        Flux.just("A", "B", "C", "D", "F")
                .hasElement("Z")
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => false

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Flux.flatMap

Transform the elements emitted by this Flux asynchronously into Publishers, then flatten these inner publishers into a single Flux through merging, which allow them to interleave.

There are three dimensions to this operator that can be compared with #flatMapSequential(Function) flatMapSequential and #concatMap(Function) concatMap:

  • Generation of inners and subscription: this operator is eagerly subscribing to its inners.
  • Ordering of the flattened values: this operator does not necessarily preserve original ordering, as inner element are flattened as they arrive.
  • Interleaving: this operator lets values from different inners interleave (similar to merging the inner sequences).

คล้าย ๆ map คือ ทำการแปลง (Transform) ข้อมูลก่อนส่งออกมา แต่เป็นแบบ Asyncronous

@Slf4j
public class FluxFlatMapExample {
    
    public static void main(String[] args) {
        Flux.just(1, 2, 3, 4, 5)
                .flatMap(number -> {
                    return Flux.create(callback -> {
                        try {
                            log.info("wait 3 seconds... at " + LocalDateTime.now());
                            Thread.sleep(3000);
                        } catch (InterruptedException ex) {
                            
                        }
                        callback.next(number * 10);
                        callback.complete();
                    });
                })
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- wait 3 seconds... at 2019-07-25T18:19:24.491  
- message => 10  
- wait 3 seconds... at 2019-07-25T18:19:27.495  
- message => 20  
- wait 3 seconds... at 2019-07-25T18:19:30.496  
- message => 30  
- wait 3 seconds... at 2019-07-25T18:19:33.498  
- message => 40  
- wait 3 seconds... at 2019-07-25T18:19:36.499  
- message => 50

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Flux.mergeWith

Merge data from this Flux and a Publisher into an interleaved merged sequence. Unlike #concatWith(Publisher) concat, inner sources are subscribed to eagerly.

ทำการ merge 2 Publisher (ได้ทั้ง Flux และ Mono) เข้าด้วยกัน

  • example 1
@Slf4j
public class FluxMergeWithExample1 {
    
    public static void main(String[] args) {
        Flux<String> flux1 = Flux.just("1", "2", "3");
        Flux<String> flux2 = Flux.just("A", "B", "C", "D");
        flux1.mergeWith(flux2)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 1  
- message => 2  
- message => 3  
- message => A  
- message => B  
- message => C  
- message => D
  • example 2
@Slf4j
public class FluxMergeWithExample2 {
    
    public static void main(String[] args) {
        Flux<String> flux = Flux.just("1", "2", "3");
        Mono<String> mono = Mono.just("A");
        flux.mergeWith(mono)
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => 1  
- message => 2  
- message => 3  
- message => A

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ข้อควรระวัง (Warning)

ข้อควรระวังในการใช้ Mono<Void>

หากมีการเรียก .map() หรือ .flatMap() หรือ .doOnNext() ต่อจาก Mono<Void> คำสั่งนั้น ๆ จะไม่ทำงาน
ให้ใช้ .then() หรือ .doOnSuccess() แทน

@Slf4j
public class MonoVoidWarningExample {

    private static Mono<Void> doSomething() {
        return Mono.fromRunnable(() -> {
            log.info("do something...");
        });
    }

    public static void main(String[] args) {
        doSomething()
                .flatMap(value -> {
                    log.info("flatMap :: value => {}", value);
                    return Mono.just(value);
                })
                .map(value -> {
                    log.info("map :: value => {}", value);
                    return value;
                })
                .doOnNext(value -> {
                    log.info("doOnNext :: value => {}", value);
                })
                .then(Mono.fromRunnable(() -> {
                    log.info("then do something ...");
                }))
                .doOnSuccess(value -> {
                    log.info("doOnSuccess :: value => {}", value);
                })
                .subscribe();
    }

}

output

- do something...  
- then do something ...  
- doOnSuccess :: value => null

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ข้อควรระวังในการใช้ .then()

ถ้างาน (task) ก่อนหน้าเกิด error หรือ exception
task ต่อ ๆ ไปจะไม่ทำงานต่อ

@Slf4j
public class MonoThenWarningExample {

    public static void main(String[] args) {
        Mono<String> task1 = Mono.fromCallable(() -> {
            log.info("do task 1");
            return "task 1";
        });
        Mono<String> task2 = Mono.fromCallable(() -> {
            log.info("do task 2");
            return "task 2";
        });
        Mono<String> task3 = Mono.defer(() -> {
            log.info("do task 3");
            //จะ Throw exception ออกไปเลย หรือ Mono.error(Exception) ก็ output แบบเดียวกัน 
            return Mono.error(new RuntimeException("something error"));
        });
        Mono<String> task4 = Mono.fromCallable(() -> {
            log.info("do task 4");
            return "task 4";
        });
        Mono<String> task5 = Mono.fromCallable(() -> {
            log.info("do task 5");
            return "task 5";
        });

        task1
                .doOnNext(value -> log.info("value => {}", value))
                .then(task2)
                .doOnNext(value -> log.info("value => {}", value))
                .then(task3)
                .doOnNext(value -> log.info("value => {}", value))
                .then(task4)
                .doOnNext(value -> log.info("value => {}", value))
                .then(task5)
                .subscribe();
    }

}

output

- do task 1  
- value => task 1  
- do task 2  
- value => task 2  
- do task 3  
java.lang.RuntimeException: something error

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ข้อควรระวังในการใช้ Flux.create

หลังจากที่ปล่อยข้อมูลด้วย .next() เสร็จแล้ว ให้เรียก .complete() ปิดท้ายด้วยเสมอ (ย้ำว่าเสมอ) **** เพื่อให้ unsubscribe และป้องกัน Memory Leak

@Slf4j
public class FluxCreateWarningExample {
    
    public static void main(String[] args) {
        Flux.create(callback -> {
            for (int i = 1; i <= 3; i++) {
                try {
                    Thread.sleep(1000L);
                } catch (InterruptedException ex) {
                   
                }
                callback.next("task " + i + " at " + LocalDateTime.now());
            }
            callback.complete();
            
        })
                .doOnNext(message -> {
                    log.info("message => {}", message);
                })
                .subscribe();
    }
    
}

output

- message => task 1 at 2019-07-31T12:22:54.985  
- message => task 2 at 2019-07-31T12:22:55.988  
- message => task 3 at 2019-07-31T12:22:56.989

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