Java 8

Reactor of Java 這一章來自於《Spring in Action, 5th》 的筆記，因為這本書講Reactor of Java講的太好了，所以作為筆記摘抄了下來。

Reactor of Java

In an imperative programming model, the code would look something like this:

String name = "Craig";String capitalName = name.toUpperCase();String greeting = "Hello, " + capitalName + "!";System.out.println(greeting);

In the imperative model, each line of code performs a step, one right after the other, and definitely in the same thread. Each step blocks the executing thread from moving to the next step until complete. In contrast, functional, reactive code could achieve the same thing like this:

Mono.just("Craig")        .map(n -> n.toUpperCase())        .map(n -> "Hello, " + n + " !")        .subscribe(System.out::println);

The Mono in the example is one of Reactor’s two core types. Flux is the other. Both are implementations of Reactive Streams’ Publisher.A Flux represents** a pipeline of zero, one, or many (potentially infinite) data items**.A Mono is a specialized reactive type that’s optimized for when the dataset is known to have no more than one data item.

CREATING FROM OBJECTS

Flux<String> fruitFlux = Flux        .just("Apple", "Orange", "Grape", "Banana", "Strawberry");        fruitFlux.subscribe(f -> System.out.println("Hello " + f));// for test        StepVerifier.create(fruitFlux)        .expectNext("Apple")        .expectNext("Orange")        .expectNext("Grape")        .expectNext("Banana")        .expectNext("Strawberry")        .verifyComplete();

CREATING FROM COLLECTIONS

Stream<String> fruitStream = Stream.of("Apple", "Orange", "Grape", "Banana", "Strawberry");        Flux<String> fruitFlux2 = Flux.fromStream(fruitStream);        fruitFlux2.subscribe(s -> System.out.println(s));        List<String> fruitList = new ArrayList<>();        fruitList.add("Apple");        fruitList.add("Orange");        fruitList.add("Grape");        fruitList.add("Banana");        fruitList.add("Strawberry");        Flux<String> fruitFlux3 = Flux.fromIterable(fruitList);        fruitFlux3.subscribe(s -> System.out.println(s));        String[] fruits = new String[] {"Apple", "Orange", "Grape", "Banana", "Strawberry" };        Flux<String> fruitFlux = Flux.fromArray(fruits);        fruitFlux.subscribe(s -> System.out.println(s));        StepVerifier.create(fruitFlux)        .expectNext("Apple")        .expectNext("Orange")        .expectNext("Grape")        .expectNext("Banana")        .expectNext("Strawberry")        .verifyComplete();

GENERATING FLUX DATA

Flux<Integer> intervalFlux =Flux.range(1, 5);intervalFlux.subscribe(integer -> System.out.println(integer));StepVerifier.create(intervalFlux).expectNext(1).expectNext(2).expectNext(3).expectNext(4).expectNext(5).verifyComplete();Flux<Long> intervalFlux =Flux.interval(Duration.ofSeconds(1)).take(5);intervalFlux.subscribe(i -> System.out.println(i));StepVerifier.create(intervalFlux).expectNext(0L).expectNext(1L).expectNext(2L).expectNext(3L).expectNext(4L).verifyComplete();

MERGING REACTIVE TYPES

Flux<String> characterFlux = Flux.just("Garfield", "Kojak", "Barbossa").delayElements(Duration.ofMillis(500));Flux<String> foodFlux = Flux.just("Lasagna", "Lollipops", "Apples").delaySubscription(Duration.ofMillis(250)).delayElements(Duration.ofMillis(500));Flux<String> mergedFlux = characterFlux.mergeWith(foodFlux);mergedFlux.subscribe(s -> System.out.println(s));StepVerifier.create(mergedFlux).expectNext("Garfield").expectNext("Lasagna").expectNext("Kojak").expectNext("Lollipops").expectNext("Barbossa").expectNext("Apples").verifyComplete();Flux<String> characterFlux = Flux.just("Garfield", "Kojak", "Barbossa");Flux<String> foodFlux = Flux.just("Lasagna", "Lollipops", "Apples");Flux<Tuple2<String, String>> zippedFlux =Flux.zip(characterFlux, foodFlux);zippedFlux.subscribe(x -> System.out.println(x));StepVerifier.create(zippedFlux).expectNextMatches(p ->p.getT1().equals("Garfield") &&p.getT2().equals("Lasagna")).expectNextMatches(p ->p.getT1().equals("Kojak") &&p.getT2().equals("Lollipops")).expectNextMatches(p ->p.getT1().equals("Barbossa") &&p.getT2().equals("Apples")).verifyComplete();Flux<String> characterFlux = Flux.just("Garfield", "Kojak", "Barbossa");Flux<String> foodFlux = Flux.just("Lasagna", "Lollipops", "Apples");Flux<String> zippedFlux =Flux.zip(characterFlux, foodFlux, (c, f) -> c + " eats " + f);zippedFlux.subscribe(x -> System.out.println(x));StepVerifier.create(zippedFlux).expectNext("Garfield eats Lasagna").expectNext("Kojak eats Lollipops").expectNext("Barbossa eats Apples").verifyComplete();

SELECTING THE FIRST REACTIVE TYPE TO PUBLISH

Flux<String> slowFlux = Flux.just("tortoise", "snail", "sloth").delaySubscription(Duration.ofMillis(100));Flux<String> fastFlux = Flux.just("hare", "cheetah", "squirrel");Flux<String> firstFlux = Flux.first(slowFlux, fastFlux);StepVerifier.create(firstFlux).expectNext("hare").expectNext("cheetah").expectNext("squirrel").verifyComplete();

FILTERING DATA FROM REACTIVE TYPES

Flux<String> skipFlux = Flux.just("one", "two", "skip a few", "ninety nine", "one hundred").skip(3);StepVerifier.create(skipFlux).expectNext("ninety nine", "one hundred").verifyComplete();Flux<String> skipFlux = Flux.just("one", "two", "skip a few", "ninety nine", "one hundred").delayElements(Duration.ofSeconds(1)).skip(Duration.ofSeconds(4));StepVerifier.create(skipFlux).expectNext("ninety nine", "one hundred").verifyComplete();Flux<String> nationalParkFlux = Flux.just("Yellowstone", "Yosemite", "Grand Canyon","Zion", "Grand Teton").take(3);StepVerifier.create(nationalParkFlux).expectNext("Yellowstone", "Yosemite", "Grand Canyon").verifyComplete();Flux<String> nationalParkFlux = Flux.just("Yellowstone", "Yosemite", "Grand Canyon","Zion", "Grand Teton").delayElements(Duration.ofSeconds(1)).take(Duration.ofMillis(3500));StepVerifier.create(nationalParkFlux).expectNext("Yellowstone", "Yosemite", "Grand Canyon").verifyComplete();Flux<String> nationalParkFlux = Flux.just("Yellowstone", "Yosemite", "Grand Canyon","Zion", "Grand Teton").filter(np -> !np.contains(" "));StepVerifier.create(nationalParkFlux).expectNext("Yellowstone", "Yosemite", "Zion").verifyComplete();Flux<String> animalFlux = Flux.just("dog", "cat", "bird", "dog", "bird", "anteater").distinct();StepVerifier.create(animalFlux).expectNext("dog", "cat", "bird", "anteater").verifyComplete();

MAPPING REACTIVE DATA

Flux<Player> playerFlux = Flux.just("Michael Jordan", "Scottie Pippen", "Steve Kerr").map(n -> {String[] split = n.split("\\s");return new Player(split[0], split[1]);});StepVerifier.create(playerFlux).expectNext(new Player("Michael", "Jordan")).expectNext(new Player("Scottie", "Pippen")).expectNext(new Player("Steve", "Kerr")).verifyComplete();Flux<Player> playerFlux = Flux.just("Michael Jordan", "Scottie Pippen", "Steve Kerr").flatMap(n -> Mono.just(n).map(p -> {String[] split = p.split("\\s");return new Player(split[0], split[1]);}).subscribeOn(Schedulers.parallel()));List<Player> playerList = Arrays.asList(new Player("Michael", "Jordan"),new Player("Scottie", "Pippen"),new Player("Steve", "Kerr"));StepVerifier.create(playerFlux).expectNextMatches(p -> playerList.contains(p)).expectNextMatches(p -> playerList.contains(p)).expectNextMatches(p -> playerList.contains(p)).verifyComplete();

BUFFERING DATA ON A REACTIVE STREAM

Flux<String> fruitFlux = Flux.just("apple", "orange", "banana", "kiwi", "strawberry");Flux<List<String>> bufferedFlux = fruitFlux.buffer(3);StepVerifier.create(bufferedFlux).expectNext(Arrays.asList("apple", "orange", "banana")).expectNext(Arrays.asList("kiwi", "strawberry")).verifyComplete();Buffering values from a reactive Flux into non-reactive List collections seems counterproductive. But when you combine buffer() with flatMap(), it enables each of the List collections to be processed in parallel:Flux.just("apple", "orange", "banana", "kiwi", "strawberry").buffer(3).flatMap(x ->Flux.fromIterable(x).map(y -> y.toUpperCase()).subscribeOn(Schedulers.parallel()).log()).subscribe();Flux<String> fruitFlux = Flux.just("apple", "orange", "banana", "kiwi", "strawberry");Mono<List<String>> fruitListMono = fruitFlux.collectList();StepVerifier.create(fruitListMono).expectNext(Arrays.asList("apple", "orange", "banana", "kiwi", "strawberry")).verifyComplete();Flux<String> animalFlux = Flux.just("aardvark", "elephant", "koala", "eagle", "kangaroo");Mono<Map<Character, String>> animalMapMono =animalFlux.collectMap(a -> a.charAt(0));StepVerifier.create(animalMapMono).expectNextMatches(map -> {returnmap.size() == 3 &&map.get('a').equals("aardvark") &&map.get('e').equals("eagle") &&map.get('k').equals("kangaroo");}).verifyComplete();Performing logic operations on reactive typesFlux<String> animalFlux = Flux.just("aardvark", "elephant", "koala", "eagle", "kangaroo");Mono<Boolean> hasAMono = animalFlux.all(a -> a.contains("a"));StepVerifier.create(hasAMono).expectNext(true).verifyComplete();Mono<Boolean> hasKMono = animalFlux.all(a -> a.contains("k"));StepVerifier.create(hasKMono).expectNext(false).verifyComplete();Flux<String> animalFlux = Flux.just("aardvark", "elephant", "koala", "eagle", "kangaroo");Mono<Boolean> hasAMono = animalFlux.any(a -> a.contains("a"));StepVerifier.create(hasAMono).expectNext(true).verifyComplete();Mono<Boolean> hasZMono = animalFlux.any(a -> a.contains("z"));StepVerifier.create(hasZMono).expectNext(false).verifyComplete();

Spring MVC change to Spring WebFlux

@GetMapping("/recent")public Iterable<Taco> recentTacos() {PageRequest page = PageRequest.of(0, 12, Sort.by("createdAt").descending());return tacoRepo.findAll(page).getContent();}@GetMapping("/recent")public Flux<Taco> recentTacos() {return Flux.fromIterable(tacoRepo.findAll()).take(12);}@PostMapping(consumes="application/json")@ResponseStatus(HttpStatus.CREATED)public Taco postTaco(@RequestBody Taco taco) {return tacoRepo.save(taco);}@PostMapping(consumes="application/json")@ResponseStatus(HttpStatus.CREATED)public Mono<Taco> postTaco(@RequestBody Mono<Taco> tacoMono) {return tacoRepo.saveAll(tacoMono).next();}public interface TacoRepositoryextends ReactiveCrudRepository<Taco, Long> {}@GetMapping("/{id}")public Taco tacoById(@PathVariable("id") Long id) {Optional<Taco> optTaco = tacoRepo.findById(id);if (optTaco.isPresent()) {return optTaco.get();}return null;}@GetMapping("/{id}")public Mono<Taco> tacoById(@PathVariable("id") Long id) {return tacoRepo.findById(id);}

WORKING WITH RXJAVA TYPES

@GetMapping("/recent")public Observable<Taco> recentTacos() {return tacoService.getRecentTacos();}@GetMapping("/{id}")public Single<Taco> tacoById(@PathVariable("id") Long id) {return tacoService.lookupTaco(id);}

Developing Reactive APIs

@Configurationpublic class RouterFunctionConfig {@Autowiredprivate TacoRepository tacoRepo;@Beanpublic RouterFunction<?> routerFunction() {return route(GET("/design/taco"), this::recents)Testing reactive controllers 279.andRoute(POST("/design"), this::postTaco);}public Mono<ServerResponse> recents(ServerRequest request) {return ServerResponse.ok().body(tacoRepo.findAll().take(12), Taco.class);}public Mono<ServerResponse> postTaco(ServerRequest request) {Mono<Taco> taco = request.bodyToMono(Taco.class);Mono<Taco> savedTaco = tacoRepo.save(taco);return ServerResponse.created(URI.create("http://localhost:8080/design/taco/" +savedTaco.getId())).body(savedTaco, Taco.class);}}

Test Reactive Rest APIs

// Test Get MethodTaco[] tacos = {testTaco(1L), testTaco(2L),testTaco(3L), testTaco(4L),testTaco(5L), testTaco(6L),testTaco(7L), testTaco(8L),testTaco(9L), testTaco(10L),testTaco(11L), testTaco(12L),testTaco(13L), testTaco(14L),testTaco(15L), testTaco(16L)};Flux<Taco> tacoFlux = Flux.just(tacos);TacoRepository tacoRepo = Mockito.mock(TacoRepository.class);when(tacoRepo.findAll()).thenReturn(tacoFlux);WebTestClient testClient = WebTestClient.bindToController(new DesignTacoController(tacoRepo)).build();testClient.get().uri("/design/recent").exchange().expectStatus().isOk().expectBody().jsonPath("$").isArray().jsonPath("$").isNotEmpty().jsonPath("$[0].id").isEqualTo(tacos[0].getId().toString()).jsonPath("$[0].name").isEqualTo("Taco 1").jsonPath("$[1].id").isEqualTo(tacos[1].getId().toString()).jsonPath("$[1].name").isEqualTo("Taco 2").jsonPath("$[11].id").isEqualTo(tacos[11].getId().toString()).jsonPath("$[11].name").isEqualTo("Taco 12").jsonPath("$[12]").doesNotExist().jsonPath("$[12]").doesNotExist();// Test POST MethodTacoRepository tacoRepo = Mockito.mock(TacoRepository.class);Mono<Taco> unsavedTacoMono = Mono.just(testTaco(null));Taco savedTaco = testTaco(null);savedTaco.setId(1L);Mono<Taco> savedTacoMono = Mono.just(savedTaco);when(tacoRepo.save(any())).thenReturn(savedTacoMono);WebTestClient testClient = WebTestClient.bindToController(new DesignTacoController(tacoRepo)).build();testClient.post().uri("/design").contentType(MediaType.APPLICATION_JSON).body(unsavedTacoMono, Taco.class).exchange().expectStatus().isCreated().expectBody(Taco.class).isEqualTo(savedTaco);// Testing with a live server@RunWith(SpringRunner.class)@SpringBootTest(webEnvironment=WebEnvironment.RANDOM_PORT)public class DesignTacoControllerWebTest {@Autowiredprivate WebTestClient testClient;@Testpublic void shouldReturnRecentTacos() throws IOException {testClient.get().uri("/design/recent").accept(MediaType.APPLICATION_JSON).exchange().expectStatus().isOk().expectBody().jsonPath("$[?(@.id == 'TACO1')].name").isEqualTo("Carnivore").jsonPath("$[?(@.id == 'TACO2')].name").isEqualTo("Bovine Bounty").jsonPath("$[?(@.id == 'TACO3')].name").isEqualTo("Veg-Out");}}

Consume Reactive APIs

Flux ingredients = WebClient.create().get().uri("http://localhost:8080/ingredients").retrieve().bodyToFlux(Ingredient.class);ingredients.subscribe(i -> { ...})Flux<Ingredient> ingredients = WebClient.create().get().uri("http://localhost:8080/ingredients").retrieve().bodyToFlux(Ingredient.class);ingredients.timeout(Duration.ofSeconds(1)).subscribe(i -> { ... },e -> {// handle timeout error})//Handing errorsingredientMono.subscribe(ingredient -> {// handle the ingredient data...},error-> {// deal with the error...});Mono<Ingredient> ingredientMono = webClient.get().uri("http://localhost:8080/ingredients/{id}", ingredientId).retrieve().onStatus(HttpStatus::is4xxClientError,response -> Mono.just(new UnknownIngredientException())).bodyToMono(Ingredient.class);

Java 9

jshell

無法用單個下劃線作為變數名稱

int _ = 3; // java9 or above , error

 String a = Objects.requireNonNullElse(m,"Bc"); // 若m不為null，則a = m，若m為null，則a = "Bc"

-cp, -classpath, --class-path(Java9新增)

Multi-Release JAR Files

--release--class-path instead of -classpath--version instead of -version--module-path option has a shortcut -p

更多，見jeps

Java8中，介面可以有靜態方法的預設實現，例：

public interface Test {    public static void print() {        System.out.println("interface print");    }    default void pout() {        System.out.println();    }}

Java9中，可以支援private的靜態方法實現。

public interface Test {    private static void print() {        System.out.println("interface print");    }    static void pout() {        print();    }}

Optional.ofNullable(date).orElseGet(() -> newDate()); // date為null，才會執行newDate()方法，否則不執行newDate()方法Optional.ofNullable(date).orElse(newDate()); // 無論date是否為null，都會執行newDate()方法

Java7中，可以使用try-with-Resources

try(Resouce res = ...) {    work with res}

res.close()會被自動執行

例：

try (var in = new Scanner(new FileInputStream("C:\\Users\\Young\\Desktop\\新建資料夾\\1.tx.txt"), StandardCharsets.UTF_8);             var out = new PrintWriter("C:\\Users\\Young\\Desktop\\新建資料夾\\out.txt", StandardCharsets.UTF_8)) {            while (in.hasNext()) {                out.println(in.next().toUpperCase());            }        }

in 和 out執行完畢後都會自動關閉資源

在Java9 中，你可以在try中預先宣告資源例：

  public static void printAll(String[] lines, PrintWriter out) {       try (out) { // effectively final variable           for (String line : lines) {               out.println(line);           } // out.close() called here       }   }

StackWalker用法示例

public class App {    /**     * Computes the factorial of a number     *     * @param n a non-negative integer     * @return n! = 1 * 2 * . . . * n     */    public static int factorial(int n) {        System.out.println("factorial(" + n + "):");        var walker = StackWalker.getInstance();        walker.forEach(System.out::println);        int r;        if (n <= 1) {            r = 1;        } else {            r = n * factorial(n - 1);        }        System.out.println("return " + r);        return r;    }    public static void main(String[] args) {        try (var in = new Scanner(System.in)) {            System.out.print("Enter n: ");            int n = in.nextInt();            factorial(n);        }    }}

Java 9 expands the use of the diamond syntax to situations where it was previously not accepted. For example , you can now use diamonds with anonymous subclasses.

ArrayList<String> list = new ArrayList<>(){            @Override            public String get(int index) {                return super.get(index).replaceAll(".","*");            }        };

Java 10

無需定義變數型別，透過var關鍵字+初始化的值，可以推測出變數型別

var a = 2; // a表示intvar b = "hello"; // b 表示Stringvar date = new java.util.Date();var obj = new Custome(); // 自定義物件

Java 11

String repeated = "Java".repeat(3); // 三個Java字串連線

JDK提供了jdeprscan 來檢查你的程式碼是否使用了deprecated的方法

專題Lambda Expression

Method ReferenceEquivalent Lambda ExpressionNotesseparator::equalsx -> separator.equals(x)This is a method expression with an object and an instance method. The lambda parameter is passed as the explicit parameter of the methodString::trimx -> x.trim()This is a method expression with a class and an instance method. The lambda parameter becomes the explicit parameter of the methodString::concat(x, y) -> x.concat(y)Again, we have an instance method, but this time, with an explicit parameter. As before, the first lambda parameter becomes the implicit parameter, and the remaining ones are passed to the methodInteger::valueOfx -> Integer::valueOf(x)This is a method expression with a static method. The lambda parameter is passed to the static methodInteger::sum(x, y) -> Integer::sum(x, y)This is another static method, but this time with two parameters. Both lambda parameters are passed to the static method. The Integer.sum method was specifically created to be used as a method reference. As a lmbda, you could just write (x, y)->x + yInteger::newx -> new Integer(x)This is a constructor reference. The lambda parameters are passed to the constructorInteger[]::newn -> new Integer[n]This is an array constructor reference. The lambda paramter is the array length

Functional InterfaceParameter TypesReturn TypesAbstract Method NameDescriptionOther MethodRunnablenonevoidrunRuns an action without arguments or return valueSuppliernoneTgetSupplies a value of type TConsumerTvoidacceptConsumes a value of type TandThenBiConsumer<T,U>T,UvoidacceptConsumes value of types T and UandThenFunction<T,R>TRapplyA function with argument of type Tcompose, andThen, identityBiFunction<T,U,R>T,URapplyA function with arguments of types T and UandThenUnaryOperatorTTapplyA unary operator on the type Tcompose, andThen, identityBinaryOperatorT,TTapplyA binary operator on the type TandThen, maxBy, minByPredicateTbooleantestA boolean-valued functionand, or, negate, isEqualBiPredicate<T,U>T,UbooleantestA boolean-valued function with two argumnetsand,or,negate

Functional interfaces for Primitive Types

p, q is int ,long double; P, Q is Int, Long, Double

Functional InterfaceParameter TypesReturn TypesAbstract Method NameBooleanSuppliernonebooleangetAsBooleanPSuppliernonepgetAsPPConsumerpvoidacceptObjPConsumerT,pvoidacceptPFunctionpTapplyPToQFunctionpqapplyAsQToPFunctionTpapplyAsPToPBiFunction<T,U>T,UpapplyAsPPUnaryOperatorppapplyAsPPBinaryOperatorp,ppapplyAsPPPredicatepbooleantest

Service LoadersProxiesLoggingGeneric ProgrammingE for the element type of a collectionK and V for key and value type of a tableT(and the neighboring letters U and S, if neccessary) for "any type at all"

Pair<String> a = new Pair<>("A", "B");Pair<Double> b = new Pair<>(1.1, 1.11);System.out.println(a.getClass() == b.getClass()); // TRUE

in Java8

public static <T> Pair<T> makePair(Supplier<T> constr) {    return new Pair<>(constr.get(), constr.get());}Pair<String> p = Pair.makePair(String:new);

In general, there is no relationship between Pair<S> and Pair<T>, no matter how S and T are related.

BUT

var managerBuiddies = new Pair<Manager>(ceo, cfo);Pair<? extends Employee> buddies = managerBuddies;

CollectionsConcurrencyStream

Java 8

// 流操作List<Integer> list = new ArrayList<>();list.add(1);list.add(2);list.parallelStream().filter(i -> i > 1).count();list.stream().filter(i -> i > 1).count();Stream<String> words = Stream.of(contents.split(","));// 建立流var limits = new BigInteger("1000");Stream<BigInteger> integerStream = Stream.iterate(BigInteger.ZERO, n -> n.compareTo(limits) < 0, n -> n.add(BigInteger.ONE));System.out.println(integerStream.count());

如果我們持有的iterable物件不是集合，那麼可以透過下面的呼叫將其轉換成一個流

StreamSupport.stream(iterable.spliterator(),false);

如果我們持有的是Iterator物件，並且希望得到一個由它的結果構成的流，那麼可以使用下面的語句

StreamSupport.stream(Spliterators.spliteratorUnknowSize(iterator, Spliterator.ORDERED),false);

至關重要的是，在執行流的操作時，我們並沒有修改流背後的集合。記住，流並沒有收集其資料，資料一直儲存在單獨的集合中

Optional

String result = optionalString.orElse(""); // The wrapped string , or "" if noneString result = optionalString.orElseGet(() -> System.getProperty("myapp.default"));String result = optionalString.orElseThrow(IllegalStateException::new);

消費Optinal值

optionalValue.ifPresent(v -> result.add(v));optionalValue.ifPresentOrElse(v -> System.out.println("Found" + v),()-> logger.warning("no match"));

管道化Optional

Optional<String> transformed = optionalString.filter(s -> s.length() >= 8).map(String::toUpperCase);

in Java9

// 如果optionalString的值存在，那麼result為optionalString，如果值不存在，那麼就會計算lambda表示式，並使用計算出來的結果Optional<String> transformed = optionalString.or(() -> alternatives.stream().findFirst());