java1.8新特性 CompletableFuture 使用案例详解

清疚 2022-10-29 01:50 190阅读 0赞

# CompletableFuture 方法详解 #

## 1、runAsync 和 supplyAsync方法 ##

CompletableFuture 提供了四个静态方法来创建一个异步操作。

public static CompletableFuture<Void> runAsync(Runnable runnable);
 public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor);
 public static CompletableFuture supplyAsync(Supplier supplier);
 public static CompletableFuture supplyAsync(Supplier supplier, Executor executor);

没有指定Executor的方法会使用ForkJoinPool.commonPool() 作为它的线程池执行异步代码。如果指定线程池，则使用指定的线程池运行。以下所有的方法都类同。

*  runAsync方法不支持返回值。
 *  supplyAsync可以支持返回值。

### 示例 ###

![20210207172841459.png][]

public static void main(String[] args) throws Exception {
 CompletableFuture<Long> future = CompletableFuture.supplyAsync(() -> {
 long start = System.currentTimeMillis();
 try {
 TimeUnit.SECONDS.sleep(1);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("运行结束");
 return System.currentTimeMillis() - start;
 });
 
 System.out.println("运行时间 => " + future.get());
 }

![20210207172829976.png][]

## 2、计算结果完成时的回调方法 ##

当CompletableFuture的计算结果完成，或者抛出异常的时候，可以执行特定的Action。主要是下面的方法：

public CompletableFuture<T> whenComplete(BiConsumer<? super T,? super Throwable> action);
 public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T,? super Throwable> action);
 public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T,? super Throwable> action, Executor executor);
 public CompletableFuture<T> exceptionally(Function<Throwable,? extends T> fn);

可以看到Action的类型是BiConsumer<? super T,? super Throwable>它可以处理正常的计算结果，或者异常情况。

whenComplete 和 whenCompleteAsync 的区别：  
whenComplete：是执行当前任务的线程执行继续执行 whenComplete 的任务。  
whenCompleteAsync：是执行把 whenCompleteAsync 这个任务继续提交给线程池来进行执行。

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
 try {
 TimeUnit.SECONDS.sleep(1);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 if (new Random().nextInt() % 2 >= 0) {
 int i = 12 / 0;
 }
 System.out.println("运行结束");
 });
 
 future.whenComplete((t, action) -> System.out.println("执行完成！"));
 future.exceptionally(t -> {
 System.out.println("执行失败！" + t.getMessage());
 return null;
 });
 
 TimeUnit.SECONDS.sleep(2);
 }

执行成功结果

![20210207172755368.png][]

异常结果

![20210207172810227.png][]

## 3、 thenApply 方法 ##

当一个线程依赖另一个线程时，可以使用 thenApply 方法来把这两个线程串行化。

public CompletableFuture thenApply(Function<? super T,? extends U> fn);
 public CompletableFuture thenApplyAsync(Function<? super T,? extends U> fn);
 public CompletableFuture thenApplyAsync(Function<? super T,? extends U> fn, Executor executor);

Function<? super T,? extends U> 
T：上一个任务返回结果的类型 
U：当前任务的返回值类型

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Long> future = CompletableFuture.supplyAsync(() -> {
 long result = new Random().nextInt(100);
 System.out.println("结果1 => " + result);
 return result;
 }).thenApply(t -> {
 long result = t * 5;
 System.out.println("结果2 => " + result);
 return result;
 });
 
 long result = future.get();
 System.out.println(result);
 }

![20210207172715625.png][]

第二个任务依赖第一个任务的结果。

## 4、 handle 方法 ##

handle 是执行任务完成时对结果的处理。  
handle 方法和 thenApply 方法处理方式基本一样。不同的是 handle 是在任务完成后再执行，还可以处理异常的任务。thenApply 只可以执行正常的任务，任务出现异常则不执行 thenApply 方法。

public CompletionStage handle(BiFunction<? super T, Throwable, ? extends U> fn);
 public CompletionStage handleAsync(BiFunction<? super T, Throwable, ? extends U> fn);
 public CompletionStage handleAsync(BiFunction<? super T, Throwable, ? extends U> fn,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
 int i = 10 / 0;
 return new Random().nextInt(10);
 }).handle((param, throwable) -> {
 int result = -1;
 if (throwable == null) {
 result = param * 2;
 } else {
 System.out.println(throwable.getMessage());
 }
 return result;
 });
 System.out.println(future.get());
 }

![20210207172648306.png][]

从示例中可以看出，在 handle 中可以根据任务是否有异常来进行做相应的后续处理操作。而 thenApply 方法，如果上个任务出现错误，则不会执行 thenApply 方法。

## 5、 thenAccept 消费处理结果 ##

接收任务的处理结果，并消费处理，无返回结果。

public CompletionStage<Void> thenAccept(Consumer<? super T> action);
 public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action);
 public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Void> future = CompletableFuture.supplyAsync(() -> 
 new Random().nextInt(10))
 .thenAccept(t -> System.out.println("结果为 => " + t));
 future.get();
 }

![20210207172549111.png][]

从示例代码中可以看出，该方法只是消费执行完成的任务，并可以根据上面的任务返回的结果进行处理。并没有后续的输错操作。

## 6、thenRun 方法 ##

跟 thenAccept 方法不一样的是，不关心任务的处理结果。只要上面的任务执行完成，就开始执行 thenAccept 。

public CompletionStage<Void> thenRun(Runnable action);
 public CompletionStage<Void> thenRunAsync(Runnable action);
 public CompletionStage<Void> thenRunAsync(Runnable action,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Void> future = CompletableFuture.supplyAsync(() ->
 new Random().nextInt(10))
 .thenRun(() -> System.out.println("thenRun ..."));
 future.get();
 }

![20210207172445303.png][]

该方法同 thenAccept 方法类似。不同的是上个任务处理完成后，并不会把计算的结果传给 thenRun 方法。只是处理玩任务后，执行 thenAccept 的后续操作。

## 7、thenCombine 合并任务 ##

thenCombine 会把 两个 CompletionStage 的任务都执行完成后，把两个任务的结果一块交给 thenCombine 来处理。

public <U,V> CompletionStage<V> thenCombine(CompletionStage<? extends U> other,BiFunction<? super T,? super U,? extends V> fn);
 public <U,V> CompletionStage<V> thenCombineAsync(CompletionStage<? extends U> other,BiFunction<? super T,? super U,? extends V> fn);
 public <U,V> CompletionStage<V> thenCombineAsync(CompletionStage<? extends U> other,BiFunction<? super T,? super U,? extends V> fn,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "hello");
 CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> "world");
 CompletableFuture<String> result = future1.thenCombine(future2, (t, u) -> t + " " + u);
 System.out.println(result.get());
 }

![20210207172430979.png][]

## 8、thenAcceptBoth ##

当两个CompletionStage都执行完成后，把结果一块交给thenAcceptBoth来进行消耗

public CompletionStage<Void> thenAcceptBoth(CompletionStage<? extends U> other,BiConsumer<? super T, ? super U> action);
 public CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,BiConsumer<? super T, ? super U> action);
 public CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,BiConsumer<? super T, ? super U> action, Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future1 => " + t);
 return t;
 });
 
 CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future2 => " + t);
 return t;
 });
 TimeUnit.SECONDS.sleep(3);
 future1.thenAcceptBoth(future2, (t, u) -> System.out.println("future1 => " + t + "; future2 => " + u + ";"));
 }

![20210207172407705.png][]

## 9、applyToEither 方法 ##

两个CompletionStage，谁执行返回的结果快，我就用那个CompletionStage的结果进行下一步的转化操作。

public CompletionStage applyToEither(CompletionStage<? extends T> other,Function<? super T, U> fn);
 public CompletionStage applyToEitherAsync(CompletionStage<? extends T> other,Function<? super T, U> fn);
 public CompletionStage applyToEitherAsync(CompletionStage<? extends T> other,Function<? super T, U> fn,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future1 => " + t);
 return t;
 });
 CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future2 => " + t);
 return t;
 });
 
 CompletableFuture<Integer> result = future1.applyToEither(future2, t -> {
 System.out.println(t);
 return t * 2;
 });
 
 System.out.println(result.get());
 }

![20210207172045172.png][]

## 10、acceptEither 方法 ##

两个CompletionStage，谁执行返回的结果快，我就用那个CompletionStage的结果进行下一步的消耗操作。

public CompletionStage<Void> acceptEither(CompletionStage<? extends T> other,Consumer<? super T> action);
 public CompletionStage<Void> acceptEitherAsync(CompletionStage<? extends T> other,Consumer<? super T> action);
 public CompletionStage<Void> acceptEitherAsync(CompletionStage<? extends T> other,Consumer<? super T> action,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future1 => " + t);
 return t;
 });
 
 CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future2 => " + t);
 return t;
 });
 future1.acceptEither(future2, System.out::println);
 }

![20210207172023781.png][]

## 11、runAfterEither 方法 ##

两个CompletionStage，任何一个完成了都会执行下一步的操作（Runnable）

public CompletionStage<Void> runAfterEither(CompletionStage<?> other,Runnable action);
 public CompletionStage<Void> runAfterEitherAsync(CompletionStage<?> other,Runnable action);
 public CompletionStage<Void> runAfterEitherAsync(CompletionStage<?> other,Runnable action,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future1 => " + t);
 return t;
 });
 
 CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future2 => " + t);
 return t;
 });
 future1.runAfterEither(future2, () -> System.out.println("任务有一方已经完成"));
 }

![20210207172003258.png][]

## 12、runAfterBoth ##

两个CompletionStage，都完成了计算才会执行下一步的操作（Runnable）

public CompletionStage<Void> runAfterBoth(CompletionStage<?> other,Runnable action);
 public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,Runnable action);
 public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,Runnable action,Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future1 => " + t);
 return t;
 });
 
 CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("future2 => " + t);
 return t;
 });
 future1.runAfterBoth(future2, () -> System.out.println("任务已经全部完成"));
 }

![20210207171947438.png][]

## 13、thenCompose 方法 ##

thenCompose 方法允许你对两个 CompletionStage 进行流水线操作，第一个操作完成时，将其结果作为参数传递给第二个操作。

public CompletableFuture thenCompose(Function<? super T, ? extends CompletionStage> fn);
 public CompletableFuture thenComposeAsync(Function<? super T, ? extends CompletionStage> fn);
 public CompletableFuture thenComposeAsync(Function<? super T, ? extends CompletionStage> fn, Executor executor);

### 示例 ###

public static void main(String[] args) throws Exception {
 CompletableFuture<Integer> f = CompletableFuture.supplyAsync(() -> {
 int t = new Random().nextInt(3);
 System.out.println("t1 => " + t);
 return t;
 }).thenCompose(param -> CompletableFuture.supplyAsync(() -> {
 int t = param * 2;
 System.out.println("t2 => " + t);
 return t;
 }));
 System.out.println("thenCompose 结果 : " + f.get());
 }

![20210207171827673.png][]

## 14、cancel 与 isDone 方法 ##

cancel 终止方法运行 会抛出 CancellationException 完成此CompletableFuture。尚未完成的从属CompletableFuture也将异常完成。

isDone 如果以任何方式（通常，异常或通过取消）完成，则返回 true。

public boolean cancel(boolean mayInterruptIfRunning);
    public boolean isDone();

### 示例 ###

private static void cancel() throws Exception {
 CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
 try {
 TimeUnit.SECONDS.sleep(10);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("运行结束");
 });
 
 future.whenComplete((t, action) -> System.out.println("执行完成！"));
 future.exceptionally(t -> {
 t.printStackTrace();
 System.out.println("执行失败！" + t.getMessage());
 return null;
 });
 
 TimeUnit.SECONDS.sleep(2);
 
 future.cancel(true);
 
 System.out.println(future.isDone());
 }

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