与CompletableFuture有关的一些知识

Java1.5引入了Future,在1.8中又引入了CompletableFuture。他的出现可以使我们更好的去对任务进行编排,合理的使用会极大的缩减多任务的处理时间,达到事半功倍的目的。下面让我们一块来看一下与它相关的一些知识。

1 Java中的函数式编程

在看CompletableFuture了解一点Java中函数式编程相关的知识会更有用一些。

在Java1.8中引入了@FunctionalInterface,同时也在java.util.function包中引入了很多函数式接口。我们来看几个常见的:

1.1 Consumer

Consumer消费者,关联的方法为void accept(T t),有参无返回值。

@FunctionalInterface
public interface Consumer {

    void accept(T t);

1.2 Function

Function函数,关联方法为R apply(T t),有参有返回值。

@FunctionalInterface
public interface Function {

    R apply(T t);

1.3 Predicate

Predicate谓语(断言?),关联方法为boolean test(T t),有参有返回值(bool)。

@FunctionalInterface
public interface Predicate {

    boolean test(T t);

1.4 Supplier

Supplier供应者,关联方法为T get(),无参有返回值。

@FunctionalInterface
public interface Supplier {

    T get();
}

2 CompletableFuture

后续的内容主要关注点会放在其内部各种API上面。在后续的方法中,因为CompletableFuture实现了CompletionStage使其具有链式调用能力。大多数方法都会类似如下:

  • xxx(Function func): 会上文的同一线程去执行当前任务
  • xxxAsync(Function func): 会启用一个新线程去执行当前任务
  • xxxAsync(Function func, Executor executor): 会使用executor线程池中的新线程去执行当前任务

2.1 CompletableFuture的创建

  • supplyAsync: 以Supplier为参,有返回值。
public static  CompletableFuture supplyAsync(Supplier supplier)
public static  CompletableFuture supplyAsync(Supplier supplier, Executor executor)
  • runAsync: 以Runnable为参,返回值为Void
public static CompletableFuture runAsync(Runnable runnable)
public static CompletableFuture runAsync(Runnable runnable, Executor executor)
  • completedFuture: 创建一个已有值且完成的CompletableFuture。
public static  CompletableFuture completedFuture(U value)
  • failedFuture: 创建一个已出现异常且完成的CompletableFuture。
public static  CompletableFuture failedFuture(Throwable ex)

可以看到runAsyncsupplyAsync都提供了额外参数Executor,在未指定executor时,默认会使用ForkJoinPool.commonPool()来执行异步任务,而Parallel Stream默认情况下也会使用该线程池,共用的话可能会导致非核心业务抢占核心业务的执行。一般来说都会使用自定义线程池来执行这些任务。

2.2 CompletableFuture的后续操作

2.2.1 执行一个任务(1)
  • apply: 有参有返回值。会将前面的返回值作为当前函数的输入。
public  CompletableFuture thenApply(Function fn)
public  CompletableFuture thenApplyAsync(Function fn)
public  CompletableFuture thenApplyAsync(Function fn, Executor executor)
  • accept: 有参无返回值。会将前面的输出作为当前函数的输入。
public CompletableFuture thenAccept(Consumer action)
public CompletableFuture thenAcceptAsync(Consumer action)
public CompletableFuture thenAcceptAsync(Consumer action, Executor executor)
  • run: 编排执行另外一个毫无相关的任务
public CompletableFuture thenRun(Runnable action)
public CompletableFuture thenRunAsync(Runnable action)
public CompletableFuture thenRunAsync(Runnable action, Executor executor)

示例

@Test
void testApply() throws ExecutionException, InterruptedException {
    final CompletableFuture future = CompletableFuture
            .supplyAsync(() -> 1)
            .thenApply((val) -> val + 1);
    assertEquals(2, future.get());
}

@Test
void testAccept() {
    final CompletableFuture future = CompletableFuture
            .supplyAsync(() -> 1)
            .thenAccept((val) -> {
                assertEquals(1, val);
            });
    future.join();
}

@Test
void testRun() {
    final CompletableFuture future = CompletableFuture
            .supplyAsync(() -> 1)
            .thenRun(() -> {});
    future.join();
}
2.2.1 执行一个任务(2)
  • whenComplete: 当任务完成时执行该方法,将返回值及异常传入BiConsumer(有参无返回值)
public CompletableFuture whenComplete(BiConsumer action)
public CompletableFuture whenCompleteAsync(BiConsumer action)
public CompletableFuture whenCompleteAsync(BiConsumer action, Executor executor)
  • exceptionally: 当相应的任务出现异常时会调用该方法,将异常作为参数传进来
public CompletableFuture exceptionally(Function fn)
public CompletableFuture exceptionallyAsync(Function fn)
public CompletableFuture exceptionallyAsync(Function fn, Executor executor)
  • handler: 接收处理前面任务的结果BiFunction(有参有返回值)
public  CompletableFuture handle(BiFunction fn)
public  CompletableFuture handleAsync(BiFunction fn)
public  CompletableFuture handleAsync(BiFunction fn, Executor executor)
2.2.2 执行两个任务其中一个即可
  • applyToEither: 执行俩任务,返回其中一个未出现异常的,并将该输出作为参数传入后续Function(有参有返回值)
public  CompletableFuture applyToEither(CompletionStage other, Function fn)
public  CompletableFuture applyToEitherAsync(CompletionStage other, Function fn)
public  CompletableFuture applyToEitherAsync(CompletionStage other, Function fn, Executor executor)
  • acceptEither: 执行俩任务,返回其中一个未出现异常的,并将该输出作为参数传入后续Consumer(有参无返回值)
public CompletableFuture acceptEither(CompletionStage other, Consumer action) 
public CompletableFuture acceptEitherAsync(CompletionStage other, Consumer action)
public CompletableFuture acceptEitherAsync(CompletionStage other, Consumer action, Executor executor)
  • runAfterEither: 执行俩任务,返回其中一个未出现异常的,并将该输出作为参数传入后续Runnable(无参无返回值)
public CompletableFuture runAfterEither(CompletionStage other, Runnable action)
public CompletableFuture runAfterEitherAsync(CompletionStage other, Runnable action)
public CompletableFuture runAfterEitherAsync(CompletionStage other, Runnable action, Executor executor)
2.2.3 执行两个任务全部完成
  • thenCombine: 执行两个任务,并将这俩任务作为参数输入BiFunction(有参有返回值)
public  CompletableFuture thenCombine(CompletionStage other, BiFunction fn)
public  CompletableFuture thenCombineAsync(CompletionStage other, BiFunction fn)
public  CompletableFuture thenCombineAsync(CompletionStage other, BiFunction fn, Executor executor)
  • thenAcceptBoth: 执行两个任务,并将这俩任务作为参数输入BiConsumer(有参无返回值)
public  CompletableFuture thenAcceptBoth(CompletionStage other, BiConsumer action)
public  CompletableFuture thenAcceptBothAsync(CompletionStage other, BiConsumer action)
public  CompletableFuture thenAcceptBothAsync(CompletionStage other, BiConsumer action, Executor executor)
  • runAfterBoth: 执行两个任务,然后执行Runnable(无参无返回值)
public CompletableFuture runAfterBoth(CompletionStage other, Runnable action)
public CompletableFuture runAfterBothAsync(CompletionStage other, Runnable action)
public CompletableFuture runAfterBothAsync(CompletionStage other, Runnable action, Executor executor)
2.2.4 执行两个有依赖的任务
  • thenCompose: 前一个任务的结果,要给第二个任务作为参数
public  CompletableFuture thenCompose(Function> fn)
public  CompletableFuture thenComposeAsync(Function> fn)
public  CompletableFuture thenComposeAsync(Function> fn, Executor executor)
2.2.5 组合更多的任务
  • allOf: 等待所有任务都完成
public static CompletableFuture allOf(CompletableFuture... cfs)
  • anyOf: 等待完成任意一任务即可
public static CompletableFuture anyOf(CompletableFuture... cfs) 
 

2.3 结果获取

2.3.1 join

任务完成时获取数据,或者异常时抛出异常。不过要注意的是,它抛出的异常是(unchecked) exceptionCancellationExceptionCompletionException

public T join()
2.3.2 get

在获取数据时: get()等待任务完成然后读取结果; get(long timeout, TimeUnit unit)有限时间内等待完成读取结果,或抛出TimeoutException异常; getNow(T valueIfAbsent)立即获取结果,或者缺省值;

public T get() throws InterruptedException, ExecutionException
public T get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException 
public T getNow(T valueIfAbsent)

3 后记

今天主要熟悉了CompletableFuture的绝大部分API,只有在合适的场景去使用才能融会贯通。后面会结合实际场景来讲讲如何使用它来做任务编排。


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