基础篇：异步编程不会？我教你啊！CompletableFuture

前言

以前需要异步执行一个任务时，一般是用Thread或者线程池Executor去创建。如果需要返回值，则是调用Executor.submit获取Future。但是多个线程存在依赖组合，我们又能怎么办？可使用同步组件CountDownLatch、CyclicBarrier等；其实有简单的方法，就是用CompeletableFuture

• 线程任务的创建
• 线程任务的串行执行
• 线程任务的并行执行
• 处理任务结果和异常
• 多任务的简单组合
• 取消执行线程任务
• 任务结果的获取和完成与否判断

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1 创建异步线程任务

根据supplier创建CompletableFuture任务

//使用内置线程ForkJoinPool.commonPool()，根据supplier构建执行任务
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier)
//指定自定义线程，根据supplier构建执行任务
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor)

根据runnable创建CompletableFuture任务

//使用内置线程ForkJoinPool.commonPool()，根据runnable构建执行任务
public static CompletableFuture<Void> runAsync(Runnable runnable)
//指定自定义线程，根据runnable构建执行任务
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)

• 使用示例

ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<Void> rFuture = CompletableFuture
        .runAsync(() -> System.out.println("hello siting"), executor);
//supplyAsync的使用
CompletableFuture<String> future = CompletableFuture
        .supplyAsync(() -> {
            System.out.print("hello ");
            return "siting";
        }, executor);

//阻塞等待，runAsync 的future 无返回值，输出null
System.out.println(rFuture.join());
//阻塞等待
String name = future.join();
System.out.println(name);
executor.shutdown(); // 线程池需要关闭
--------输出结果--------
hello siting
null
hello siting

常量值作为CompletableFuture返回

//有时候是需要构建一个常量的CompletableFuture
public static <U> CompletableFuture<U> completedFuture(U value)

2 线程串行执行

任务完成则运行action，不关心上一个任务的结果，无返回值

public CompletableFuture<Void> thenRun(Runnable action)
public CompletableFuture<Void> thenRunAsync(Runnable action)
//action用指定线程池执行
public CompletableFuture<Void> thenRunAsync(Runnable action, Executor executor)

• 使用示例

CompletableFuture<Void> future = CompletableFuture
        .supplyAsync(() -> "hello siting", executor)
        .thenRunAsync(() -> System.out.println("OK"), executor);
executor.shutdown();
--------输出结果--------
OK

任务完成则运行action，依赖上一个任务的结果，无返回值

public CompletableFuture<Void> thenAccept(Consumer<? super T> action)
public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action)
//action用指定线程池执行
public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action, Executor executor)

• 使用示例

ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<Void> future = CompletableFuture
        .supplyAsync(() -> "hello siting", executor)
        .thenAcceptAsync(System.out::println, executor);
executor.shutdown();
--------输出结果--------
hello siting

任务完成则运行fn，依赖上一个任务的结果，有返回值

public <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn)
public <U> CompletableFuture<U> thenApplyAsync(Function<? super T,? extends U> fn)    
//fn用指定线程池执行
public <U> CompletableFuture<U> thenApplyAsync(Function<? super T,? extends U> fn, Executor executor)

• 使用示例

ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<String> future = CompletableFuture
        .supplyAsync(() -> "hello world", executor)
        .thenApplyAsync(data -> {
            System.out.println(data); return "OK";
        }, executor);
System.out.println(future.join());
executor.shutdown();
--------输出结果--------
hello world
OK

thenCompose - 任务完成则运行fn，依赖上一个任务的结果，有返回值

• 类似thenApply（区别是thenCompose的返回值是CompletionStage，thenApply则是返回 U），提供该方法为了和其他CompletableFuture任务更好地配套组合使用

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

• 使用示例

//第一个异步任务，常量任务
CompletableFuture<String> f = CompletableFuture.completedFuture("OK");
//第二个异步任务
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<String> future = CompletableFuture
        .supplyAsync(() -> "hello world", executor)
        .thenComposeAsync(data -> {
            System.out.println(data); return f; //使用第一个任务作为返回
        }, executor);
System.out.println(future.join());
executor.shutdown();
--------输出结果--------
hello world
OK

3 线程并行执行

两个CompletableFuture并行执行完，然后执行action，不依赖上两个任务的结果，无返回值

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

• 使用示例

//第一个异步任务，常量任务
CompletableFuture<String> first = CompletableFuture.completedFuture("hello world");
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<Void> future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // () -> System.out.println("OK") 是第三个任务
        .runAfterBothAsync(first, () -> System.out.println("OK"), executor);
executor.shutdown();
--------输出结果--------
OK

两个CompletableFuture并行执行完，然后执行action，依赖上两个任务的结果，无返回值

//调用方任务和other并行完成后执行action，action再依赖消费两个任务的结果，无返回值
public <U> CompletableFuture<Void> thenAcceptBoth(CompletionStage<? extends U> other,
        BiConsumer<? super T, ? super U> action)
//两个任务异步完成，fn再依赖消费两个任务的结果，无返回值，使用默认线程池
public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,
        BiConsumer<? super T, ? super U> action)  
//两个任务异步完成，fn（用指定线程池执行）再依赖消费两个任务的结果，无返回值                
public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,
        BiConsumer<? super T, ? super U> action, Executor executor) 

• 使用示例

//第一个异步任务，常量任务
CompletableFuture<String> first = CompletableFuture.completedFuture("hello world");
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<Void> future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // (w, s) -> System.out.println(s) 是第三个任务
        .thenAcceptBothAsync(first, (s, w) -> System.out.println(s), executor);
executor.shutdown();
--------输出结果--------
hello siting

两个CompletableFuture并行执行完，然后执行fn，依赖上两个任务的结果，有返回值

//调用方任务和other并行完成后，执行fn，fn再依赖消费两个任务的结果，有返回值
public <U,V> CompletableFuture<V> thenCombine(CompletionStage<? extends U> other, 
		BiFunction<? super T,? super U,? extends V> fn)
//两个任务异步完成，fn再依赖消费两个任务的结果，有返回值，使用默认线程池
public <U,V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other,
        BiFunction<? super T,? super U,? extends V> fn)   
//两个任务异步完成，fn（用指定线程池执行）再依赖消费两个任务的结果，有返回值        
public <U,V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other,
        BiFunction<? super T,? super U,? extends V> fn, Executor executor)         

• 使用示例

//第一个异步任务，常量任务
CompletableFuture<String> first = CompletableFuture.completedFuture("hello world");
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<String> future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // (w, s) -> System.out.println(s) 是第三个任务
        .thenCombineAsync(first, (s, w) -> {
            System.out.println(s);
            return "OK";
        }, executor);
System.out.println(future.join());
executor.shutdown();
--------输出结果--------
hello siting
OK

4 线程并行执行，谁先执行完则谁触发下一任务（二者选其最快）

上一个任务或者other任务完成, 运行action，不依赖前一任务的结果，无返回值

public CompletableFuture<Void> runAfterEither(CompletionStage<?> other, Runnable action)   
public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other, Runnable action)
//action用指定线程池执行
public CompletableFuture<Void> runAfterEitherAsync(CompletionStage<?> other,
		Runnable action, Executor executor)

• 使用示例

//第一个异步任务，休眠1秒，保证最晚执行晚
CompletableFuture<String> first = CompletableFuture.supplyAsync(()->{
    try{ Thread.sleep(1000); }catch (Exception e){}
    System.out.println("hello world");
    return "hello world";
});
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<Void> future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() ->{
            System.out.println("hello siting");
            return "hello siting";
        } , executor)
        //() ->  System.out.println("OK") 是第三个任务
        .runAfterEitherAsync(first, () ->  System.out.println("OK") , executor);
executor.shutdown();
--------输出结果--------
hello siting
OK

上一个任务或者other任务完成, 运行action，依赖最先完成任务的结果，无返回值

public CompletableFuture<Void> acceptEither(CompletionStage<? extends T> other,
		Consumer<? super T> action)
public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other,
		Consumer<? super T> action, Executor executor)       
//action用指定线程池执行
public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other,
		Consumer<? super T> action, Executor executor)     

• 使用示例

//第一个异步任务，休眠1秒，保证最晚执行晚
CompletableFuture<String> first = CompletableFuture.supplyAsync(()->{
    try{ Thread.sleep(1000);  }catch (Exception e){}
    return "hello world";
});
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<Void> future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // data ->  System.out.println(data) 是第三个任务
        .acceptEitherAsync(first, data ->  System.out.println(data) , executor);
executor.shutdown();
--------输出结果--------
hello siting        

上一个任务或者other任务完成, 运行fn，依赖最先完成任务的结果，有返回值

public <U> CompletableFuture<U> applyToEither(CompletionStage<? extends T> other,
		Function<? super T, U> fn) 
public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other,
		Function<? super T, U> fn)         
//fn用指定线程池执行
public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other,
		Function<? super T, U> fn, Executor executor)         

• 使用示例

//第一个异步任务，休眠1秒，保证最晚执行晚
CompletableFuture<String> first = CompletableFuture.supplyAsync(()->{
    try{ Thread.sleep(1000);  }catch (Exception e){}
    return "hello world";
});
ExecutorService executor = Executors.newSingleThreadExecutor();
CompletableFuture<String> future = CompletableFuture
        //第二个异步任务
        .supplyAsync(() -> "hello siting", executor)
        // data ->  System.out.println(data) 是第三个任务
        .applyToEitherAsync(first, data ->  {
            System.out.println(data);
            return "OK";
        } , executor);
System.out.println(future);
executor.shutdown();
--------输出结果--------
hello siting
OK

5 处理任务结果或者异常

exceptionally-处理异常

public CompletableFuture<T> exceptionally(Function<Throwable, ? extends T> fn)

• 如果之前的处理环节有异常问题，则会触发exceptionally的调用相当于 try...catch
• 使用示例

CompletableFuture<Integer> first = CompletableFuture
        .supplyAsync(() -> {
            if (true) {
                throw new RuntimeException("main error!");
            }
            return "hello world";
        })
        .thenApply(data -> 1)
        .exceptionally(e -> {
            e.printStackTrace(); // 异常捕捉处理，前面两个处理环节的日常都能捕获
            return 0;
        });

handle-任务完成或者异常时运行fn，返回值为fn的返回

• 相比exceptionally而言，即可处理上一环节的异常也可以处理其正常返回值

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

• 使用示例

CompletableFuture<Integer> first = CompletableFuture
        .supplyAsync(() -> {
            if (true) { throw new RuntimeException("main error!"); }
            return "hello world";
        })
        .thenApply(data -> 1)
        .handleAsync((data,e) -> {
            e.printStackTrace(); // 异常捕捉处理
            return data;
        });
System.out.println(first.join());
--------输出结果--------
java.util.concurrent.CompletionException: java.lang.RuntimeException: main error!
	... 5 more
null

whenComplete-任务完成或者异常时运行action，有返回值

• whenComplete与handle的区别在于，它不参与返回结果的处理，把它当成监听器即可
• 即使异常被处理，在CompletableFuture外层，异常也会再次复现
• 使用whenCompleteAsync时，返回结果则需要考虑多线程操作问题，毕竟会出现两个线程同时操作一个结果

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)        

• 使用示例

CompletableFuture<AtomicBoolean> first = CompletableFuture
        .supplyAsync(() -> {
            if (true) {  throw new RuntimeException("main error!"); }
            return "hello world";
        })
        .thenApply(data -> new AtomicBoolean(false))
        .whenCompleteAsync((data,e) -> {
            //异常捕捉处理, 但是异常还是会在外层复现
            System.out.println(e.getMessage());
        });
first.join();
--------输出结果--------
java.lang.RuntimeException: main error!
Exception in thread "main" java.util.concurrent.CompletionException: java.lang.RuntimeException: main error!
	... 5 more

6 多个任务的简单组合

public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs)
public static CompletableFuture<Object> anyOf(CompletableFuture<?>... cfs)

• 使用示例

 CompletableFuture<Void> future = CompletableFuture
        .allOf(CompletableFuture.completedFuture("A"),
                CompletableFuture.completedFuture("B"));
//全部任务都需要执行完
future.join();
CompletableFuture<Object> future2 = CompletableFuture
        .anyOf(CompletableFuture.completedFuture("C"),
                CompletableFuture.completedFuture("D"));
//其中一个任务行完即可
future2.join();

8 取消执行线程任务

// mayInterruptIfRunning 无影响；如果任务未完成,则返回异常
public boolean cancel(boolean mayInterruptIfRunning) 
//任务是否取消
public boolean isCancelled()

• 使用示例

CompletableFuture<Integer> future = CompletableFuture
        .supplyAsync(() -> {
            try { Thread.sleep(1000);  } catch (Exception e) { }
            return "hello world";
        })
        .thenApply(data -> 1);

System.out.println("任务取消前:" + future.isCancelled());
// 如果任务未完成,则返回异常,需要对使用exceptionally，handle 对结果处理
future.cancel(true);
System.out.println("任务取消后:" + future.isCancelled());
future = future.exceptionally(e -> {
    e.printStackTrace();
    return 0;
});
System.out.println(future.join());
--------输出结果--------
任务取消前:false
任务取消后:true
java.util.concurrent.CancellationException
	at java.util.concurrent.CompletableFuture.cancel(CompletableFuture.java:2276)
	at Test.main(Test.java:25)
0

9 任务的获取和完成与否判断

// 任务是否执行完成
public boolean isDone()
//阻塞等待 获取返回值
public T join()
// 阻塞等待 获取返回值,区别是get需要返回受检异常
public T get()
//等待阻塞一段时间，并获取返回值
public T get(long timeout, TimeUnit unit)
//未完成则返回指定value
public T getNow(T valueIfAbsent)
//未完成，使用value作为任务执行的结果，任务结束。需要future.get获取
public boolean complete(T value)
//未完成，则是异常调用,返回异常结果，任务结束
public boolean completeExceptionally(Throwable ex)
//判断任务是否因发生异常结束的
public boolean isCompletedExceptionally()
//强制地将返回值设置为value，无论该之前任务是否完成；类似complete
public void obtrudeValue(T value)
//强制地让异常抛出，异常返回，无论该之前任务是否完成；类似completeExceptionally
public void obtrudeException(Throwable ex) 

• 使用示例

CompletableFuture<Integer> future = CompletableFuture
        .supplyAsync(() -> {
            try { Thread.sleep(1000);  } catch (Exception e) { }
            return "hello world";
        })
        .thenApply(data -> 1);

System.out.println("任务完成前:" + future.isDone());
future.complete(10);
System.out.println("任务完成后:" + future.join());
--------输出结果--------
任务完成前:false
任务完成后:10

欢迎指正文中错误