JUC并发—14.Future模式和异步编程分析

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1.FutureTask(Future/Callable)的使用例子

2.FutureTask(Future/Callable)的实现原理

3.FutureTask(Future/Callable)的源码分析

4.CompletableFuture的基本介绍

5.CompletionStage方法及作用说明

6.CompletableFuture的实现原理分析

7.CompletableFuture的核心源码分析

1.FutureTask(Future/Callable)的使用例子

Future/Callable实现了一个异步执行并带有返回结果的功能。Future表示获取一个异步执行的结果，Callable表示一个异步执行的任务，Callable会产生一个结果并给到Future。

Future/Callable的使用例子如下：

public class FutureCallableExample {     static class CalculationCallable implements Callable<Integer> {         private int x;         private int y;          public CalculationCallable(int x, int y) {             this.x = x;             this.y = y;         }          @Override         public Integer call() throws Exception {             System.out.println("开始执行：" + new Date());             TimeUnit.SECONDS.sleep(2);//模拟任务执行的耗时             return x + y;         }     }      public static void main(String[] args) throws ExecutionException, InterruptedException {         CalculationCallable calculationCallable = new CalculationCallable(1, 2);         FutureTask<Integer> futureTask = new FutureTask<>(calculationCallable);         new Thread(futureTask).start();         System.out.println("开始执行futureTask：" + new Date());         Integer rs = futureTask.get();         System.out.println("执行结果：" + rs);         System.out.println("结束执行futureTask：" + new Date());     } }

首先定义一个CalculationCallable类。该类实现了Callable接口，并重写了call()方法，它的功能就是定义一个具有返回值的任务。

然后用FutureTask声明一个带有返回值的任务，把CalculationCallable作为构造参数传递进去。

FutureTask实现了Future接口和Runnable接口。我们知道线程执行完之后是不可能获得一个返回值的。Future之所以能够获得返回值，是因为在线程执行中做了相关处理。FutureTask就是用来获得线程执行结果的。

接着把FutureTask作为一个任务传入Thread的构造方法，让线程去执行。FutureTask既然实现了Runnable接口，创建FutureTask时又把实现了Callable接口的任务传递到其构造方法中，那么FutureTask的run()方法中会调用Callable接口的call()方法的实现，最终在获得返回值之后保存到某个属性中。

最后使用FutureTask.get()方法来获得返回值，这个get()方法是个阻塞方法。当线程还没有执行完FutureTask之前，主线程会阻塞在get()方法中。直到FutureTask执行结束，主线程才会被唤醒。

2.FutureTask(Future/Callable)的实现原理

(1)FutureTask的类关系

(2)FutureTask的实现核心

(1)FutureTask的类关系

Runnable接口的实现可以被线程执行，Future接口提供了获取线程执行结果的方法。RunnableFuture接口同时继承了Runnable接口和Future接口，而FutureTask类则实现了RunnableFuture接口。

创建FutureTask类实例时，会传入Callable接口的实现类实例作为构造参数，也就是FutureTask类会封装Callable接口的实现类。这样在启动线程后执行FutureTask类重写Runnable接口的run()方法时，FutureTask类实例就会把执行Callable接口call()方法的运行结果保存起来，然后通过Future接口提供的get()方法来获取运行结果。

一.FutureTask的类关系源码

//A cancellable asynchronous computation. //This class provides a base implementation of Future, with methods to start and cancel a computation,  //query to see if the computation is complete, and retrieve the result of the computation. //The result can only be retrieved when the computation has completed;  //the get methods will block if the computation has not yet completed. //Once the computation has completed, the computation cannot be restarted or cancelled  //(unless the computation is invoked using #runAndReset).  //A FutureTask can be used to wrap a Callable or Runnable object.   //Because FutureTask implements Runnable,  //a FutureTask can be submitted to an Executor for execution.  //In addition to serving as a standalone class,  //this class provides protected functionality that may be useful when creating customized task classes. public class FutureTask<V> implements RunnableFuture<V> {     ...     //Creates a FutureTask that will, upon running, execute the given Callable.     public FutureTask(Callable<V> callable) {         if (callable == null) {             throw new NullPointerException();         }         this.callable = callable;         this.state = NEW;//ensure visibility of callable     }     ... }  //A Future that is Runnable. //Successful execution of the run method causes completion of the Future and allows access to its results. public interface RunnableFuture<V> extends Runnable, Future<V> {     //Sets this Future to the result of its computation unless it has been cancelled.     void run(); }  //A Future represents the result of an asynchronous computation. //Methods are provided to check if the computation is complete,  //to wait for its completion, and to retrieve the result of the computation.   //The result can only be retrieved using method get when the computation has completed,  //blocking if necessary until it is ready. //Cancellation is performed by the cancel method.   //Additional methods are provided to determine if the task completed normally or was cancelled.  //Once a computation has completed, the computation cannot be cancelled. //If you would like to use a Future for the sake of cancellability but not provide a usable result,  //you can declare types of the form Future<?> and return null as a result of the underlying task. public interface Future<V> {     //用来取消任务，取消成功则返回true，取消失败则返回false     //mayInterruptIfRunning参数表示是否允许取消正在执行却没有执行完毕的任务，设为true，则表示可以取消正在执行过程中的任务     //如果任务已完成，则无论mayInterruptIfRunning为true还是false，此方法都返回false，即如果取消已经完成的任务会返回false     //如果任务正在执行，若mayInterruptIfRunning设置为true，则返回true，若mayInterruptIfRunning设置为false，则返回false     //如果任务还没有执行，则无论mayInterruptIfRunning为true还是false，肯定返回true     boolean cancel(boolean mayInterruptIfRunning);      //表示任务是否被取消成功，如果在任务正常完成前被取消成功，则返回true     boolean isCancelled();      //表示任务是否已经完成，若任务完成，则返回true     boolean isDone();      //获取执行结果，如果最终结果还没得出该方法会产生阻塞，直到任务执行完毕返回结果     V get() throws InterruptedException, ExecutionException;      //获取执行结果，如果在指定时间内，还没获取到结果，则抛出TimeoutException     V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException; }  @FunctionalInterface public interface Runnable {     public abstract void run(); }

二.FutureTask的类关系图

(2)FutureTask的实现核心

一.FutureTask本身是一个线程

通过new Thread(new FutureTask(callable)).start()来启动，必然会执行FutureTask实现Runnable接口的run()方法。

二.Runnable接口的run()方法是没有返回值的

实际上返回值是由Callable接口的call()方法提供，所以调用FutureTask的run()方法，会触发调用Callable的call()方法。

三.通过Future接口的get()方法阻塞式获得返回值

如果在FutureTask的run()方法中调用Callable接口的call()方法执行任务时，需要比较长的时间，那么为了能够正确获得返回值，Future接口的get()方法必须实现阻塞，直到call()方法执行完毕。

四.需要一个队列来保存阻塞的线程

涉及线程阻塞和唤醒，要使用LockSupport来阻塞和唤醒队列中的线程。

3.FutureTask(Future/Callable)的源码分析

(1)FutureTask的核心属性

(2)FutureTask的run()方法

(3)FutureTask的get()方法

(4)FutureTask的finishCompletion()方法

(5)FutureTask的实现原理总结

(1)FutureTask的核心属性

一.state

代表任务在运行过程中的状态(7种)。

二.callable

当前要执行的任务。

三.outcome

任务的执行结果，通过Future.get()获取的值。

四.runner

当前执行callable任务的线程。

五.waiter

用来保存所有等待任务执行结束的线程的单向链表。

public class FutureTask<V> implements RunnableFuture<V> {     //The run state of this task, initially NEW.      //The run state transitions to a terminal state only in methods set, setException, and cancel.     //During completion, state may take on transient values of COMPLETING (while outcome is being set)      //or INTERRUPTING (only while interrupting the runner to satisfy a cancel(true)).      //Transitions from these intermediate to final states use cheaper ordered/lazy writes      //because values are unique and cannot be further modified.     //Possible state transitions:     //NEW(初始状态) -> COMPLETING(正在设置任务结果) -> NORMAL，这是任务正常执行完毕时状态的变更流程     //NEW(初始状态) -> COMPLETING(正在设置任务结果) -> EXCEPTIONAL，这是任务执行异常时状态的变更流程     //NEW(初始状态) -> CANCELLED(任务被取消)，这是调用了Future.cancel()方法     //NEW(初始状态) -> INTERRUPTING(正在中断执行任务的线程) -> INTERRUPTED(任务被中断)     //代表任务在运行过程中的状态(7种)     private volatile int state;     private static final int NEW          = 0;     private static final int COMPLETING   = 1;     private static final int NORMAL       = 2;     private static final int EXCEPTIONAL  = 3;     private static final int CANCELLED    = 4;     private static final int INTERRUPTING = 5;     private static final int INTERRUPTED  = 6;      //The underlying callable; nulled out after running     //当前要执行的任务     private Callable<V> callable;          //The result to return or exception to throw from get()     //任务的执行结果，通过Future.get()获取的值     private Object outcome;          //The thread running the callable; CASed during run()     //当前执行callable任务的线程     private volatile Thread runner;          //Treiber stack of waiting threads     //用来保存所有等待任务执行结束的线程的单向链表     private volatile WaitNode waiters;     ... }

(2)FutureTask的run()方法

使用线程来执行FutureTask任务时，比如new Thread(new FutureTask(callable)).start()，会回调FutureTask的run()方法。

FutureTask的run()方法的执行流程如下：

首先判断当前状态是否为NEW，并使用CAS设置runner属性为当前线程。如果当前状态不是NEW或者CAS设置失败，则说明已经有其他线程正在执行当前任务了，于是直接返回。然后获取通过构造方法传入的Callable接口的实现类实例callable，接着调用Callable接口的实现类实例callable中的call()方法获得执行结果，最后调用FutureTask的set()方法把执行结果保存到outcome属性中。

public class FutureTask<V> implements RunnableFuture<V> {     ...     //代表任务在运行过程中的状态(7种)     private volatile int state;          //当前要执行的任务     private Callable<V> callable;          //任务的执行结果，通过Future.get()获取的值     private Object outcome;          //当前执行callable任务的线程     private volatile Thread runner;          //用来保存所有等待任务执行结束的线程的单向链表     private volatile WaitNode waiters;          private static final sun.misc.Unsafe UNSAFE;     private static final long stateOffset;     private static final long runnerOffset;     private static final long waitersOffset;     static {         try {             UNSAFE = sun.misc.Unsafe.getUnsafe();             Class<?> k = FutureTask.class;             stateOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("state"));             runnerOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("runner"));             waitersOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("waiters"));         } catch (Exception e) {             throw new Error(e);         }     }          public void run() {         //首先判断当前状态是否为NEW，并使用CAS把runner属性设置为当前线程         //如果当前状态不是NEW或者CAS设置失败，说明已经有其他线程正在执行当前任务了，于是直接返回         if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread())) {              return;         }         try {             //获取通过构造方法传入的Callable接口的实现类实例callable             Callable<V> c = callable;             if (c != null && state == NEW) {                 V result;                 boolean ran;                 try {                     //然后调用callable中的call()方法获得执行结果                     result = c.call();                     ran = true;                 } catch (Throwable ex) {                     result = null;                     ran = false;                     setException(ex);                 }                 if (ran) {                     //调用set()方法把执行结果保存到outcome属性中                     set(result);                 }             }         } finally {             //runner must be non-null until state is settled to prevent concurrent calls to run()             runner = null;             //state must be re-read after nulling runner to prevent leaked interrupts             int s = state;             if (s >= INTERRUPTING) {                 handlePossibleCancellationInterrupt(s);             }         }     }          //Sets the result of this future to the given value unless this future has already been set or has been cancelled.     //This method is invoked internally by the run method upon successful completion of the computation.     protected void set(V v) {         //CAS修改任务状态为COMPLETING         if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {             //把调用call()方法获取到的结果保存到outcome             outcome = v;             //CAS修改任务状态为NORMAL             UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state             finishCompletion();         }     }     ... }

(3)FutureTask的get()方法

FutureTask的get()方法的逻辑很简单，如果当前状态不是COMPLETING，就调用awaitDone()方法让当前线程阻塞等待，直到任务执行完成。其中awaitDone()方法的返回值表示任务的状态，当任务进入终止状态后，会调用reports()方法，根据状态类型来决定是返回运行结果还是抛异常。

在FutureTask的awaitDone()方法中会进行自旋。首先如果检测到线程被中断，则把加入等待队列中的线程移除。然后如果发现任务已经进入终止状态，则直接返回任务状态。如果任务正在设置执行结果，则通过Thread.yield()让出当前线程的CPU资源。

当FutureTask.awaitDone()方法第一次调用时，在第一次for循环中会初始化一个WaitNode结点，这个WaitNode结点便保存了调用FutureTask.get()方法的线程。在第二次for循环中会通过CAS按头插法将WaitNode结点插入waiters链表。在之后的for循环中，也就是当前线程已经加入了等待队列后，如果发现任务还没有执行完成，则通过LockSupport的方法阻塞线程。

注意，被阻塞的线程在如下两种情况下会被唤醒：

一.任务执行完成后，在set()方法中调用finishCompletion()方法

二.线程被中断，在awaitDone()方法中执行中断检测if(Thread.interrupted())

public class FutureTask<V> implements RunnableFuture<V> {     ...     //代表任务在运行过程中的状态(7种)     private volatile int state;     //当前要执行的任务     private Callable<V> callable;     //任务的执行结果，通过Future.get()获取的值     private Object outcome;     //当前执行callable任务的线程     private volatile Thread runner;     //用来保存所有等待任务执行结束的线程的单向链表     private volatile WaitNode waiters;          public V get() throws InterruptedException, ExecutionException {         int s = state;         if (s <= COMPLETING) {             s = awaitDone(false, 0L);         }         return report(s);     }          //Awaits completion or aborts on interrupt or timeout.     //@param timed true if use timed waits     //@param nanos time to wait, if timed     //@return state upon completion     private int awaitDone(boolean timed, long nanos) throws InterruptedException {         //阻塞超时时间，timed表示是否传递阻塞时间的参数         final long deadline = timed ? System.nanoTime() + nanos : 0L;         WaitNode q = null;         boolean queued = false;         for (;;) {//自旋             //如果检测到线程被中断，则把加入等待队列中的线程移除             if (Thread.interrupted()) {                 removeWaiter(q);                 //抛出中断异常                 throw new InterruptedException();             }             int s = state;             //如果任务已经进入终止状态，则直接返回任务状态             if (s > COMPLETING) {                 if (q != null) {                     q.thread = null;                 }                 return s;             } else if (s == COMPLETING) {// cannot time out yet                 //如果任务正在设置执行结果，则通过Thread.yield()让出当前线程的CPU资源                 Thread.yield();             } else if (q == null) {                 //awaitDone()方法被第一次调用时，q == null为true                 //此时会初始化一个WaitNode结点并赋值给q，这个WaitNode结点保存了调用FutureTask.get()的线程                  q = new WaitNode();             } else if (!queued) {                 //awaitDone()方法被第一次调用时，进入的第二次for循环                 //便会通过CAS将q结点按头插法插入waiters单向链表中                 queued = UNSAFE.compareAndSwapObject(this, waitersOffset, q.next = waiters, q);             } else if (timed) {                 //如果当前线程加入等待队列后，任务还没有执行完成，则通过LockSupport的方法阻塞线程                 nanos = deadline - System.nanoTime();                 if (nanos <= 0L) {                     removeWaiter(q);                     return state;                 }                 LockSupport.parkNanos(this, nanos);             } else {                 LockSupport.park(this);             }         }     }          //Returns result or throws exception for completed task.     //@param s completed state value     @SuppressWarnings("unchecked")     private V report(int s) throws ExecutionException {         Object x = outcome;         if (s == NORMAL) {             return (V)x;         }         if (s >= CANCELLED) {             throw new CancellationException();         }         throw new ExecutionException((Throwable)x);     }          static final class WaitNode {         volatile Thread thread;         volatile WaitNode next;         WaitNode() {             thread = Thread.currentThread();         }     }          private void removeWaiter(WaitNode node) {         if (node != null) {             node.thread = null;             retry:             for (;;) {//restart on removeWaiter race                 for (WaitNode pred = null, q = waiters, s; q != null; q = s) {                     s = q.next;                     if (q.thread != null) {                         pred = q;                     } else if (pred != null) {                         pred.next = s;                         if (pred.thread == null) {// check for race                             continue retry;                         }                     } else if (!UNSAFE.compareAndSwapObject(this, waitersOffset, q, s)) {                         continue retry;                     }                 }                 break;             }         }     }     ... }

(4)FutureTask的finishCompletion()方法

当Callable任务执行完成后，FutureTask的set()方法会调用finishCompletion()方法唤醒链表中的阻塞线程。

public class FutureTask<V> implements RunnableFuture<V> {     ...     //代表任务在运行过程中的状态(7种)     private volatile int state;     //当前要执行的任务     private Callable<V> callable;     //任务的执行结果，通过Future.get()获取的值     private Object outcome;     //当前执行callable任务的线程     private volatile Thread runner;     //用来保存所有等待任务执行结束的线程的单向链表     private volatile WaitNode waiters;          ...     //Removes and signals all waiting threads, invokes done(), and nulls out callable.     private void finishCompletion() {         for (WaitNode q; (q = waiters) != null;) {             if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {                 for (;;) {                     Thread t = q.thread;                     if (t != null) {                         q.thread = null;                         LockSupport.unpark(t);                     }                     WaitNode next = q.next;                     if (next == null) {                         break;                     }                     q.next = null; // unlink to help gc                     q = next;                 }                 break;             }         }         done();         callable = null;     }     ... }

(5)FutureTask的实现原理总结

FutureTask实现了Runnable和Future接口，FutureTask表示一个带有状态及执行结果的任务，而任务执行结果的获取是基于阻塞的方式来实现的。

在Callable接口的call()方法没有返回结果之前，其他线程调用FutureTask的get()方法获取结果时，FutureTask会构建一个waiters链表，把当前线程存储到链表中并通过LockSupport进行阻塞，直到call()方法返回后把结果设置到outcome属性以及唤醒阻塞的线程。

(6)FutureTask的局限性

局限性一：

在获取异步任务的执行结果时，要么调用get()方法阻塞等待返回结果，要么耗费CPU资源通过轮询调用FutureTask.isDone()方法来判断任务的执行状态，然后再调用get()方法获取返回结果。

局限性二：

FutureTask没有提供通知机制，没有办法知道任务什么时候执行完成。

4.CompletableFuture的基本介绍

(1)CompletableFuture的介绍

(2)CompletableFuture的类关系图

(3)CompletableFuture的方法说明

(1)CompletableFuture的介绍

CompletableFuture针对Future做了改进，也就是在异步任务执行完成后，主线程如果需要依赖该任务的执行结果来继续后面的操作，则可以不用通过等待来实现，只需向CompletableFuture传入一个回调对象。当异步任务执行完毕后，便会自动调用该回调对象(异步回调通知功能)。

CompletableFuture还提供了非常强大的功能。对于回调对象的执行，可以放到非任务线程中，也可以放到任务线程中。CompletableFuture提供了函数式编程能力，简化了异步编程的复杂性。还提供了多个CompletableFuture的组合与转化功能。

(2)CompletableFuture的类关系

CompletableFuture类实现了Future和CompletionStage这两个接口，其中Future接口提供了获取任务执行结果及任务执行状态的功能，CompletionStage接口表示任务执行的一个阶段。CompletionStage接口定义了很多方法，比如thenApply()、thenAccept()等。通过这些方法可以实现多个任务之间的时序关系，比如串行、并行、聚合等。

因此CompletableFuture既提供了Future阻塞式获取结果 + 任务状态的功能，也提供了CompletionStage的任务执行后触发回调 + 多个任务聚合的功能。

(3)CompletableFuture的方法说明

一.构建CompletableFuture的静态方法

二.runAsync()和supplyAsync()静态方法

三.allOf()和anyOf()静态方法

四.主动获取任务执行结果的方法

一.构建CompletableFuture的静态方法

CompletableFuture提供了4个静态方法来构建一个异步事件。由于传递进CompletableFuture这4个方法的任务需要异步执行，所以默认会使用ForkJoinPool.commonPool()提供的线程池来执行异步任务，当然也可以自定义一个线程池传入这些静态方法来执行异步任务。

//A Future that may be explicitly completed (setting its value and status),  //and may be used as a CompletionStage, //supporting dependent functions and actions that trigger upon its completion. public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     ...     //Returns a new CompletableFuture that is asynchronously completed      //by a task running in the ForkJoinPool#commonPool()      //with the value obtained by calling the given Supplier.     //@param supplier a function returning the value to be used to complete the returned CompletableFuture      //@param <U> the function's return type     //@return the new CompletableFuture     //带有返回值的异步执行方法，传入一个函数式接口，返回一个新的CompletableFuture对象     //默认使用ForkJoinPool.commonPool()作为线程池执行异步任务     public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {         return asyncSupplyStage(asyncPool, supplier);     }      //Returns a new CompletableFuture that is asynchronously completed      //by a task running in the given executor with the value obtained by calling the given Supplier.     //@param supplier a function returning the value to be used to complete the returned CompletableFuture     //@param executor the executor to use for asynchronous execution     //@param <U> the function's return type     //@return the new CompletableFuture     //带有返回值的异步执行方法，传入一个函数式接口 + 一个线程池，返回一个新的CompletableFuture对象     //多了一个Executor参数，表示使用自定义线程池来执行任务     public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor) {         return asyncSupplyStage(screenExecutor(executor), supplier);     }          static <U> CompletableFuture<U> asyncSupplyStage(Executor e, Supplier<U> f) {         if (f == null) throw new NullPointerException();         CompletableFuture<U> d = new CompletableFuture<U>();         e.execute(new AsyncSupply<U>(d, f));         return d;     }      //Returns a new CompletableFuture that is asynchronously completed     //by a task running in the ForkJoinPool#commonPool() after it runs the given action.     //@param runnable the action to run before completing the returned CompletableFuture     //@return the new CompletableFuture     //不带返回值的异步执行方法，传入一个Runnable参数，返回一个新的CompletableFuture对象     //默认使用ForkJoinPool.commonPool()作为线程池执行异步任务     public static CompletableFuture<Void> runAsync(Runnable runnable) {         return asyncRunStage(asyncPool, runnable);     }      //Returns a new CompletableFuture that is asynchronously completed      //by a task running in the given executor after it runs the given action.     //@param runnable the action to run before completing the returned CompletableFuture     //@param executor the executor to use for asynchronous execution     //@return the new CompletableFuture     //不带返回值的异步执行方法，传入一个Runnable参数 + 一个线程池，返回一个新的CompletableFuture对象     //多了一个Executor参数，表示使用自定义线程池来执行任务     public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor) {         return asyncRunStage(screenExecutor(executor), runnable);     }          static CompletableFuture<Void> asyncRunStage(Executor e, Runnable f) {         if (f == null) throw new NullPointerException();         CompletableFuture<Void> d = new CompletableFuture<Void>();         e.execute(new AsyncRun(d, f));         return d;     }     ... }

二.runAsync()和supplyAsync()静态方法

下面使用runAsync()方法来构建一个异步执行事件，由于runAsync()方法是没有返回值的，所以get()这个阻塞等待任务执行完成的方法返回的还是null。

CompletableFuture cf = CompletableFuture.runAsync(() -> {     System.out.println(Thread.currentThread().getName() + ":异步执行一个任务"); }); cf.get();//阻塞等待任务执行完成

下面使用supplyAsync()方法来构建一个异步执行事件，由于supplyAsync()方法具有返回值，所以get()方法会返回"supplyAsync"。

CompletableFuture cf = CompletableFuture.supplyAsync(() -> {     System.out.println("supplyAsync"); }); cf.get();//阻塞等待任务执行完成

三.allOf()和anyOf()静态方法

allOf()方法接收多个CompletableFuture的无返回值的任务。当所有的任务都执行结束后，返回一个新的CompletableFuture对象。

allOf()方法相当于实现了等待多个任务执行结束后再返回的功能，并且接收的CompletableFuture任务是通过runAsync()方法构建的。之所以无返回值，是因为当多个任务都具有返回值时get()方法不知取哪个。

public class CompletableFutureExample {     public static void main(String[] args) throws ExecutionException, InterruptedException {         CompletableFuture<Void> v1 = CompletableFuture.runAsync(() -> {             System.out.println("任务v1没有返回值");         });         CompletableFuture<Void> v2 = CompletableFuture.runAsync(() -> {             System.out.println("任务v2没有返回值");         });         //通过join()方法让主线程阻塞等待allOf()方法中的所有任务都执行完成后再继续执行         CompletableFuture.allOf(v1, v2).join();     } }

anyOf()方法接收多个CompletableFuture的带有返回值的任务。当任何一个任务执行完成后，返回一个新的CompletableFuture对象。

anyOf()方法实现了等待多个任务中任何一个任务执行结束便返回的功能，接收的CompletableFuture任务是通过supplyAsync()方法构建的。

public class CompletableFutureExample {     public static void main(String[] args) throws ExecutionException, InterruptedException {         CompletableFuture<String> v1 = CompletableFuture.supplyAsync(() -> {             return "任务v1的返回值";         });         CompletableFuture<String> v2 = CompletableFuture.supplyAsync(() -> {             return "任务v2的返回值";         });         //通过join()方法让主线程阻塞等待anyOf()方法中的任何一个任务执行完成后再继续执行         CompletableFuture.anyOf(v1, v2).thenAccept(value -> System.out.println(value)).join();     } }

四.主动获取任务执行结果的方法

由于CompletableFuture实现了Future接口，所以它可以像Future那样主动通过阻塞或轮询的方式来获得执行结果。比如可以通过get()方法阻塞式获取异步任务的执行结果(可中断)，比如也可以通过join()方法阻塞式获取异步任务的执行结果(不可中断)，此外通过complete()方法可实现线程间的数据传递 + 唤醒被get()阻塞的线程。

public class CompleteMethodExample {     public static class ClientThread implements Runnable {         private CompletableFuture completableFuture;                  public ClientThread(CompletableFuture completableFuture) {             this.completableFuture = completableFuture;         }                  @Override         public void run() {             log.info(Thread.currentThread().getName() + ":" + completableFuture.get());         }     }          public static void main(String[] args) {         //在ClientThread线程中使用completableFuture.get()获取返回值时，         //由于传入的cf并没有使用runAsync()等方法构建具体的异步任务，         //所以ClientThread线程中的completableFuture.get()方法必然会阻塞；         CompletableFuture cf = new CompletableFuture();         new Thread(new ClientThread(cf)).start();         new Thread(new ClientThread(cf)).start();         System.out.println("此时两个客户端线程正在被get()方法阻塞");         //通过compelete()方法来完成cf任务，并且设置了任务的返回结果为"finish"         cf.complete("finish");//此时会将值为"finish"传入两个线程，并唤醒这两个线程     } }

5.CompletionStage方法及作用说明

(1)CompletionStage示例

(2)CompletionStage的方法概述

(3)有传参但没返回值的方法

(4)有传参且有返回值的方法

(5)没传参也没返回值的方法

(6)组合起来串行执行的方法

(7)异常处理方法

(1)CompletionStage示例

CompletionStage表示任务执行的一个阶段，每个异步任务都会返回一个新的CompletionStage对象，可针对多个CompletionStage对象进行串行、并行、聚合等操作。简单来说，CompletionStage就是实现异步任务执行后的自动回调功能。

下面的CompletionStage例子：首先需要调用一个远程方法获得结果，然后把返回结果保存到数据库。所以代码中先定义一个异步任务处理远程调用，并返回CompletionStage，接着调用thenAccept()方法把第一步的执行结果保存到数据库中。

public class CompletionStageExample {     public static void main(String[] args) {         CompletionStage<String> cf = CompletableFuture.supplyAsync(() -> "远程调用的返回结果");         cf.thenAccept(result -> {             System.out.println("第一个异步任务的返回值是：" + result);             System.out.println("把result保存到数据库");         });     } }

可以看见和Future明显不一样的地方就是：thenAccept()方法中传入的回调对象是第一个异步任务执行完后自动触发的，不需要像Future那样去阻塞当前线程等待返回结果，还可以使用thenAcceptAsync()方法让保存到数据库的任务使用独立线程池。

(2)CompletionStage的方法概述

CompletionStage总共提供了38个方法来实现多个任务的串行、并行、聚合等功能，这些方法可以按功能进行如下的分类：

一.有传参但没返回值的方法

二.有传参且有返回值的方法

三.没传参也没返回值的方法

四.组合起来串行执行的方法

五.异常处理的方法

注意：Accept关键字有传参没有返回值，Run关键字没传参没返回值。

(3)有传参但没返回值的方法

有传参但没返回值的方法就是：用上一个异步任务的结果作为当前方法的参数进行下一步运算，并且当前方法会产生一个新的没有返回值的CompletionStage对象。有传参但没返回值的方法都包含Accept关键字。

一.依赖单个CompletionStage任务完成

thenAccept()相关方法用上一个任务的执行结果作为参数执行当前的action，这些方法接收的参数是一个函数式接口Consumer，表示一个待执行的任务。这些方法的返回值是CompletionStage，表示没有返回值。

注意：方法以Async结尾，表示使用单独的线程池来执行action，否则使用执行当前任务的线程来执行action。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that, when this stage completes normally,      //is executed with this stage's result as the argument to the supplied action.     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> thenAccept(Consumer<? super T> action);      //Returns a new CompletionStage that, when this stage completes normally,      //is executed using this stage's default asynchronous execution facility,      //with this stage's result as the argument to the supplied action.     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action);      //Returns a new CompletionStage that, when this stage completes normally,      //is executed using the supplied Executor,      //with this stage's result as the argument to the supplied action.     //@param action the action to perform before completing the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@return the new CompletionStage     public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action, Executor executor);     ... }  public class CompletionStageExample {     //当cf实例的任务执行完成后，会回调传入thenAcceptAsync()方法中的回调函数     //其中回调函数的result表示cf异步任务的返回结果     public static void main(String[] args) throws IOException, ExecutionException, InterruptedException {          CompletableFuture<String> cf = CompletableFuture.supplyAsync(() -> "thenAccept message");         cf.thenAcceptAsync((result) -> {             System.out.println(Thread.currentThread().getName() + "第一个异步任务的返回值:" + result);         });     } }

二.依赖两个CompletionStage任务都完成

thenAcceptBoth()相关方法提供了与thenAccept()相关方法类似的功能。不同点在于thenAcceptBoth()相关方法多了一个CompletionStage参数，表示当两个CompletionStage任务都完成后，才执行后面的action。而且这个action可以接收两个参数，这两个参数分别表示两个任务的返回值。thenAcceptBoth()相关方法相当于实现了两个异步任务的组合。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when this and the other given stage both complete normally,      //is executed with the two results as arguments to the supplied action.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@param <U> the type of the other CompletionStage's result     //@return the new CompletionStage     public <U> CompletionStage<Void> thenAcceptBoth(CompletionStage<? extends U> other, BiConsumer<? super T, ? super U> action);       //Returns a new CompletionStage that,      //when this and the other given stage complete normally,      //is executed using this stage's default asynchronous execution facility,      //with the two results as arguments to the supplied action.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@param <U> the type of the other CompletionStage's result     //@return the new CompletionStage     public <U> CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T, ? super U> action);       //Returns a new CompletionStage that,      //when this and the other given stage complete normally,      //is executed using the supplied executor,      //with the two results as arguments to the supplied function.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@param <U> the type of the other CompletionStage's result     //@return the new CompletionStage     public <U> CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T, ? super U> action, Executor executor);      ... }  public class ThenAcceptBothExample {     //task1和task2都执行完成后，会得到两个任务的返回值AcceptBoth和message，     //接着开始执行thenAcceptBoth()中的action，     //这个action会接收前面两个任务的执行结果r1和r2，并最终打印出：执行结果为"AcceptBoth+message"     public static void main(String[] args) {         CompletableFuture<String> task1 = CompletableFuture.supplyAsync(() -> "AcceptBoth");         CompletableFuture<String> task2 = CompletableFuture.supplyAsync(() -> "message");         task1.thenAcceptBoth(task2, (r1, r2) -> {             System.out.println("执行结果" + r1 + "+" + r2);         });                 //或者采用Fluent风格来写         //CompletableFuture.supplyAsync(() -> "AcceptBoth").thenAcceptBoth(         //    CompletableFuture.supplyAsync(() -> "message"), (r1, r2) -> {         //      System.out.println("执行结果：" + r1 + ", " + r2);         //    }         //);     } }

三.依赖两个CompletionStage任务中的任何一个完成

acceptEither()相关方法和thenAcceptBoth()相关方法几乎一样。它同样接收两个CompletionStage任务，但是只需要保证其中一个任务完成，就会回调acceptEither()方法中传入的action任务。这两个CompletionStage任务谁先完成就会获得谁的返回值，作为参数传给后续的action任务。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //is executed with the corresponding result as argument to the supplied action.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> acceptEither(CompletionStage<? extends T> other, Consumer<? super T> action);      //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //is executed using this stage's default asynchronous execution facility,      //with the corresponding result as argument to the supplied action.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> acceptEitherAsync(CompletionStage<? extends T> other, Consumer<? super T> action);      //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //is executed using the supplied executor,      //with the corresponding result as argument to the supplied function.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@return the new CompletionStage     public CompletionStage<Void> acceptEitherAsync(CompletionStage<? extends T> other, Consumer<? super T> action, Executor executor);      ... }

(4)有传参且有返回值的方法

有传参且有返回值的方法就是：用上一个异步任务的执行结果作为当前方法的参数进行下一步计算，并且当前方法会产生一个新的有返回值的CompletionStage对象。

一.依赖单个CompletionStage任务完成

thenApply()这一组方法的功能是等上一个CompletionStage任务执行完后，就会把执行结果传递给函数fn，将函数fn作为一个新的执行任务去执行，最后返回一个新的有返回值的CompletionStage对象。

其中以Async结尾的方法表示函数fn这个任务将采用单独的线程池来执行。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that, when this stage completes normally,      //is executed with this stage's result as the argument to the supplied function.     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> thenApply(Function<? super T,? extends U> fn);      //Returns a new CompletionStage that, when this stage completes normally,      //is executed using this stage's default asynchronous execution facility,      //with this stage's result as the argument to the supplied function.     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> thenApplyAsync(Function<? super T,? extends U> fn);      //Returns a new CompletionStage that, when this stage completes normally,      //is executed using the supplied Executor,      //with this stage's result as the argument to the supplied function.     //@param fn the function to use to compute the value of the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> thenApplyAsync(Function<? super T,? extends U> fn, Executor executor);      ... }

二.依赖两个CompletionStage任务都完成

thenCombine()这一组方法的功能类似于thenAcceptBoth()方法。它表示两个CompletionStage任务并行执行结束后，把这两个CompletionStage任务的执行结果传递给函数fn，函数fn执行后返回一个新的有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when this and the other given stage both complete normally,      //is executed with the two results as arguments to the supplied function.     //@param other the other CompletionStage     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the type of the other CompletionStage's result     //@param <V> the function's return type     //@return the new CompletionStage     public <U,V> CompletionStage<V> thenCombine(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn);      //Returns a new CompletionStage that,      //when this and the other given stage complete normally,      //is executed using this stage's default asynchronous execution facility,      //with the two results as arguments to the supplied function.     //@param other the other CompletionStage     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the type of the other CompletionStage's result     //@param <V> the function's return type     //@return the new CompletionStage     public <U,V> CompletionStage<V> thenCombineAsync(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn);      //Returns a new CompletionStage that,      //when this and the other given stage complete normally,      //is executed using the supplied executor,      //with the two results as arguments to the supplied function.       //@param other the other CompletionStage     //@param fn the function to use to compute the value of the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@param <U> the type of the other CompletionStage's result     //@param <V> the function's return type     //@return the new CompletionStage     public <U,V> CompletionStage<V> thenCombineAsync(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn, Executor executor);      ... }  public class ThenCombineExample {     public static void main(String[] args) {         CompletableFuture<String> task1 = CompletableFuture.supplyAsync(() -> "Combine");         CompletableFuture<String> task2 = CompletableFuture.supplyAsync(() -> "message");         CompletableFuture<String> cf = task1.thenCombineAsync(task2, (r1, r2) -> {             System.out.println("执行结果：" + r1 + ", " + r2);             return r1 + r2;         });         System.out.println(cf.get());                //或者采用Fluent风格来写         //CompletableFuture cf = CompletableFuture.supplyAsync(() -> "Combine").thenCombineAsync(         //    CompletableFuture.supplyAsync(() -> "message"), (r1, r2) -> {         //        System.out.println("执行结果：" + r1 + ", " + r2);         //        return r1 + r2;         //    }         //);         //System.out.println(cf.get());     } }

三.依赖两个CompletionStage任务中的任何一个完成

applyToEither()方法表示两个CompletionStage任务中任意一个任务完成后，都执行传入applyToEither()方法中的函数fn，函数fn执行后返回一个新的有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //is executed with the corresponding result as argument to the supplied function.     //@param other the other CompletionStage     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> applyToEither(CompletionStage<? extends T> other, Function<? super T, U> fn);      //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //is executed using this stage's default asynchronous execution facility,      //with the corresponding result as argument to the supplied function.     //@param other the other CompletionStage     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T, U> fn);      //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //is executed using the supplied executor,      //with the corresponding result as argument to the supplied function.     //@param other the other CompletionStage     //@param fn the function to use to compute the value of the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T, U> fn, Executor executor);      ... }

(5)没传参也没返回值的方法

没传参也没返回值的方法就是：当前方法不依赖上一个异步任务的执行结果，只要上一个异步任务执行完成就执行当前方法，并且当前方法会产生一个新的没有返回值的CompletionStage对象，没传参也没返回值的方法都包含Run关键字。

一.依赖单个CompletionStage任务完成

thenRun()方法只要上一个阶段的任务执行完成后，便立即执行指定action。thenRunAsync()表示采用ForkjoinPool.commonPool()线程池来执行action，action执行完成后会返回一个新的没有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when this stage completes normally, executes the given action.     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> thenRun(Runnable action);      //Returns a new CompletionStage that, when this stage completes normally,      //executes the given action using this stage's default asynchronous execution facility.     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> thenRunAsync(Runnable action);      //Returns a new CompletionStage that, when this stage completes normally,      //executes the given action using the supplied Executor.     //@param action the action to perform before completing the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@return the new CompletionStage     public CompletionStage<Void> thenRunAsync(Runnable action, Executor executor);     ... }

二.依赖两个CompletionStage任务都完成

runAfterBoth()方法接收一个CompletionStage任务。该方法要保证两个CompletionStage任务都完成，再执行指定的action。action执行完成后会返回一个新的没有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when this and the other given stage both complete normally, executes the given action.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> runAfterBoth(CompletionStage<?> other, Runnable action);      //Returns a new CompletionStage that,      //when this and the other given stage complete normally,      //executes the given action using this stage's default asynchronous execution facility.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other, Runnable action);      //Returns a new CompletionStage that,      //when this and the other given stage complete normally,      //executes the given action using the supplied executor.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@return the new CompletionStage     public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other, Runnable action, Executor executor);      ... }

三.依赖两个CompletionStage任务中的任何一个完成

runAfterEither()方法接收一个CompletionStage任务。它只需要保证两个任务中任意一个任务执行完成，即可执行指定的action，action执行完成后会返回一个新的没有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when either this or the other given stage complete normally, executes the given action.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> runAfterEither(CompletionStage<?> other, Runnable action);      //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //executes the given action using this stage's default asynchronous execution facility.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@return the new CompletionStage     public CompletionStage<Void> runAfterEitherAsync(CompletionStage<?> other, Runnable action);      //Returns a new CompletionStage that,      //when either this or the other given stage complete normally,      //executes the given action using the supplied executor.     //@param other the other CompletionStage     //@param action the action to perform before completing the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@return the new CompletionStage     public CompletionStage<Void> runAfterEitherAsync(CompletionStage<?> other, Runnable action, Executor executor);      ... }

(6)组合起来串行执行的方法

thenCompose()是多任务组合方法，它的作用是把两个CompletionStage任务进行组合达到串行执行的目的，也就是把第一个任务的执行结果作为参数传递给第二个任务执行。

thenCompose()方法有点类似于thenCombine()方法，但thenCompose()方法中的两个任务存在先后关系，而thenCombine()方法中的两个任务是并行执行的。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that, when this stage completes normally,      //is executed with this stage as the argument to the supplied function.     //@param fn the function returning a new CompletionStage     //@param <U> the type of the returned CompletionStage's result     //@return the CompletionStage     public <U> CompletionStage<U> thenCompose(Function<? super T, ? extends CompletionStage<U>> fn);      //Returns a new CompletionStage that, when this stage completes normally,      //is executed using this stage's default asynchronous execution facility,     //with this stage as the argument to the supplied function.     //@param fn the function returning a new CompletionStage     //@param <U> the type of the returned CompletionStage's result     //@return the CompletionStage     public <U> CompletionStage<U> thenComposeAsync(Function<? super T, ? extends CompletionStage<U>> fn);      //Returns a new CompletionStage that, when this stage completes normally,      //is executed using the supplied Executor,      //with this stage's result as the argument to the supplied function.     //@param fn the function returning a new CompletionStage     //@param executor the executor to use for asynchronous execution     //@param <U> the type of the returned CompletionStage's result     //@return the CompletionStage     public <U> CompletionStage<U> thenComposeAsync(Function<? super T, ? extends CompletionStage<U>> fn, Executor executor);      ... }  public class ThenComposeExample {     //下面使用supplyAsync()方法构建了一个异步带返回值的任务，返回值为"Compose Message"；     //接着使用thenCompose()方法组合另外一个任务，并把前面任务的返回值r作为参数传递给第二个任务     //在第二个任务中同样使用supplyAsync()方法构建了一个新的任务将参数r转为大写     //最后thenCompose()方法返回一个新的没有返回值的CompletionStage对象     public static void main(String[] args) {         CompletableFuture<String> task1 = CompletableFuture.supplyAsync(() -> "Compose Message");         CompletableFuture<String> cf = task1.thenCompose(r -> CompletableFuture.supplyAsync(() -> r.toUpperCase()));         System.out.println(cf.get());                //或者采用Fluent风格来写         //CompletableFuture cf = CompletableFuture.supplyAsync(() -> "Compose Message")         //     .thenCompose(r -> CompletableFuture.supplyAsync(() -> r.toUpperCase())         //);         //System.out.println(cf.get());     } }

(7)异常处理方法

上述介绍的方法都是CompletionStage任务正常执行时的处理方法。如果依赖的前一个任务出现异常，那么会导致后续的任务无法正常执行。比如下述代码，如果前置任务cf出现异常，那么会影响后置任务的执行。

public class RunAfterBothExample {     public static void main(String[] args) {         CompletableFuture cf = CompletableFuture.supplyAsync(() -> {             throw new RuntimeException("Exception");         }).runAfterBoth(CompletableFuture.supplyAsync(() -> "Message"), () -> {             System.out.println("Done");         });         System.out.println(cf.get());     } }

CompletionStage提供了3类异常处理的方法。

一.whenComplete()方法

whenComplete()这一组方法表示的是：不论前置的CompletionStage任务是正常执行结束还是出现异常，都能触发执行指定action，最后返回一个没返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage with the same result or exception as this stage,      //that executes the given action when this stage completes.     //@param action the action to perform     //@return the new CompletionStage     public CompletionStage<T> whenComplete(BiConsumer<? super T, ? super Throwable> action);      //Returns a new CompletionStage with the same result or exception as this stage,      //that executes the given action using this stage's default asynchronous execution facility when this stage completes.     //@param action the action to perform     //@return the new CompletionStage     public CompletionStage<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action);      //Returns a new CompletionStage with the same result or exception as this stage,      //that executes the given action using the supplied Executor when this stage completes.     //@param action the action to perform     //@param executor the executor to use for asynchronous execution     //@return the new CompletionStage     public CompletionStage<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action, Executor executor);      ... }

二.handle()方法

handle()这一组方法表示的是：不论前置的CompletionStage任务是正常执行结束还是出现异常，都会执行其中的函数fn，最后返回一个有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when this stage completes either normally or exceptionally,      //is executed with this stage's result and exception as arguments to the supplied function.     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> handle(BiFunction<? super T, Throwable, ? extends U> fn);      //Returns a new CompletionStage that,      //when this stage completes either normally or exceptionally,      //is executed using this stage's default asynchronous execution facility,      //with this stage's result and exception as arguments to the supplied function.     //@param fn the function to use to compute the value of the returned CompletionStage     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> handleAsync(BiFunction<? super T, Throwable, ? extends U> fn);      //Returns a new CompletionStage that,      //when this stage completes either normally or exceptionally,      //is executed using the supplied executor,      //with this stage's result and exception as arguments to the supplied function.     //@param fn the function to use to compute the value of the returned CompletionStage     //@param executor the executor to use for asynchronous execution     //@param <U> the function's return type     //@return the new CompletionStage     public <U> CompletionStage<U> handleAsync(BiFunction<? super T, Throwable, ? extends U> fn, Executor executor);      ... }  public class HandleExample {     public static void main(String[] args) throws ExecutionException, InterruptedException {         CompletableFuture cf = CompletableFuture.supplyAsync(() -> {             throw new RuntimeException("Exception");         }).handleAsync((r, th) -> {            return th != null ? "出现异常" : "正常执行";         });         System.out.println(cf.get());     } }

三.exceptionally()方法

exceptionally()方法接收一个函数fn，当上一个CompletionStage任务出现异常时，会把该异常作为参数传递给fn，最后返回一个有返回值的CompletionStage对象。

public interface CompletionStage<T> {     ...     //Returns a new CompletionStage that,      //when this stage completes exceptionally,      //is executed with this stage's exception as the argument to the supplied function.     //Otherwise, if this stage completes normally,      //then the returned stage also completes normally with the same value.     //@param fn the function to use to compute the value of the returned CompletionStage if this CompletionStage completed exceptionally     //@return the new CompletionStage     public CompletionStage<T> exceptionally(Function<Throwable, ? extends T> fn);     ... }  public class ExceptionallyExample {     public static void main(String[] args) throws ExecutionException, InterruptedException {         CompletableFuture cf = CompletableFuture.supplyAsync(() -> {             throw new RuntimeException("Exception");         }).exceptionally(e -> {             log.error(e);             return "ExceptionallyExample";         });         System.out.println(cf.get());     } }

6.CompletableFuture的实现原理分析

(1)CompletableFuture实现回调的例子

(2)CompletableFuture如何存储任务

(3)Completion的几个实现类

(4)Completion的栈结构存储回调任务

(5)Completion中的回调任务的执行和总结

(1)CompletableFuture实现回调的例子

CompletableFuture实现了Future接口和CompletionStage接口，CompletionStage接口为CompletableFuture提供了丰富的异步回调接口，CompletableFuture可以使用这些接口来实现复杂的异步计算工作。

下面是一个使用CompletableFuture回调的例子。其中构建了两个CompletionStage任务，第一个任务是返回"thenAccept message"字符串，第二个任务是打印第一个任务的返回值。注意：Accept关键字有传参没有返回值，Run关键字没传参没返回值。

这两个任务建立了串行执行的关系，第二个任务相当于第一个任务执行结束后的异步回调，并且多个CompletionStage任务可以使用链式风格串联。

public class CompletionStageExample {     public static void main(String[] args) throws InterruptedException, ExecutionException {         CompletableFuture<Void> cf = CompletableFuture.supplyAsync(() -> "thenAccept message")             .thenAcceptAsync((result) -> {                 System.out.println("第一个异步任务的返回值：" + result);             });         cf.get();     } }

(2)CompletableFuture如何存储任务

一.CompletableFuture的成员变量

CompletableFuture的成员变量只有两个：result和stack。

public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     ...     //表示CompletionStage任务的返回结果或者一个异常的封装对象AltResult     volatile Object result;//Either the result or boxed AltResult          //表示依赖操作栈的栈顶，链式调用中传递的任务都会被压入这个stack中     volatile Completion stack;//Top of Treiber stack of dependent actions     ... }

二.表示具体执行任务的Completion

成员变量stack是一个存储Completion对象的Treiber Stack结构，Treiber Stack是一种基于CAS机制实现的无锁并发栈。

Completion表示一个具体的执行任务。每个回调任务都会封装成Completion对象，然后放入Treiber Stack中。Completion中的成员变量next保存了栈中的下一个回调任务。

public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     ...     abstract static class Completion extends ForkJoinTask<Void> implements Runnable, AsynchronousCompletionTask {         volatile Completion next;//Treiber stack link          //Performs completion action if triggered, returning a dependent that may need propagation, if one exists.         //@param mode SYNC, ASYNC, or NESTED         abstract CompletableFuture<?> tryFire(int mode);          //Returns true if possibly still triggerable. Used by cleanStack.         abstract boolean isLive();          public final void run() {             tryFire(ASYNC);         }          public final boolean exec() {             tryFire(ASYNC);             return true;         }          public final Void getRawResult() {             return null;         }          public final void setRawResult(Void v) {                }     }     ... }

(3)Completion的几个实现类

一.UniCompletion

当使用如thenRun()、thenApply()等方法处理单个任务的，那么这些任务就会封装成UniCompletion对象。

二.CoCompletion

当使用如thenCombine()、applyToEither()等方法处理两个任务的，那么这些任务会封装成CoCompletion对象。

三.Signaller

当使用如get()、join()等方法处理任务时，那么调用方也会作为任务被封装成Signaller对象。

(4)Completion的栈结构存储回调任务

以如下创建一个CompletableFuture任务为例，说明在CompletableFuture中是如何存储这些回调任务的。注意：该例子在Debug中没有发现baseFuture的成员变量stack的变化。

public class CompletionStackExample {     public static void main(String[] args) throws InterruptedException, ExecutionException {         //创建一个CompletableFuture任务对象         CompletableFuture<String> baseFuture = CompletableFuture.supplyAsync(() -> {             try {                 System.out.println("开始执行入栈baseFuture的第一个异步任务");                 Thread.sleep(5000);                 System.out.println("第一个异步任务执行完毕");             } catch (Exception e) {             }             return "BaseFuture";         });         System.out.println("主线程第一次打印");          baseFuture.thenApply(r -> {             System.out.println("开始执行入栈baseFuture的第二个异步任务");             try {                 Thread.sleep(5000);                 System.out.println("第二个异步任务执行完毕");             } catch (Exception e) {             }             return "Then Apply";         });//输出结果中没有"Then Apply"，因为没有任务使用"Then Apply"这个返回值         System.out.println("主线程第二次打印");          baseFuture.thenAccept(r -> {             System.out.println("开始执行入栈baseFuture的第三个异步任务");             try {                 Thread.sleep(5000);                 System.out.println("第三个异步任务执行完毕: " + r);             } catch (Exception e) {             }         }).thenAccept(Void -> {             System.out.println("baseFuture的第三个异步任务返回的新CompletableFuture，入栈第一个异步任务");             try {                 Thread.sleep(5000);                 System.out.println("第三个异步任务的子任务执行完毕");             } catch (Exception e) {             }         });         System.out.println("主线程第三次打印");          baseFuture.thenApply(r -> {             System.out.println("开始执行入栈baseFuture的第四个异步任务");             try {                 Thread.sleep(5000);                 System.out.println("第四个异步任务执行完毕");             } catch (Exception e) {             }             return "Apply Message";         }).thenAccept(r -> {             System.out.println("baseFuture的第四个异步任务返回的新CompletableFuture，入栈第一个异步任务");             try {                 Thread.sleep(5000);                 System.out.println("第四个异步任务的子任务执行完毕: " + r);             } catch (Exception e) {             }         });          System.out.println("主线程第四次打印");         System.out.println("finish: " + baseFuture.get());     }     //输出的结果如下：     //主线程第一次打印     //开始执行入栈baseFuture的第一个异步任务     //主线程第二次打印     //主线程第三次打印     //主线程第四次打印     //第一个异步任务执行完毕     //开始执行入栈baseFuture的第四个异步任务     //开始执行入栈baseFuture的第三个异步任务     //第四个异步任务执行完毕     //第三个异步任务执行完毕: BaseFuture     //baseFuture的第三个异步任务返回的新CompletableFuture，入栈第一个异步任务     //baseFuture的第四个异步任务返回的新CompletableFuture，入栈第一个异步任务     //第三个异步任务的子任务执行完毕     //第四个异步任务的子任务执行完毕: Apply Message     //开始执行入栈baseFuture的第二个异步任务     //第二个异步任务执行完毕     //finish: BaseFuture }

一.第一阶段的Completion Stack结构

主线程第一次打印和第二次打印执行完成后，会创建如下图所示的结构。此时Completion类型是UniCompletion，因为thenApply()方法只接收一个任务。

二.第二阶段的Completion Stack结构

主线程第三次打印执行完成后，就会创建如下图所示的结构。

首先使用baseFuture.thenAccept()方法在baseFuture上增加一个回调，此时会把这个回调对应的Completion压入baseFuture的stack的栈顶。

然后会产生一个新的CompletableFuture对象实例继续执行thenAccept()，由于这个新的CompletableFuture对象实例是在栈顶的Completion中产生的，因此在栈顶的Completion中会有一个dep属性指向这个新的对象实例。

在新的CompletableFuture对象中又调用thenAccept()来构建一个回调任务，所以又会有一个新的Completion Stack结构。

三.第三阶段的Completion Stack结构

主线程第四次打印执行完成后，就会创建如下图所示的结构。

首先是在baseFuture上使用thenApply()方法创建一个带有返回值的回调，这个回调对应的Completion同样会压入baseFuture的stack的栈顶。然后同样会创建一个新的CompletableFuture对象实例。接着在这个新的对象实例中继续使用thenAccept()方法添加另外一个回调，这个回调对应的Completion会压入新的CompletableFuture的stack的栈顶。

(5)Completion中的回调任务的执行和总结

从Completion Stack的栈顶中逐个出栈来执行。

如果当前出栈的Completion存在一个子Completion Stack，那么就优先执行这一条链路的Completion任务。

CompletableFuture中的回调任务，是基于Completion来实现的。针对CompletionStage中不同类型的方法，Completion有不同的子类处理。

Completion表示一个具体的回调任务，这些Completion采用了一种Treiber Stack结构来存储。由于每个Completion都可能会产生新的CompletableFuture，所以整个结构看起来像一棵很深的树。

7.CompletableFuture的核心源码分析

(1)CompletableFuture的核心源码

(2)CompletableFuture对象的创建

(3)Completion Stack的构建

(4)Completion任务的执行流程

(5)Completion任务的执行结果获取

(6)总结

(1)CompletableFuture的核心源码

CompletableFuture的源码主要分四部分：

一.CompletableFuture对象的创建

二.Completion Stack的构建

三.get()方法获取任务处理结果时阻塞和唤醒线程

四.当前置任务执行完成后，Completion Stack的执行流程

(2)CompletableFuture对象的创建

假设使用supplyAsync()方法来创建一个CompletableFuture对象。那么在执行supplyAsync()方法时触发调用的asyncSupplyStage()方法中，便会使用线程池来执行一个由AsyncSupply()构造方法构建的任务，这个线程池默认情况下是由ForkJoinPool的commonPool()方法返回的。

当线程池执行由AsyncSupply()构造方法构建的任务时，会调用AsyncSupply的run()方法来执行具体的任务。

在AsyncSupply的run()方法中：首先会使用f.get()来获得Supplier这个函数式接口的执行结果，然后通过执行CompletableFuture的completeValue()方法，把执行结果通过CAS设置到CompletableFuture的成员变量result中。最后调用CompletableFuture的postComplete()方法表示执行完成，该postComplete()方法会执行Completion Stack中的所有回调任务。

//Represents a supplier of results. //There is no requirement that a new or distinct result be returned each time the supplier is invoked. //This is a functional interface whose functional method is get(). //@param <T> the type of results supplied by this supplier @FunctionalInterface public interface Supplier<T> {     //Gets a result.     //@return a result     T get(); }  public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     volatile Object result;       // Either the result or boxed AltResult     volatile Completion stack;    // Top of Treiber stack of dependent actions          //Returns a new CompletableFuture that is asynchronously completed by a task      //running in the ForkJoinPool#commonPool() with the value obtained by calling the given Supplier.     //@param supplier a function returning the value to be used to complete the returned CompletableFuture     //@param <U> the function's return type     //@return the new CompletableFuture     public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {         return asyncSupplyStage(asyncPool, supplier);     }          static <U> CompletableFuture<U> asyncSupplyStage(Executor e, Supplier<U> f) {         if (f == null) throw new NullPointerException();         CompletableFuture<U> d = new CompletableFuture<U>();         //使用线程池来执行一个由AsyncSupply()方法构建的任务         e.execute(new AsyncSupply<U>(d, f));         //返回一个新的CompletableFuture对象         return d;     }          static final class AsyncSupply<T> extends ForkJoinTask<Void> implements Runnable, AsynchronousCompletionTask {         CompletableFuture<T> dep;         Supplier<T> fn;         AsyncSupply(CompletableFuture<T> dep, Supplier<T> fn) {             this.dep = dep;             this.fn = fn;         }                 public final Void getRawResult() {             return null;         }         public final void setRawResult(Void v) {                      }         public final boolean exec() {             run();             return true;         }                 public void run() {             CompletableFuture<T> d;             Supplier<T> f;             if ((d = dep) != null && (f = fn) != null) {                 dep = null; fn = null;                 if (d.result == null) {                     try {                         //首先使用f.get()来获得Supplier这个函数式接口中的执行结果                         //然后通过执行CompletableFuture的completeValue()方法，                         //把执行结果设置到CompletableFuture的成员变量result中；                         d.completeValue(f.get());                     } catch (Throwable ex) {                         d.completeThrowable(ex);                     }                 }                 //最后调用CompletableFuture的postComplete()方法执行Completion Stack中的所有回调任务                 d.postComplete();             }         }     }          //Completes with a non-exceptional result, unless already completed.     final boolean completeValue(T t) {         return UNSAFE.compareAndSwapObject(this, RESULT, null, (t == null) ? NIL : t);     }          //Pops and tries to trigger all reachable dependents. Call only when known to be done.     final void postComplete() {         //On each step, variable f holds current dependents to pop and run.           //It is extended along only one path at a time, pushing others to avoid unbounded recursion.         CompletableFuture<?> f = this;         Completion h;         while ((h = f.stack) != null || (f != this && (h = (f = this).stack) != null)) {             CompletableFuture<?> d;             Completion t;             if (f.casStack(h, t = h.next)) {                 if (t != null) {                     if (f != this) {                         pushStack(h);                         continue;                     }                     h.next = null;    // detach                 }                 f = (d = h.tryFire(NESTED)) == null ? this : d;             }         }     }          private static final sun.misc.Unsafe UNSAFE;     private static final long RESULT;     private static final long STACK;     private static final long NEXT;     static {         try {             final sun.misc.Unsafe u;             UNSAFE = u = sun.misc.Unsafe.getUnsafe();             Class<?> k = CompletableFuture.class;             RESULT = u.objectFieldOffset(k.getDeclaredField("result"));             STACK = u.objectFieldOffset(k.getDeclaredField("stack"));             NEXT = u.objectFieldOffset(Completion.class.getDeclaredField("next"));         } catch (Exception x) {             throw new Error(x);         }     }     ... }

(3)Completion Stack的构建

假设已经使用了CompletableFuture的supplyAsync()方法创建了源任务，接着需要使用CompletionStage的thenApply()等方法来构建回调任务。

源任务 -> Supplier接口的实现类对象(get()方法)，回调任务 -> Function接口的实现类对象(apply()方法)。

CompletableFuture的thenApply()方法会触发执行uniApplyStage()方法。在uniApplyStage()方法中，首先会创建一个新的CompletableFuture对象，然后根据CompletableFuture的uniApply()方法判断源任务是否已经完成。如果源任务已经完成，则不需要入栈，直接执行回调任务的apply()方法。如果源任务还没执行完成，才将回调任务封装为UniApply对象并入栈。

源任务还没执行完成的处理过程具体如下：首先把回调任务封装成一个UniApply对象，然后调用CompletableFuture的push()方法，把UniApply对象压入源任务所在CompletableFuture对象中的stack的栈顶，最后调用UniApply的tryFire()方法来尝试执行该回调任务。

注意：UniApply对象其实是一个Completion对象，因为UniApply类继承自UniCompletion类，而UniCompletion类又继承自Completion类。

//Represents a function that accepts one argument and produces a result. //This is a functional interface whose functional method is apply(Object). //@param <T> the type of the input to the function //@param <R> the type of the result of the function @FunctionalInterface public interface Function<T, R> {     //Applies this function to the given argument.     //@param t the function argument     //@return the function result     R apply(T t);     ... }      public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     volatile Object result;       // Either the result or boxed AltResult     volatile Completion stack;    // Top of Treiber stack of dependent actions          public <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn) {         return uniApplyStage(null, fn);     }          private <V> CompletableFuture<V> uniApplyStage(Executor e, Function<? super T,? extends V> f) {         if (f == null) throw new NullPointerException();         //创建一个新的CompletableFuture对象         CompletableFuture<V> d =  new CompletableFuture<V>();         //根据CompletableFuture的uniApply()方法判断源任务是否已经完成         //如果源任务已经完成，则不需要入栈         if (e != null || !d.uniApply(this, f, null)) {             //首先把回调任务f封装成一个UniApply对象             UniApply<T,V> c = new UniApply<T,V>(e, d, this, f);             //然后调用CompletableFuture的push()方法             //把UniApply对象压入源任务所在的CompletableFuture对象中的stack的栈顶             push(c);             //最后调用UniApply的tryFire()方法来尝试执行该回调任务             c.tryFire(SYNC);         }         return d;     }          final <S> boolean uniApply(CompletableFuture<S> a, Function<? super S,? extends T> f, UniApply<S,T> c) {          Object r;         Throwable x;         //如果任务还没完成(result == null)，直接返回false         if (a == null || (r = a.result) == null || f == null) {             return false;         }         tryComplete: if (result == null) {             //判断result是否为异常类型             if (r instanceof AltResult) {                 if ((x = ((AltResult)r).ex) != null) {                     //如果result是异常类型，则使用completeThrowable()方法处理，并返回true                     completeThrowable(x, r);                     break tryComplete;                 }                 r = null;             }             //如果result不为空，任务已经执行完成，并且没有出现异常             try {                 if (c != null && !c.claim()) {                     return false;                 }                 //把源任务的执行结果s作为参数传给回调任务f                 //直接执行回调任务的apply()方法，并将结果设置到CompletableFuture对象的成员变量result中                 @SuppressWarnings("unchecked") S s = (S) r;                 completeValue(f.apply(s));              } catch (Throwable ex) {                 completeThrowable(ex);             }         }         return true;     }          //Pushes the given completion (if it exists) unless done.     final void push(UniCompletion<?,?> c) {         if (c != null) {             while (result == null && !tryPushStack(c)) {                 lazySetNext(c, null); // clear on failure             }         }     }          final boolean tryPushStack(Completion c) {         Completion h = stack;         lazySetNext(c, h);         return UNSAFE.compareAndSwapObject(this, STACK, h, c);     }          static void lazySetNext(Completion c, Completion next) {         UNSAFE.putOrderedObject(c, NEXT, next);     }          ...          static final class UniApply<T,V> extends UniCompletion<T,V> {         Function<? super T,? extends V> fn;         UniApply(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src, Function<? super T,? extends V> fn) {             super(executor, dep, src);             this.fn = fn;         }         //尝试执行当前CompletableFuture中的Completion         final CompletableFuture<V> tryFire(int mode) {             CompletableFuture<V> d;             CompletableFuture<T> a;             //执行CompletableFuture的uniApply()方法尝试执行回调任务             if ((d = dep) == null || !d.uniApply(a = src, fn, mode > 0 ? null : this)) {                 return null;             }             dep = null;             src = null;             fn = null;             //执行CompletableFuture的postFire()方法             return d.postFire(a, mode);         }     }          final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {         if (a != null && a.stack != null) {             if (mode < 0 || a.result == null) {                 a.cleanStack();             } else {                 a.postComplete();             }         }         if (result != null && stack != null) {             if (mode < 0) {                 return this;             } else {                 postComplete();             }         }         return null;     }          abstract static class UniCompletion<T,V> extends Completion {         Executor executor;//执行当前任务的线程池         CompletableFuture<V> dep;//构建当前任务的CompletableFuture对象         CompletableFuture<T> src;//指向源任务          UniCompletion(Executor executor, CompletableFuture<V> dep, CompletableFuture<T> src) {             this.executor = executor;             this.dep = dep;             this.src = src;         }          //判断是否使用单独的线程池来执行任务         final boolean claim() {             Executor e = executor;             if (compareAndSetForkJoinTaskTag((short)0, (short)1)) {                 if (e == null) {                     return true;                 }                 executor = null; // disable                 e.execute(this);             }             return false;         }         //判断任务是否存活         final boolean isLive() {             return dep != null;         }     } }

(4)Completion任务的执行流程

一.CompletableFuture的postComplete()方法

CompletableFuture中的任务完成后即源任务完成后，会通过CompletableFuture.postComplete()方法来完成后置逻辑，也就是把当前CompletableFuture.stack中存储的Completion逐项出栈执行。

postComplete()方法会触发stack中所有可执行的回调任务Completion，该方法会遍历整个stack，并通过Completion任务的tryFire()方法来尝试执行。

public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     volatile Object result;     volatile Completion stack;          public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {         return asyncSupplyStage(asyncPool, supplier);     }          static <U> CompletableFuture<U> asyncSupplyStage(Executor e, Supplier<U> f) {         if (f == null) throw new NullPointerException();         CompletableFuture<U> d = new CompletableFuture<U>();         //使用线程池来执行一个由AsyncSupply()方法构建的任务         e.execute(new AsyncSupply<U>(d, f));         //返回一个新的CompletableFuture对象         return d;     }          static final class AsyncSupply<T> extends ForkJoinTask<Void> implements Runnable, AsynchronousCompletionTask {         CompletableFuture<T> dep;         Supplier<T> fn;         AsyncSupply(CompletableFuture<T> dep, Supplier<T> fn) {             this.dep = dep;             this.fn = fn;         }         ...         public void run() {             CompletableFuture<T> d;             Supplier<T> f;             if ((d = dep) != null && (f = fn) != null) {                 dep = null; fn = null;                 if (d.result == null) {                     try {                         //首先使用f.get()来获得Supplier这个函数式接口中的执行结果                         //然后通过执行CompletableFuture的completeValue()方法，                         //把执行结果设置到CompletableFuture的成员变量result中；                         d.completeValue(f.get());                     } catch (Throwable ex) {                         d.completeThrowable(ex);                     }                 }                 //最后调用CompletableFuture的postComplete()方法执行Completion Stack中的所有回调任务                 d.postComplete();             }         }     }          final boolean completeValue(T t) {         return UNSAFE.compareAndSwapObject(this, RESULT, null, (t == null) ? NIL : t);     }          final void postComplete() {         CompletableFuture<?> f = this;         Completion h;         //如果stack不为空，则不断循环从stack中出栈Completion任务         while ((h = f.stack) != null || (f != this && (h = (f = this).stack) != null)) {             CompletableFuture<?> d;             Completion t;             //通过CAS逐个取出stack中的Completion任务并重置stack             if (f.casStack(h, t = h.next)) {                 if (t != null) {                     //表示h.tryFire()返回了另外一个CompleableFuture对象                     if (f != this) {                         pushStack(h);                         continue;                     }                     h.next = null;    // detach                 }                 //执行指定Completion的tryFire()方法，比如UniApply.tryFire()方法                 f = (d = h.tryFire(NESTED)) == null ? this : d;             }         }     }     ... }

二.Completion任务的执行流程图

(5)Completion任务的执行结果获取

可以通过get()或join()方法获取CompletableFuture的执行结果。当任务还没执行结束时(r == null)，则调用waitingGet()方法进行阻塞等待。主要会先自旋256次判断执行是否结束，如果不是才挂起线程进行阻塞，从而避免直接挂起线程带来的性能开销。

public class CompletableFuture<T> implements Future<T>, CompletionStage<T> {     ...     //Waits if necessary for this future to complete, and then returns its result.     public T get() throws InterruptedException, ExecutionException {         Object r;         return reportGet((r = result) == null ? waitingGet(true) : r);     }          //Returns raw result after waiting, or null if interruptible and interrupted.     private Object waitingGet(boolean interruptible) {         Signaller q = null;         boolean queued = false;         int spins = -1;         Object r;         while ((r = result) == null) {             if (spins < 0) {                 spins = (Runtime.getRuntime().availableProcessors() > 1) ? 1 << 8 : 0;             } else if (spins > 0) {                 if (ThreadLocalRandom.nextSecondarySeed() >= 0) {                     --spins;                 }             } else if (q == null) {                 q = new Signaller(interruptible, 0L, 0L);             } else if (!queued) {                 queued = tryPushStack(q);             } else if (interruptible && q.interruptControl < 0) {                 q.thread = null;                 cleanStack();                 return null;             } else if (q.thread != null && result == null) {                 try {                     ForkJoinPool.managedBlock(q);                 } catch (InterruptedException ie) {                     q.interruptControl = -1;                 }             }         }         if (q != null) {             q.thread = null;             if (q.interruptControl < 0) {                 if (interruptible) {                     r = null; // report interruption                 } else {                     Thread.currentThread().interrupt();                 }             }         }         postComplete();         return r;     }     ... }

(6)总结

CompletableFuture的核心在于CompletionStage，CompletionStage提供了最基础的异步回调机制。也就是主线程不需要通过阻塞方式来等待异步任务的执行结果，而是当异步任务执行完成后主动通知来触发执行下一个任务。此外，CompletionStage全部采用了函数式接口的方式来实现，可以通过链式的方式来对多个CompletionStage进行组合。

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