JUC并发—14.Future模式和异步编程分析
大纲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;}@FunctionalInterfacepublic 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@FunctionalInterfacepublic 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@FunctionalInterfacepublic 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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