深入学习java源码之CompletableFuture.reportGet()与CompletableFuture.supplyAsync()

异步计算

• 所谓异步调用其实就是实现一个可无需等待被调用函数的返回值而让操作继续运行的方法。在 Java 语言中，简单的讲就是另启一个线程来完成调用中的部分计算，使调用继续运行或返回，而不需要等待计算结果。但调用者仍需要取线程的计算结果。

• JDK5新增了Future接口，用于描述一个异步计算的结果。虽然 Future 以及相关使用方法提供了异步执行任务的能力，但是对于结果的获取却是很不方便，只能通过阻塞或者轮询的方式得到任务的结果。阻塞的方式显然和我们的异步编程的初衷相违背，轮询的方式又会耗费无谓的 CPU 资源，而且也不能及时地得到计算结果。

• 以前我们获取一个异步任务的结果可能是这样写的

Future 接口的局限性

Future接口可以构建异步应用，但依然有其局限性。它很难直接表述多个Future 结果之间的依赖性。实际开发中，我们经常需要达成以下目的：

1. 将多个异步计算的结果合并成一个

2. 等待Future集合中的所有任务都完成

3. Future完成事件（即，任务完成以后触发执行动作）

4. 。。。

函数式编程

CompletionStage

• CompletionStage代表异步计算过程中的某一个阶段，一个阶段完成以后可能会触发另外一个阶段

• 一个阶段的计算执行可以是一个Function，Consumer或者Runnable。比如：stage.thenApply(x -> square(x)).thenAccept(x -> System.out.print(x)).thenRun(() -> System.out.println())

• 一个阶段的执行可能是被单个阶段的完成触发，也可能是由多个阶段一起触发

CompletableFuture

• 在Java8中，CompletableFuture提供了非常强大的Future的扩展功能，可以帮助我们简化异步编程的复杂性，并且提供了函数式编程的能力，可以通过回调的方式处理计算结果，也提供了转换和组合 CompletableFuture 的方法。
• 它可能代表一个明确完成的Future，也有可能代表一个完成阶段（ CompletionStage ），它支持在计算完成以后触发一些函数或执行某些动作。
• 它实现了Future和CompletionStage接口

创建CompletableFuture

thenApply

当前阶段正常完成以后执行，而且当前阶段的执行的结果会作为下一阶段的输入参数。thenApplyAsync默认是异步执行的。这里所谓的异步指的是不在当前线程内执行。

thenApply相当于回调函数（callback）

thenAccept与thenRun

• 可以看到，thenAccept和thenRun都是无返回值的。如果说thenApply是不停的输入输出的进行生产，那么thenAccept和thenRun就是在进行消耗。它们是整个计算的最后两个阶段。

• 同样是执行指定的动作，同样是消耗，二者也有区别：

  • thenAccept接收上一阶段的输出作为本阶段的输入 　　

  • thenRun根本不关心前一阶段的输出，根本不不关心前一阶段的计算结果，因为它不需要输入参数

thenCombine整合两个计算结果

例如，此阶段与其它阶段一起完成，进而触发下一阶段：

whenComplete

例子

事实上，如果每个操作都很简单的话（比如：上面的例子中按照id去查）没有必要用这种多线程异步的方式，因为创建线程还需要时间，还不如直接同步执行来得快。

事实证明，只有当每个操作很复杂需要花费相对很长的时间（比如，调用多个其它的系统的接口；比如，商品详情页面这种需要从多个系统中查数据显示的）的时候用CompletableFuture才合适，不然区别真的不大，还不如顺序同步执行。

java源码

package java.util.concurrent;
import java.util.function.Supplier;
import java.util.function.Consumer;
import java.util.function.BiConsumer;
import java.util.function.Function;
import java.util.function.BiFunction;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.Executor;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.CancellationException;
import java.util.concurrent.CompletionException;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.locks.LockSupport;

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

    final boolean internalComplete(Object r) { // CAS from null to r
        return UNSAFE.compareAndSwapObject(this, RESULT, null, r);
    }

    final boolean casStack(Completion cmp, Completion val) {
        return UNSAFE.compareAndSwapObject(this, STACK, cmp, val);
    }

    /** Returns true if successfully pushed c onto stack. */
    final boolean tryPushStack(Completion c) {
        Completion h = stack;
        lazySetNext(c, h);
        return UNSAFE.compareAndSwapObject(this, STACK, h, c);
    }

    final void pushStack(Completion c) {
        do {} while (!tryPushStack(c));
    }

    /* ------------- Encoding and decoding outcomes -------------- */

    static final class AltResult { // See above
        final Throwable ex;        // null only for NIL
        AltResult(Throwable x) { this.ex = x; }
    }

    /** The encoding of the null value. */
    static final AltResult NIL = new AltResult(null);

    /** Completes with the null value, unless already completed. */
    final boolean completeNull() {
        return UNSAFE.compareAndSwapObject(this, RESULT, null,
                                           NIL);
    }

    /** Returns the encoding of the given non-exceptional value. */
    final Object encodeValue(T t) {
        return (t == null) ? NIL : t;
    }

    /** Completes with a non-exceptional result, unless already completed. */
    final boolean completeValue(T t) {
        return UNSAFE.compareAndSwapObject(this, RESULT, null,
                                           (t == null) ? NIL : t);
    }

    static AltResult encodeThrowable(Throwable x) {
        return new AltResult((x instanceof CompletionException) ? x :
                             new CompletionException(x));
    }

    /** Completes with an exceptional result, unless already completed. */
    final boolean completeThrowable(Throwable x) {
        return UNSAFE.compareAndSwapObject(this, RESULT, null,
                                           encodeThrowable(x));
    }

    static Object encodeThrowable(Throwable x, Object r) {
        if (!(x instanceof CompletionException))
            x = new CompletionException(x);
        else if (r instanceof AltResult && x == ((AltResult)r).ex)
            return r;
        return new AltResult(x);
    }

    final boolean completeThrowable(Throwable x, Object r) {
        return UNSAFE.compareAndSwapObject(this, RESULT, null,
                                           encodeThrowable(x, r));
    }

    Object encodeOutcome(T t, Throwable x) {
        return (x == null) ? (t == null) ? NIL : t : encodeThrowable(x);
    }

    static Object encodeRelay(Object r) {
        Throwable x;
        return (((r instanceof AltResult) &&
                 (x = ((AltResult)r).ex) != null &&
                 !(x instanceof CompletionException)) ?
                new AltResult(new CompletionException(x)) : r);
    }

    final boolean completeRelay(Object r) {
        return UNSAFE.compareAndSwapObject(this, RESULT, null,
                                           encodeRelay(r));
    }

    private static <T> T reportGet(Object r)
        throws InterruptedException, ExecutionException {
        if (r == null) // by convention below, null means interrupted
            throw new InterruptedException();
        if (r instanceof AltResult) {
            Throwable x, cause;
            if ((x = ((AltResult)r).ex) == null)
                return null;
            if (x instanceof CancellationException)
                throw (CancellationException)x;
            if ((x instanceof CompletionException) &&
                (cause = x.getCause()) != null)
                x = cause;
            throw new ExecutionException(x);
        }
        @SuppressWarnings("unchecked") T t = (T) r;
        return t;
    }

    private static <T> T reportJoin(Object r) {
        if (r instanceof AltResult) {
            Throwable x;
            if ((x = ((AltResult)r).ex) == null)
                return null;
            if (x instanceof CancellationException)
                throw (CancellationException)x;
            if (x instanceof CompletionException)
                throw (CompletionException)x;
            throw new CompletionException(x);
        }
        @SuppressWarnings("unchecked") T t = (T) r;
        return t;
    }

    /* ------------- Async task preliminaries -------------- */
    public static interface AsynchronousCompletionTask {
    }

    private static final boolean useCommonPool =
        (ForkJoinPool.getCommonPoolParallelism() > 1);

    private static final Executor asyncPool = useCommonPool ?
        ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();

    /** Fallback if ForkJoinPool.commonPool() cannot support parallelism */
    static final class ThreadPerTaskExecutor implements Executor {
        public void execute(Runnable r) { new Thread(r).start(); }
    }

    static Executor screenExecutor(Executor e) {
        if (!useCommonPool && e == ForkJoinPool.commonPool())
            return asyncPool;
        if (e == null) throw new NullPointerException();
        return e;
    }

    // Modes for Completion.tryFire. Signedness matters.
    static final int SYNC   =  0;
    static final int ASYNC  =  1;
    static final int NESTED = -1;

    /* ------------- Base Completion classes and operations -------------- */

    @SuppressWarnings("serial")
    abstract static class Completion extends ForkJoinTask<Void>
        implements Runnable, AsynchronousCompletionTask {
        volatile Completion next;      // Treiber stack link

        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) {}
    }

    static void lazySetNext(Completion c, Completion next) {
        UNSAFE.putOrderedObject(c, NEXT, next);
    }

    final void postComplete() {
        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;
            }
        }
    }

    /** Traverses stack and unlinks dead Completions. */
    final void cleanStack() {
        for (Completion p = null, q = stack; q != null;) {
            Completion s = q.next;
            if (q.isLive()) {
                p = q;
                q = s;
            }
            else if (p == null) {
                casStack(q, s);
                q = stack;
            }
            else {
                p.next = s;
                if (p.isLive())
                    q = s;
                else {
                    p = null;  // restart
                    q = stack;
                }
            }
        }
    }

    /* ------------- One-input Completions -------------- */

    /** A Completion with a source, dependent, and executor. */
    @SuppressWarnings("serial")
    abstract static class UniCompletion<T,V> extends Completion {
        Executor executor;                 // executor to use (null if none)
        CompletableFuture<V> dep;          // the dependent to complete
        CompletableFuture<T> src;          // source for action

        UniCompletion(Executor executor, CompletableFuture<V> dep,
                      CompletableFuture<T> src) {
            this.executor = executor; this.dep = dep; this.src = src;
        }

        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;
        }

        final boolean isLive() { return dep != null; }
    }

    /** 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 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;
    }

    @SuppressWarnings("serial")
    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;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d; CompletableFuture<T> a;
            if ((d = dep) == null ||
                !d.uniApply(a = src, fn, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final <S> boolean uniApply(CompletableFuture<S> a,
                               Function<? super S,? extends T> f,
                               UniApply<S,T> c) {
        Object r; Throwable x;
        if (a == null || (r = a.result) == null || f == null)
            return false;
        tryComplete: if (result == null) {
            if (r instanceof AltResult) {
                if ((x = ((AltResult)r).ex) != null) {
                    completeThrowable(x, r);
                    break tryComplete;
                }
                r = null;
            }
            try {
                if (c != null && !c.claim())
                    return false;
                @SuppressWarnings("unchecked") S s = (S) r;
                completeValue(f.apply(s));
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private <V> CompletableFuture<V> uniApplyStage(
        Executor e, Function<? super T,? extends V> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<V> d =  new CompletableFuture<V>();
        if (e != null || !d.uniApply(this, f, null)) {
            UniApply<T,V> c = new UniApply<T,V>(e, d, this, f);
            push(c);
            c.tryFire(SYNC);
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniAccept<T> extends UniCompletion<T,Void> {
        Consumer<? super T> fn;
        UniAccept(Executor executor, CompletableFuture<Void> dep,
                  CompletableFuture<T> src, Consumer<? super T> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d; CompletableFuture<T> a;
            if ((d = dep) == null ||
                !d.uniAccept(a = src, fn, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final <S> boolean uniAccept(CompletableFuture<S> a,
                                Consumer<? super S> f, UniAccept<S> c) {
        Object r; Throwable x;
        if (a == null || (r = a.result) == null || f == null)
            return false;
        tryComplete: if (result == null) {
            if (r instanceof AltResult) {
                if ((x = ((AltResult)r).ex) != null) {
                    completeThrowable(x, r);
                    break tryComplete;
                }
                r = null;
            }
            try {
                if (c != null && !c.claim())
                    return false;
                @SuppressWarnings("unchecked") S s = (S) r;
                f.accept(s);
                completeNull();
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private CompletableFuture<Void> uniAcceptStage(Executor e,
                                                   Consumer<? super T> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<Void> d = new CompletableFuture<Void>();
        if (e != null || !d.uniAccept(this, f, null)) {
            UniAccept<T> c = new UniAccept<T>(e, d, this, f);
            push(c);
            c.tryFire(SYNC);
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniRun<T> extends UniCompletion<T,Void> {
        Runnable fn;
        UniRun(Executor executor, CompletableFuture<Void> dep,
               CompletableFuture<T> src, Runnable fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<Void> tryFire(int mode) {
            CompletableFuture<Void> d; CompletableFuture<T> a;
            if ((d = dep) == null ||
                !d.uniRun(a = src, fn, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final boolean uniRun(CompletableFuture<?> a, Runnable f, UniRun<?> c) {
        Object r; Throwable x;
        if (a == null || (r = a.result) == null || f == null)
            return false;
        if (result == null) {
            if (r instanceof AltResult && (x = ((AltResult)r).ex) != null)
                completeThrowable(x, r);
            else
                try {
                    if (c != null && !c.claim())
                        return false;
                    f.run();
                    completeNull();
                } catch (Throwable ex) {
                    completeThrowable(ex);
                }
        }
        return true;
    }

    private CompletableFuture<Void> uniRunStage(Executor e, Runnable f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<Void> d = new CompletableFuture<Void>();
        if (e != null || !d.uniRun(this, f, null)) {
            UniRun<T> c = new UniRun<T>(e, d, this, f);
            push(c);
            c.tryFire(SYNC);
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniWhenComplete<T> extends UniCompletion<T,T> {
        BiConsumer<? super T, ? super Throwable> fn;
        UniWhenComplete(Executor executor, CompletableFuture<T> dep,
                        CompletableFuture<T> src,
                        BiConsumer<? super T, ? super Throwable> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<T> tryFire(int mode) {
            CompletableFuture<T> d; CompletableFuture<T> a;
            if ((d = dep) == null ||
                !d.uniWhenComplete(a = src, fn, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final boolean uniWhenComplete(CompletableFuture<T> a,
                                  BiConsumer<? super T,? super Throwable> f,
                                  UniWhenComplete<T> c) {
        Object r; T t; Throwable x = null;
        if (a == null || (r = a.result) == null || f == null)
            return false;
        if (result == null) {
            try {
                if (c != null && !c.claim())
                    return false;
                if (r instanceof AltResult) {
                    x = ((AltResult)r).ex;
                    t = null;
                } else {
                    @SuppressWarnings("unchecked") T tr = (T) r;
                    t = tr;
                }
                f.accept(t, x);
                if (x == null) {
                    internalComplete(r);
                    return true;
                }
            } catch (Throwable ex) {
                if (x == null)
                    x = ex;
            }
            completeThrowable(x, r);
        }
        return true;
    }

    private CompletableFuture<T> uniWhenCompleteStage(
        Executor e, BiConsumer<? super T, ? super Throwable> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<T> d = new CompletableFuture<T>();
        if (e != null || !d.uniWhenComplete(this, f, null)) {
            UniWhenComplete<T> c = new UniWhenComplete<T>(e, d, this, f);
            push(c);
            c.tryFire(SYNC);
        }
        return d;
    }

    @SuppressWarnings("serial")
    static final class UniHandle<T,V> extends UniCompletion<T,V> {
        BiFunction<? super T, Throwable, ? extends V> fn;
        UniHandle(Executor executor, CompletableFuture<V> dep,
                  CompletableFuture<T> src,
                  BiFunction<? super T, Throwable, ? extends V> fn) {
            super(executor, dep, src); this.fn = fn;
        }
        final CompletableFuture<V> tryFire(int mode) {
            CompletableFuture<V> d; CompletableFuture<T> a;
            if ((d = dep) == null ||
                !d.uniHandle(a = src, fn, mode > 0 ? null : this))
                return null;
            dep = null; src = null; fn = null;
            return d.postFire(a, mode);
        }
    }

    final <S> boolean uniHandle(CompletableFuture<S> a,
                                BiFunction<? super S, Throwable, ? extends T> f,
                                UniHandle<S,T> c) {
        Object r; S s; Throwable x;
        if (a == null || (r = a.result) == null || f == null)
            return false;
        if (result == null) {
            try {
                if (c != null && !c.claim())
                    return false;
                if (r instanceof AltResult) {
                    x = ((AltResult)r).ex;
                    s = null;
                } else {
                    x = null;
                    @SuppressWarnings("unchecked") S ss = (S) r;
                    s = ss;
                }
                completeValue(f.apply(s, x));
            } catch (Throwable ex) {
                completeThrowable(ex);
            }
        }
        return true;
    }

    private <V> CompletableFuture<V> uniHandleStage(
        Executor e, BiFunction<? super T, Throwable, ? extends V> f) {
        if (f == null) throw new NullPointerException();
        CompletableFuture<V> d = new CompletableFuture<V>();
        if (e != null || !d.uniHandle(this, f, null)) {
            UniHandle<T,V> c = new UniHandle<T,V>(e,