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
    ...
}

abstract static class Completion extends ForkJoinTask<Void>
    implements Runnable, AsynchronousCompletionTask {
    volatile Completion next;      // Treiber stack link
    ...
}

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
    ...
}

public static void main(String[] args) {
    CompletableFuture<String> baseFuture = CompletableFuture.completedFuture("Base Future");
    log.info(baseFuture.thenApply((r) -> r + " Then Apply").join());
    baseFuture.thenAccept((r) -> log.info(r)).thenAccept((Void) -> log.info("Void"));
}

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();
    // 創(chuàng)建一個(gè)新的CompletableFuture對象
    CompletableFuture<V> d =  new CompletableFuture<V>();
    // e：如果是異步調(diào)用直接執(zhí)行代碼塊
    // !d.uniApply：執(zhí)行任務(wù)，如果返回false則任務(wù)未執(zhí)行需入棧
    if (e != null || !d.uniApply(this, f, null)) {
        UniApply<T,V> c = new UniApply<T,V>(e, d, this, f);
        // 創(chuàng)建出新的UniApply對象進(jìn)行入棧
        push(c);
        // 嘗試執(zhí)行任務(wù)
        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;
    // 任務(wù)未完成結(jié)果為null直接返回false
    if (a == null || (r = a.result) == null || f == null)
        return false;
    // 驗(yàn)證是否出現(xiàn)異常結(jié)果，如有則任務(wù)執(zhí)行結(jié)束
    tryComplete: if (result == null) {
        if (r instanceof AltResult) {
            if ((x = ((AltResult)r).ex) != null) {
                completeThrowable(x, r);
                break tryComplete;
            }
            r = null;
        }
        try {
            // 異步執(zhí)行任務(wù)
            if (c != null && !c.claim())
                // 任務(wù)未執(zhí)行返回false
                return false;
            @SuppressWarnings("unchecked") S s = (S) r;
            // 任務(wù)執(zhí)行完成將結(jié)果寫入result
            completeValue(f.apply(s));
        } catch (Throwable ex) {
            completeThrowable(ex);
        }
    }
    return true;
}

public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action) {
    return uniAcceptStage(asyncPool, action);
}

private CompletableFuture<Void> uniAcceptStage(Executor e, Consumer<? super T> f) {
    if (f == null) throw new NullPointerException();
    CompletableFuture<Void> d = new CompletableFuture<Void>();
    // 如果是異步任務(wù)，這里的參數(shù)e不會為空，也就是會將任務(wù)執(zhí)行壓入棧頂
    if (e != null || !d.uniAccept(this, f, null)) {
        UniAccept<T> c = new UniAccept<T>(e, d, this, f);
        push(c);
        // 重點(diǎn)還是這個(gè)入口
        c.tryFire(SYNC);
    }
    return d;
}

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;
        // dep為空即任務(wù)已被執(zhí)行過，直接返回null
        // uniAccept()結(jié)果為false，可能是任務(wù)執(zhí)行中未完成，也可能是由線程池中的其他任務(wù)執(zhí)行完成
        if ((d = dep) == null || !d.uniAccept(a = src, fn, mode > 0 ? null : this))
            return null;
        dep = null; src = null; fn = null;
        // 說明當(dāng)前線程執(zhí)行了該任務(wù)，返回結(jié)果繼續(xù)執(zhí)行前一個(gè)任務(wù)
        return d.postFire(a, mode);
    }
}

final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {
    if (a != null && a.stack != null) {
        // postComplete調(diào)用過來的，或者上一個(gè)任務(wù)執(zhí)行完成，清空棧數(shù)據(jù)，否則調(diào)用postComplete完成任務(wù)
        if (mode < 0 || a.result == null)
            a.cleanStack();
        else
            // 完成任務(wù)執(zhí)行并進(jìn)行出棧
            a.postComplete();
    }
    if (result != null && stack != null) {
        if (mode < 0)
            // postComplete調(diào)用過來的任務(wù)已完成
            return this;
        else
            // 完成任務(wù)執(zhí)行并進(jìn)行出棧
            postComplete();
    }
    return null;
}