Java CompletableFuture实现原理分析详解

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();
    // 创建一个新的CompletableFuture对象
    CompletableFuture<V> d =  new CompletableFuture<V>();
    // e：如果是异步调用直接执行代码块
    // !d.uniApply：执行任务，如果返回false则任务未执行需入栈
    if (e != null || !d.uniApply(this, f, null)) {
        UniApply<T,V> c = new UniApply<T,V>(e, d, this, f);
        // 创建出新的UniApply对象进行入栈
        push(c);
        // 尝试执行任务
        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;
    // 任务未完成结果为null直接返回false
    if (a == null || (r = a.result) == null || f == null)
        return false;
    // 验证是否出现异常结果，如有则任务执行结束
    tryComplete: if (result == null) {
        if (r instanceof AltResult) {
            if ((x = ((AltResult)r).ex) != null) {
                completeThrowable(x, r);
                break tryComplete;
            }
            r = null;
        }
        try {
            // 异步执行任务
            if (c != null && !c.claim())
                // 任务未执行返回false
                return false;
            @SuppressWarnings("unchecked") S s = (S) r;
            // 任务执行完成将结果写入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>();
    // 如果是异步任务，这里的参数e不会为空，也就是会将任务执行压入栈顶
    if (e != null || !d.uniAccept(this, f, null)) {
        UniAccept<T> c = new UniAccept<T>(e, d, this, f);
        push(c);
        // 重点还是这个入口
        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为空即任务已被执行过，直接返回null
        // uniAccept()结果为false，可能是任务执行中未完成，也可能是由线程池中的其他任务执行完成
        if ((d = dep) == null || !d.uniAccept(a = src, fn, mode > 0 ? null : this))
            return null;
        dep = null; src = null; fn = null;
        // 说明当前线程执行了该任务，返回结果继续执行前一个任务
        return d.postFire(a, mode);
    }
}

final CompletableFuture<T> postFire(CompletableFuture<?> a, int mode) {
    if (a != null && a.stack != null) {
        // postComplete调用过来的，或者上一个任务执行完成，清空栈数据，否则调用postComplete完成任务
        if (mode < 0 || a.result == null)
            a.cleanStack();
        else
            // 完成任务执行并进行出栈
            a.postComplete();
    }
    if (result != null && stack != null) {
        if (mode < 0)
            // postComplete调用过来的任务已完成
            return this;
        else
            // 完成任务执行并进行出栈
            postComplete();
    }
    return null;
}