来自 Callable 的 CompletableFuture？

Question

今天我尝试了 Java 8 中的“新”CompletableFuture，当我没有找到 runAsync(Callable) 方法时，我感到很困惑。 我可以自己做，如下图所示，但是为什么（对我来说非常明显和有用的实用方法）丢失了？ 我错过了什么吗？

public static <T> CompletableFuture<T> asFuture(Callable<? extends T> callable, Executor executor) {
    CompletableFuture<T> future = new CompletableFuture<>();
    executor.execute(() -> {
        try {
            future.complete(callable.call());
        } catch (Throwable t) {
            future.completeExceptionally(t);
        }
    });
    return future;
}

Answer 1

你应该使用supplyAsync(Supplier<U>)

一般来说，lambdas 和检查异常不能很好地协同工作，CompletableFuture 通过设计避免检查异常。虽然在你的情况下应该没问题。

相关话题：

http://cs.oswego.edu/pipermail/concurrency-interest/2012-December/010486.html

http://cs.oswego.edu/pipermail/concurrency-interest/2014-August/012911.html

Answer 2

对于那些需要问题中提供的功能的人：我对其进行了一些更改，以理想地反映现有 CompletableFuture.runAsync 和 CompletableFuture.supplyAsync 函数的行为。

// Java 8 (Java 9+ version below)
import java.util.concurrent.Callable;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;
import java.util.function.Supplier;

/**
 * {@link CompletableFuture} utils.
 *
 * @author stonar96
 *
 * @see CompletableFuture
 */
public final class CompletableFutureUtils {
    /**
     * Delegates the given Callable to
     * {@link CompletableFuture#supplyAsync(Supplier)}, handles checked exceptions
     * accordingly to unchecked exceptions and associates a new CompletableFuture.
     *
     * @param callable a function returning the value to be used to complete the
     *                 returned CompletableFuture
     * @param          <U> the function's return type
     * @return the new associated CompletableFuture
     * @see CompletableFuture#supplyAsync(Supplier)
     */
    public static <U> CompletableFuture<U> callAsync(Callable<U> callable) {
        return completeAsync(new CompletableFuture<>(), callable);
    }

    /**
     * Delegates the given Callable to
     * {@link CompletableFuture#supplyAsync(Supplier)}, handles checked exceptions
     * accordingly to unchecked exceptions and associates the given
     * CompletableFuture.
     *
     * @param result   the CompletableFuture to be associated
     * @param callable a function returning the value to be used to complete the
     *                 returned CompletableFuture
     * @param          <T> the function's return type
     * @return the given associated CompletableFuture
     * @see CompletableFuture#supplyAsync(Supplier)
     */
    public static <T> CompletableFuture<T> completeAsync(CompletableFuture<T> result, Callable<? extends T> callable) {
        if (result == null) {
            throw new NullPointerException();
        }

        CompletableFuture<T> delegate = CompletableFuture.supplyAsync(callable == null ? null : () -> {
            try {
                return callable.call();
            } catch (Throwable t) {
                if (t instanceof Error) {
                    throw (Error) t;
                }

                if (t instanceof RuntimeException) {
                    throw (RuntimeException) t;
                }

                result.completeExceptionally(t);
            }

            return null;
        });

        if (delegate == null) {
            return null;
        }

        result.whenComplete((v, t) -> {
            if (t == null) {
                delegate.complete(v);
                return;
            }

            delegate.completeExceptionally(t);
        });
        delegate.whenComplete((v, t) -> {
            if (t == null) {
                result.complete(v);
                return;
            }

            result.completeExceptionally(t);
        });
        return result;
    }

    /**
     * Delegates the given Callable and Executor to
     * {@link CompletableFuture#supplyAsync(Supplier, Executor)}, handles checked
     * exceptions accordingly to unchecked exceptions and associates a new
     * CompletableFuture.
     *
     * @param callable 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 associated CompletableFuture
     * @see CompletableFuture#supplyAsync(Supplier, Executor)
     */
    public static <U> CompletableFuture<U> callAsync(Callable<U> callable, Executor executor) {
        return completeAsync(new CompletableFuture<>(), callable, executor);
    }

    /**
     * Delegates the given Callable and Executor to
     * {@link CompletableFuture#supplyAsync(Supplier, Executor)}, handles checked
     * exceptions accordingly to unchecked exceptions and associates the given
     * CompletableFuture.
     *
     * @param result   the CompletableFuture to be associated
     * @param callable a function returning the value to be used to complete the
     *                 returned CompletableFuture
     * @param executor the executor to use for asynchronous execution
     * @param          <T> the function's return type
     * @return the given associated CompletableFuture
     * @see CompletableFuture#supplyAsync(Supplier, Executor)
     */
    public static <T> CompletableFuture<T> completeAsync(CompletableFuture<T> result, Callable<? extends T> callable, Executor executor) {
        if (result == null) {
            throw new NullPointerException();
        }

        CompletableFuture<T> delegate = CompletableFuture.supplyAsync(callable == null ? null : () -> {
            try {
                return callable.call();
            } catch (Throwable t) {
                if (t instanceof Error) {
                    throw (Error) t;
                }

                if (t instanceof RuntimeException) {
                    throw (RuntimeException) t;
                }

                result.completeExceptionally(t);
            }

            return null;
        }, executor);

        if (delegate == null) {
            return null;
        }

        result.whenComplete((v, t) -> {
            if (t == null) {
                delegate.complete(v);
                return;
            }

            delegate.completeExceptionally(t);
        });
        delegate.whenComplete((v, t) -> {
            if (t == null) {
                result.complete(v);
                return;
            }

            result.completeExceptionally(t);
        });
        return result;
    }

    private CompletableFutureUtils() {
        throw new AssertionError("CompletableFutureUtils cannot be instantiated");
    }
}

// Java 9+
import java.util.concurrent.Callable;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.Executor;
import java.util.function.Supplier;

/**
 * {@link CompletableFuture} utils.
 *
 * @author stonar96
 *
 * @see CompletableFuture
 */
public final class CompletableFutureUtils {
    /**
     * Delegates the given Callable to
     * {@link CompletableFuture#completeAsync(Supplier)} using a new
     * CompletableFuture and handles checked exceptions accordingly to unchecked
     * exceptions.
     *
     * @param callable a function returning the value to be used to complete the
     *                 returned CompletableFuture
     * @param          <U> the function's return type
     * @return the new CompletableFuture
     * @see CompletableFuture#completeAsync(Supplier)
     */
    public static <U> CompletableFuture<U> callAsync(Callable<U> callable) {
        return completeAsync(new CompletableFuture<>(), callable);
    }

    /**
     * Delegates the given Callable to
     * {@link CompletableFuture#completeAsync(Supplier)} using the given
     * CompletableFuture and handles checked exceptions accordingly to unchecked
     * exceptions.
     *
     * @param result   the CompletableFuture to be used
     * @param callable a function returning the value to be used to complete the
     *                 returned CompletableFuture
     * @param          <T> the function's return type
     * @return the given CompletableFuture
     * @see CompletableFuture#completeAsync(Supplier)
     */
    public static <T> CompletableFuture<T> completeAsync(CompletableFuture<T> result, Callable<? extends T> callable) {
        return result.completeAsync(callable == null ? null : () -> {
            try {
                return callable.call();
            } catch (Throwable t) {
                if (t instanceof Error) {
                    throw (Error) t;
                }

                if (t instanceof RuntimeException) {
                    throw (RuntimeException) t;
                }

                result.completeExceptionally(t);
            }

            return null;
        });
    }

    /**
     * Delegates the given Callable and Executor to
     * {@link CompletableFuture#completeAsync(Supplier, Executor)} using a new
     * CompletableFuture and handles checked exceptions accordingly to unchecked
     * exceptions.
     *
     * @param callable 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
     * @see CompletableFuture#completeAsync(Supplier, Executor)
     */
    public static <U> CompletableFuture<U> callAsync(Callable<U> callable, Executor executor) {
        return completeAsync(new CompletableFuture<>(), callable, executor);
    }

    /**
     * Delegates the given Callable and Executor to
     * {@link CompletableFuture#completeAsync(Supplier, Executor)} using the given
     * CompletableFuture and handles checked exceptions accordingly to unchecked
     * exceptions.
     *
     * @param result   the CompletableFuture to be used
     * @param callable a function returning the value to be used to complete the
     *                 returned CompletableFuture
     * @param executor the executor to use for asynchronous execution
     * @param          <T> the function's return type
     * @return the given CompletableFuture
     * @see CompletableFuture#completeAsync(Supplier, Executor)
     */
    public static <T> CompletableFuture<T> completeAsync(CompletableFuture<T> result, Callable<? extends T> callable, Executor executor) {
        return result.completeAsync(callable == null ? null : () -> {
            try {
                return callable.call();
            } catch (Throwable t) {
                if (t instanceof Error) {
                    throw (Error) t;
                }

                if (t instanceof RuntimeException) {
                    throw (RuntimeException) t;
                }

                result.completeExceptionally(t);
            }

            return null;
        }, executor);
    }

    private CompletableFutureUtils() {
        throw new AssertionError("CompletableFutureUtils cannot be instantiated");
    }
}

如您所见，一切都按原样委托，除了必须处理的已检查异常。