以功能方式处理异常的更好方法

java java-8 java-stream

2022-08-31 16:28:17

异常，特别是经过检查的异常，当在 Java 8 中使用 FP 成语时，可能会严重中断程序逻辑的流程。下面是一个任意示例：

String s1 = "oeu", s2 = "2";
Stream.of(s1, s2).forEach(s -> 
    System.out.println(Optional.of(s).map(Integer::parseInt).get()));

当无法解析的字符串出现异常时，上述代码将中断。但是说我只想用默认值替换它，就像我可以用：Optional

Stream.of(s1, s2).forEach(s -> 
   System.out.println(Optional.of(s)
                              .map(Integer::parseInt)
                              .orElse(-1)));

当然，这仍然会失败，因为只处理 s。我想要一些如下的东西：Optionalnull

Stream.of(s1, s2).forEach(s ->
    System.out.println(
        Exceptional.of(s)
                   .map(Integer::parseInt)
                   .handle(NumberFormatException.class, swallow())
                   .orElse(-1)));

注意：这是一个自我回答的问题。

答案 1

下面显示的是该类的完整代码。它有一个相当大的API，它是API的纯粹扩展，因此它可以在任何现有代码中直接替换它 - 除了它不是最终类的子类型。该类可以被视为与 Try monad 的关系与与 monad 的关系相同：它从中汲取灵感，但适应 Java 习语（例如实际抛出异常，甚至来自非终端操作）。ExceptionalOptionalOptionalOptionalMaybe

以下是该类遵循的一些关键准则：

与一元方法相反，不会忽略Java的异常机制;
相反，它减轻了异常和高阶函数之间的阻抗不匹配;
异常处理不是静态类型安全的（由于偷偷摸摸的抛出），但在运行时始终安全（除非在显式请求下，否则从不吞噬异常）。

该类尝试涵盖处理异常的所有典型方法：

recover使用一些提供替代值的处理代码;
flatRecover这类似于，允许返回一个新实例，该实例将被解包并适当更新当前实例的状态;flatMapExceptional
propagate一个异常，从表达式中抛出它并使调用声明此异常类型;Exceptionalpropagate
propagate在包装到另一个例外（翻译它）之后;
handle它，导致空Exceptional;
作为处理的特殊情况，它具有空的处理程序块。swallow

该方法允许人们有选择地从代码中选择要公开的已检查异常。在调用终端操作时仍未处理的异常（如）将被偷偷地抛出而不声明。这通常被认为是一种高级且危险的方法，但仍然经常被用作一种在一定程度上减轻已检查异常与未声明它们的lambda形状相结合的麻烦的方法。该课程希望提供一种更干净，更有选择性的替代方案，而不是偷偷摸摸的投掷。propagategetExceptional

/*
 * Copyright (c) 2015, Marko Topolnik. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;

public final class Exceptional<T>
{
  private final T value;
  private final Throwable exception;

  private Exceptional(T value, Throwable exc) {
    this.value = value;
    this.exception = exc;
  }

  public static <T> Exceptional<T> empty() {
    return new Exceptional<>(null, null);
  }

  public static <T> Exceptional<T> ofNullable(T value) {
    return value != null ? of(value) : empty();
  }

  public static <T> Exceptional<T> of(T value) {
    return new Exceptional<>(Objects.requireNonNull(value), null);
  }

  public static <T> Exceptional<T> ofNullableException(Throwable exception) {
    return exception != null? new Exceptional<>(null, exception) : empty();
  }

  public static <T> Exceptional<T> ofException(Throwable exception) {
    return new Exceptional<>(null, Objects.requireNonNull(exception));
  }

  public static <T> Exceptional<T> from(TrySupplier<T> supplier) {
    try {
      return ofNullable(supplier.tryGet());
    } catch (Throwable t) {
      return new Exceptional<>(null, t);
    }
  }

  public static Exceptional<Void> fromVoid(TryRunnable task) {
    try {
      task.run();
      return new Exceptional<>(null, null);
    } catch (Throwable t) {
      return new Exceptional<>(null, t);
    }
  }

  public static <E extends Throwable> Consumer<? super E> swallow() {
    return e -> {};
  }

  public T get() {
    if (value != null) return value;
    if (exception != null) sneakyThrow(exception);
    throw new NoSuchElementException("No value present");
  }

  public T orElse(T other) {
    if (value != null) return value;
    if (exception != null) sneakyThrow(exception);
    return other;
  }

  public T orElseGet(Supplier<? extends T> other) {
    if (value != null) return value;
    if (exception != null) sneakyThrow(exception);
    return other.get();
  }

  public Stream<T> stream() { 
      return value == null ? Stream.empty() : Stream.of(value); 
  }

  public<U> Exceptional<U> map(Function<? super T, ? extends U> mapper) {
    Objects.requireNonNull(mapper);
    if (value == null) return new Exceptional<>(null, exception);
    final U u;
    try {
      u = mapper.apply(value);
    } catch (Throwable exc) {
      return new Exceptional<>(null, exc);
    }
    return ofNullable(u);
  }

  public<U> Exceptional<U> flatMap(Function<? super T, Exceptional<U>> mapper) {
    Objects.requireNonNull(mapper);
    return value != null ? Objects.requireNonNull(mapper.apply(value)) : empty();
  }

  public Exceptional<T> filter(Predicate<? super T> predicate) {
    Objects.requireNonNull(predicate);
    if (value == null) return this;
    final boolean b;
    try {
      b = predicate.test(value);
    } catch (Throwable t) {
      return ofException(t);
    }
    return b ? this : empty();
  }

  public <X extends Throwable> Exceptional<T> recover(
      Class<? extends X> excType, Function<? super X, T> mapper)
  {
    Objects.requireNonNull(mapper);
    return excType.isInstance(exception) ? ofNullable(mapper.apply(excType.cast(exception))) : this;
  }

  public <X extends Throwable> Exceptional<T> recover(
      Iterable<Class<? extends X>> excTypes, Function<? super X, T> mapper)
  {
    Objects.requireNonNull(mapper);
    for (Class<? extends X> excType : excTypes)
      if (excType.isInstance(exception))
        return ofNullable(mapper.apply(excType.cast(exception)));
    return this;
  }

  public <X extends Throwable> Exceptional<T> flatRecover(
      Class<? extends X> excType, Function<? super X, Exceptional<T>> mapper)
  {
    Objects.requireNonNull(mapper);
    return excType.isInstance(exception) ? Objects.requireNonNull(mapper.apply(excType.cast(exception))) : this;
  }

  public <X extends Throwable> Exceptional<T> flatRecover(
      Iterable<Class<? extends X>> excTypes, Function<? super X, Exceptional<T>> mapper)
  {
    Objects.requireNonNull(mapper);
    for (Class<? extends X> c : excTypes)
      if (c.isInstance(exception))
        return Objects.requireNonNull(mapper.apply(c.cast(exception)));
    return this;
  }

  public <E extends Throwable> Exceptional<T> propagate(Class<E> excType) throws E {
    if (excType.isInstance(exception))
      throw excType.cast(exception);
    return this;
  }

  public <E extends Throwable> Exceptional<T> propagate(Iterable<Class<? extends E>> excTypes) throws E {
    for (Class<? extends E> excType : excTypes)
      if (excType.isInstance(exception))
        throw excType.cast(exception);
    return this;
  }

  public <E extends Throwable, F extends Throwable> Exceptional<T> propagate(
      Class<E> excType, Function<? super E, ? extends F> translator)
  throws F
  {
    if (excType.isInstance(exception))
      throw translator.apply(excType.cast(exception));
    return this;
  }

  public <E extends Throwable, F extends Throwable> Exceptional<T> propagate(
      Iterable<Class<E>> excTypes, Function<? super E, ? extends F> translator)
  throws F
  {
    for (Class<? extends E> excType : excTypes)
      if (excType.isInstance(exception))
        throw translator.apply(excType.cast(exception));
    return this;
  }

  public <E extends Throwable> Exceptional<T> handle(Class<E> excType, Consumer<? super E> action) {
    if (excType.isInstance(exception)) {
      action.accept(excType.cast(exception));
      return empty();
    }
    return this;
  }

  public <E extends Throwable> Exceptional<T> handle(Iterable<Class<E>> excTypes, Consumer<? super E> action) {
    for (Class<? extends E> excType : excTypes)
      if (excType.isInstance(exception)) {
        action.accept(excType.cast(exception));
        return empty();
      }
    return this;
  }

  public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
    if (value != null) return value;
    if (exception != null) sneakyThrow(exception);
    throw exceptionSupplier.get();
  }

  public boolean isPresent() {
    return value != null;
  }

  public void ifPresent(Consumer<? super T> consumer) {
    if (value != null)
      consumer.accept(value);
    if (exception != null) sneakyThrow(exception);
  }

  public boolean isException() {
    return exception != null;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) return true;
    return obj instanceof Exceptional && Objects.equals(value, ((Exceptional)obj).value);
  }

  @Override
  public int hashCode() {
    return Objects.hashCode(value);
  }

  @SuppressWarnings("unchecked")
  private static <T extends Throwable> void sneakyThrow(Throwable t) throws T {
    throw (T) t;
  }
}

@FunctionalInterface
public interface TrySupplier<T> {
  T tryGet() throws Throwable;
}

@FunctionalInterface
public interface TryRunnable {
  void run() throws Throwable;
}

答案 2

如果允许提供的每个功能接口引发异常，该怎么办？java.util.function

public interface ThrowingSupplier<R, X extends Throwable> {
    public R get() throws X;
}

我们可以使用一些默认方法来提供所需的行为。

您可以回退到某些默认值或操作
或者，您可以尝试执行其他可能引发异常的操作

我写了一个库，以这种方式重新定义了大多数接口。我甚至提供了一个ThrowingStream，它允许您使用这些新接口，这些新接口与常规的API相同。java.util.functionStream

@FunctionalInterface
public interface ThrowingSupplier<R, X extends Throwable> {
    public R get() throws X;

    default public Supplier<R> fallbackTo(Supplier<? extends R> supplier) {
        ThrowingSupplier<R, Nothing> t = supplier::get;
        return orTry(t)::get;
    }

    default public <Y extends Throwable> ThrowingSupplier<R, Y> orTry(
            ThrowingSupplier<? extends R, ? extends Y> supplier) {
        Objects.requireNonNull(supplier, "supplier");
        return () -> {
            try {
                return get();
            } catch (Throwable x) {
                try {
                    return supplier.get();
                } catch (Throwable y) {
                    y.addSuppressed(x);
                    throw y;
                }
            }
        };
    }
}

(没有什么是永远不能扔掉的。RuntimeException

您的原始示例将变为

ThrowingFunction<String, Integer, NumberFormatException> parse = Integer::parseInt;
Function<String, Optional<Integer>> safeParse = parse.fallbackTo(s -> null)
    .andThen(Optional::ofNullable);
Stream.of(s1, s2)
    .map(safeParse)
    .map(i -> i.orElse(-1))
    .forEach(System.out::println);