java - 以功能方式处理异常的更好方法

Question

当在 Java 8 中使用 FP 习语时，异常，尤其是已检查的异常，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，上面的代码会中断。但是假设我只想用默认值替换它，就像我可以使用 Optional:

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

当然，这仍然失败，因为 Optional 只处理 null。我想要以下内容:

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 self 回答的问题。

Answer 1

下面是 Exceptional 类的完整代码。它有一个相当大的 API，它是 Optional API 的纯扩展，因此它可以在任何现有代码中替代它——除了它不是最终 的子类型code>可选类。该类可以看作与 Try 具有相同的关系。 monad as Optional 与 Maybe monad 一起使用:它从中汲取灵感，但适应了 Java 习语(例如实际抛出异常，甚至来自非终端操作) .

这些是类(class)遵循的一些关键准则:

• 与一元方法相反，它不会忽略 Java 的异常机制；

• 反而缓解了异常与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(由于偷偷抛出)，但在运行时始终是安全的(除非明确请求，否则永远不会吞下异常)。

该类试图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，类似于 flatMap，允许返回一个新的 Exceptional 实例，该实例将被解包并适本地返回当前实例的状态已更新；
• propagate 一个异常，将其从 Exceptional 表达式中抛出，并使 propagate 调用声明此异常类型；
• propagate 包装到另一个异常后(翻译 it)；
• handle它，导致一个空的Exceptional;
• 作为处理的一种特殊情况，吞下它用一个空的处理程序 block 。

propagate 方法允许人们有选择地选择他想从他的代码中公开哪些检查异常。在调用终端操作(如 get)时仍未处理的异常将在不声明的情况下偷偷地抛出。这通常被认为是一种先进且危险的方法，但通常被用作一种在某种程度上减轻检查异常与未声明它们的 lambda 形状相结合的滋扰的方法。 Exceptional 类希望为偷偷摸摸的 throw 提供一种更清洁、更有选择性的替代方案。

---

/*
 * 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;
  }
}

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

---

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