Concurrency

CompletableFuture: join は失敗をCompletionExceptionで包む

非同期処理が失敗すると、joinは元の例外をそのままではなくCompletionExceptionのcauseとして送出します。

Java 8+

pitfallconcurrencycompletablefutureexceptionasync

01 / 01 proof

SourceCompletableFutureFailures.java

Java 21

package examples.concurrency;

import java.util.concurrent.CompletableFuture;

public final class CompletableFutureFailures {
  private CompletableFutureFailures() {}

  public static String joinFailedFuture() {
    return CompletableFuture.<String>failedFuture(new IllegalArgumentException("invalid input"))
        .join();
  }
}

Source✓Test

TestCompletableFutureFailuresTest.java

Java 21

package examples.concurrency;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertInstanceOf;
import static org.junit.jupiter.api.Assertions.assertThrows;

import java.util.concurrent.CompletionException;
import org.junit.jupiter.api.Test;

class CompletableFutureFailuresTest {
  @Test
  void joinは非同期処理の失敗をCompletionExceptionで包む() {
    CompletionException exception =
        assertThrows(CompletionException.class, CompletableFutureFailures::joinFailedFuture);

    assertInstanceOf(IllegalArgumentException.class, exception.getCause());
    assertEquals("invalid input", exception.getCause().getMessage());
  }
}

01Assertion

このテストで確認できること

failedFutureのjoinはCompletionExceptionを送出する
getCauseで元のIllegalArgumentExceptionを取り出せる

joinを呼ぶ境界ではCompletionExceptionを考慮し、原因例外を確認してアプリケーションの例外方針へ変換します。

OBSERVED RESULT

expected exception: join → CompletionException
actual:   CompletionException
expected cause: IllegalArgumentException
actual:   IllegalArgumentException

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