JavaCompletionStage沒有將例外傳播到例外

我在我的一個專案中使用異步編程。在回傳型別為 CompletionStage 的方法之一中，為了進行測驗，我正在模擬一個例外拋出，該例外不會被例外（）捕獲。下面是我根據用例創建的一些測驗代碼。

public class TestMain {
  
  TestService service = new TestService();
  
  public static void main(String[] args) throws InterruptedException {
    TestMain t = new TestMain();
    System.out.println("STARTED: THREAD "   Thread.currentThread().getName());
    t.execute("test").toCompletableFuture().join();
    System.out.println("DONE: THREAD "   Thread.currentThread().getName());
  }
  
  public CompletionStage<Void> execute(String s) {
    System.out.println("EXECUTE: THREAD "   Thread.currentThread().getName());
    return CompletableFuture.runAsync(() -> {
      System.out.println("EXECUTE-ASYNC STARTED: THREAD "   Thread.currentThread().getName());
      process(s);}).exceptionally(ex->{
        System.out.println("EXECUTE-ASYNC EXCEPTIONAL: THREAD "   Thread.currentThread().getName());
        System.err.println("EXECUTE-ASYNC EXCEPTIONAL-EXCEPTION "   ex.toString());
        if (ex.getCause()!=null)
          System.err.println("EXECUTE-ASYNC EXCEPTIONAL-EXCEPTION CAUSE "   ex.getCause().toString());
        throw ex instanceof CompletionException ? (CompletionException) ex : new CompletionException(ex);
      });
  }
  
  private CompletionStage<Void> process(String s) {
    System.out.println("EXECUTE-PROCESS: THREAD "   Thread.currentThread().getName());
//    throw new RuntimeException();
    return service.service(s);
  }

}

public class TestService {
  
  public CompletionStage<Void> service(String s) {
    System.out.println("SERVICE: THREAD "   Thread.currentThread().getName());
//    throw new RuntimeException();
    return preProcess(s).thenCompose(v -> {
      System.out.println("SERVICE-PRE PROCESS -> COMPOSE: THREAD "   Thread.currentThread().getName());
      return process(s);
    });
  }
  
  private CompletionStage<Void> preProcess(String s) {
    System.out.println("SERVICE-PRE PROCESS: THREAD "   Thread.currentThread().getName());
//    throw new RuntimeException();
    return CompletableFuture.completedFuture(null);
  }

  private CompletionStage<Void> process(String s) {
    System.out.println("SERVICE-PROCESS: THREAD "   Thread.currentThread().getName());
    throw new RuntimeException();
//    return CompletableFuture.completedFuture(null);
  }
  
}

當我運行上面給出的代碼時，輸??出顯示 TestMain.execute() 中的 exceptionly() 沒有被呼叫。

實際輸出（當從 TestService.process() 拋出例外時）：

STARTED: THREAD main
EXECUTE: THREAD main
EXECUTE-ASYNC STARTED: THREAD ForkJoinPool.commonPool-worker-1
EXECUTE-PROCESS: THREAD ForkJoinPool.commonPool-worker-1
SERVICE: THREAD ForkJoinPool.commonPool-worker-1
SERVICE-PRE PROCESS: THREAD ForkJoinPool.commonPool-worker-1
SERVICE-PRE PROCESS -> COMPOSE: THREAD ForkJoinPool.commonPool-worker-1
SERVICE-PROCESS: THREAD ForkJoinPool.commonPool-worker-1
DONE: THREAD main

但是，如果我在注釋代碼標記的任何其他地方拋出例外，則會呼叫 exceptionly()。下面是一個這樣的例子。

實際輸出（當從 TestService.preProcess() 拋出例外時）：期望上述輸出與此類似。

STARTED: THREAD main
EXECUTE: THREAD main
EXECUTE-ASYNC STARTED: THREAD ForkJoinPool.commonPool-worker-1
EXECUTE-PROCESS: THREAD ForkJoinPool.commonPool-worker-1
SERVICE: THREAD ForkJoinPool.commonPool-worker-1
SERVICE-PRE PROCESS: THREAD ForkJoinPool.commonPool-worker-1
EXECUTE-ASYNC EXCEPTIONAL: THREAD main
EXECUTE-ASYNC EXCEPTIONAL-EXCEPTION java.util.concurrent.CompletionException: java.lang.RuntimeException
EXECUTE-ASYNC EXCEPTIONAL-EXCEPTION CAUSE java.lang.RuntimeException
Exception in thread "main" java.util.concurrent.CompletionException: java.lang.RuntimeException
    at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
    at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
    at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1643)
    at java.util.concurrent.CompletableFuture$AsyncRun.exec(CompletableFuture.java:1632)
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1067)
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1703)
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:172)
Caused by: java.lang.RuntimeException
    at in.TestService.preProcess(TestService.java:19)
    at in.TestService.service(TestService.java:11)
    at in.TestMain.process(TestMain.java:34)
    at in.TestMain.lambda$0(TestMain.java:22)
    at java.util.concurrent.CompletableFuture$AsyncRun.run(CompletableFuture.java:1640)
    ... 5 more

有人可以幫忙解釋為什么會這樣嗎？我需要在 TestMain.execute().exceptionalally() 中捕獲拋出的例外，以實作我想在這里使用的重試邏輯。

提前致謝。

uj5u.com熱心網友回復：

您正在呼叫一個回傳 aCompletionStage但忽略回傳值的方法：

CompletableFuture.runAsync(() -> {
  System.out.println("EXECUTE-ASYNC STARTED: THREAD " Thread.currentThread().getName());
  process(s); // return value ignored
})

CompletableFuture.runAsync(…)如果函式/拋出例外，由創建的階段將例外完成Runnable。當您放入方法中時會發生這種throw new RuntimeException();情況process。但在另一種情況下，它不會拋出例外，而是回傳呼叫者忽略的失敗階段。

您可以將代碼更改為

CompletableFuture.runAsync(() -> {
  System.out.println("EXECUTE-ASYNC STARTED: THREAD " Thread.currentThread().getName());
  process(s).toCompletableFuture().join();
})

不要忽略回傳的階段，或者最好使用您已經在其他地方使用過的模式：

CompletableFuture.completedFuture(null).thenComposeAsync(v -> {
  System.out.println("EXECUTE-ASYNC STARTED: THREAD " Thread.currentThread().getName());
  return process(s);
})

標籤：异步 例外 java-8 完成阶段

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