java - CompletableFuture#异常地重新抛出已检查的异常

Question

假设我想在遇到特定异常时恢复一些值，否则返回带有异常的失败的 future。我期望这样的事情:

public static void main(String[] args) {
    CompletableFuture
            .supplyAsync(FuturesExample::fetchValue)
            .exceptionally(throwable -> {
                if (throwable instanceof RuntimeException) {
                    return "All good";
                }
                throw throwable; // does not compile
            });
}

public static String fetchValue() {
    // code that potentially throws an exception
    return "value";
}

如果fetchValue函数会抛出checked异常，我想在链式方法中处理它。我尝试了 return throwable 和 throw throwable ，但都没有编译。 CompletableFuture 是否为这种情况提供任何解决方案？我知道作为 exceptionly 方法参数的 Function 接口(interface)不会抛出任何异常 - 在这种情况下我只想返回已经失败的 future。我想找到使用 Java 8 的解决方案。

Answer 1

在这种情况下，不可能接收已检查的异常，因为前一阶段是基于 Supplier 的，不允许抛出已检查的异常。

因此，您可以处理所有未经检查的异常，并为应该不可能的抛出异常引发 AssertionError:

CompletableFuture
    .supplyAsync(FuturesExample::fetchValue)
    .exceptionally(throwable -> {
        if (throwable instanceof RuntimeException) {
            return "All good";
        }
        if(throwable instanceof Error) throw (Error)throwable;
        throw new AssertionError(throwable);
    });

否则，您可能会认为后续阶段以及 join() 的调用者将获得除 CompletionException 和 CancellationException 之外的所有异常无论如何，在 CompletionException 中。例如。当我使用时

public static void main(String[] args) {
    CompletableFuture<String> f = CompletableFuture
        .supplyAsync(FuturesExample::fetchValue)
        .exceptionally(throwable -> {
            if(throwable instanceof RuntimeException) {
                throw (RuntimeException)throwable;
            }
            throw new Error();
        });
    f.whenComplete((s,t) -> {
        if(t != null) {
            System.err.println("in whenComplete handler ");
            t.printStackTrace();
        }
    });
    System.err.println("calling join()");
    f.join();
}
public static String fetchValue() {
    throw new IllegalStateException("a test is going on");
}

我明白了

in whenComplete handler 
java.util.concurrent.CompletionException: java.lang.IllegalStateException: a test is going on
    at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
    at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1702)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1692)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
    at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
    at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
    at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
    at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
Caused by: java.lang.IllegalStateException: a test is going on
    at FuturesExample.fetchValue(FuturesExample.java:23)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1700)
    ... 6 more
calling join()
Exception in thread "main" java.util.concurrent.CompletionException: java.lang.IllegalStateException: a test is going on
    at java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:314)
    at java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:319)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1702)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1692)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
    at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
    at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
    at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
    at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
Caused by: java.lang.IllegalStateException: a test is going on
    at FuturesExample.fetchValue(FuturesExample.java:23)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1700)
    ... 6 more

因此，我可以使用 CompletionException 来包装任意可抛出对象，利用 CompletionException 不会再次包装的事实。所以如果我使用

public static void main(String[] args) {
    CompletableFuture<String> f = CompletableFuture
        .supplyAsync(FuturesExample::fetchValue)
        .exceptionally(throwable -> {
            if(throwable instanceof CompletionException)
                throwable = throwable.getCause();
            System.err.println("wrapping '"+throwable+"' inside exceptionally");
            throw new CompletionException(throwable);
        });
    f.whenComplete((s,t) -> {
        if(t != null) {
            System.err.println("in whenComplete handler ");
            t.printStackTrace();
        }
    });
    System.err.println("calling join()");
    f.join();
}
public static String fetchValue() {
    throw new IllegalStateException("a test is going on");
}

我明白了

wrapping 'java.lang.IllegalStateException: a test is going on' inside exceptionally
in whenComplete handler 
java.util.concurrent.CompletionException: java.lang.IllegalStateException: a test is going on
    at FuturesExample.lambda$main$0(FuturesExample.java:12)
    at java.base/java.util.concurrent.CompletableFuture.uniExceptionally(CompletableFuture.java:986)
    at java.base/java.util.concurrent.CompletableFuture$UniExceptionally.tryFire(CompletableFuture.java:970)
    at java.base/java.util.concurrent.CompletableFuture.unipush(CompletableFuture.java:589)
    at java.base/java.util.concurrent.CompletableFuture.uniExceptionallyStage(CompletableFuture.java:1002)
    at java.base/java.util.concurrent.CompletableFuture.exceptionally(CompletableFuture.java:2307)
    at FuturesExample.main(FuturesExample.java:8)
Caused by: java.lang.IllegalStateException: a test is going on
    at FuturesExample.fetchValue(FuturesExample.java:24)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1700)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1692)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
    at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
    at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
    at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
    at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)
calling join()
Exception in thread "main" java.util.concurrent.CompletionException: java.lang.IllegalStateException: a test is going on
    at FuturesExample.lambda$main$0(FuturesExample.java:12)
    at java.base/java.util.concurrent.CompletableFuture.uniExceptionally(CompletableFuture.java:986)
    at java.base/java.util.concurrent.CompletableFuture$UniExceptionally.tryFire(CompletableFuture.java:970)
    at java.base/java.util.concurrent.CompletableFuture.unipush(CompletableFuture.java:589)
    at java.base/java.util.concurrent.CompletableFuture.uniExceptionallyStage(CompletableFuture.java:1002)
    at java.base/java.util.concurrent.CompletableFuture.exceptionally(CompletableFuture.java:2307)
    at FuturesExample.main(FuturesExample.java:8)
Caused by: java.lang.IllegalStateException: a test is going on
    at FuturesExample.fetchValue(FuturesExample.java:24)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1700)
    at java.base/java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1692)
    at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
    at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
    at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
    at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
    at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)

堆栈跟踪略有不同，但对接收/捕获异常的代码没有影响，无论哪种情况，它都是包装 IllegalStateException 的 CompletionException >.

所以回到你的问题的例子，你可以使用

CompletableFuture
    .supplyAsync(FuturesExample::fetchValue)
    .exceptionally(throwable -> {
        if (throwable instanceof RuntimeException) { // includes CompletionException
            return "All good";
        }
        throw new CompletionException(throwable);
    });

由于 CompletionException 是一个 RuntimeException，因此此代码会处理它并避免将 CompletionException 包装在另一个 CompletionException 中。否则，模式将是

    .exceptionally(throwable -> {
        if (some condition) {
            return some value;
        }
        throw throwable instanceof CompletionException?
            (CompletionException)throwable: new CompletionException(throwable);
    });