How to catch exceptions from indirect thread?

Question

EDIT:

How to catch an Exception from a thread - this is not a duplication. The link refers to direct thread (son thread) which it's not the case here! We are talking about indirect (grandson thread). The son thread is a closed JAR (I do not have write access)

I have the following method: f1 which I don't have write access to (It's a closed JAR).

f1 creates and runs a task on new thread which throws exception. On the main method, I must call f1 on new thread. I need to be able to catch all the exceptions which were thrown from child threads of f1.

In other words, how do I catch exceptions from grandson thread without changing the son thread.

main method opens new thread and call:
      |
      V
f1 method opens new thread and call:
      |
      V
anonymous method which throws exception

Example code:

private static void f1() {
    Executors.newSingleThreadExecutor()
            .submit(() -> {
                try {
                    throw new Exception("exception from anonymous thread");
                } catch (Exception e) {
                    e.printStackTrace();
                    throw e;
                }
            });

}

public void main() {
    final Future<?> submit = Executors.newSingleThreadExecutor()
            .submit(() -> f1());
    try {
        submit.get();
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    }
}

ExecutionException will have the thrown exception as root cause — guido, Apr 09 '18 at 08:17
@GhostCat This is not the same question! Your duplication refers to directed thread (son thread) which it's not the case here! We are talking about indirect (grandson thread) — Ida Amit, Apr 09 '18 at 08:20
@GhostCat The OP does not ask the same question. Your link refers to a question which has to write access to the code and here the OP doesn't have. — Stav Alfi, Apr 09 '18 at 08:30
@StavAlfi It does not because the method `f1` swallows all the exceptions, it doesn't throw any exception itself, that's why in the main method we can catch nothing. — grape_mao, Apr 09 '18 at 11:42
@grape_mao do you imply that, in case I can't edit `f1`, there isn't any solution for this problem ? — Ida Amit, Apr 09 '18 at 12:18
@AlmaShvartz I can't say that for sure, maybe some hack on bytecode level. If the design of `f1` doesn't give you access to the executor/thread, nor to the task, then it doesn't want you to care about the execution. But for me it's a bad design. — grape_mao, Apr 09 '18 at 12:27
No chance. Keep in mind that by the time, your `main` method’s `submit.get()` returns, there is no guaranty that the asynchronous task spawned by `f1()` has even finished, not to speak of having delivered an inspectable exception. Even `UncaughtExceptionHandler` of the linked answer won’t help, as the exception is not uncaught. When `submit` is used, the job gets wrapped in a future which will catch any exception, so it can be propagated if `get` is called. Since no-one keeps a reference to that future, the exception is lost. Without modifying `f1()`, there’s no way. — Holger, Apr 09 '18 at 13:01

Bartosz Bilicki · Answer 1 · 2018-04-24T20:16:58.200

There is no general answer to this question. In this case exception is handled inside Executor. You can do one of

catch all exceptions that may occur inside task and return meaningfull status for exceptions caught
if task was submitted as Callable<?> then get on returned Future<?> will throw ExecutionException if task completed with exception
for uncaught exceptions you could set Uncaught Exception Handler on Thread Factory used by Executor
ThreadPoolExecutor has afterExecute(Runnable r, Throwable t) method with can handle uncaught exceptions from tasks

How to catch exceptions from indirect thread?

1 Answers1