ForkJoinPool - Error Handling

Reference

Git: ForkJoinPoolErrorMain.java
前篇: ForkJoinPool - Workstealing
Next: ForkJoinPool - Thread Management

Error Handling

如果 fork 出去的 task/action 有 exception, 呼叫 join 的時候要注意 catch exception.

不然 task/action 就死掉失控了.

而且可怕的是: 不會有 log.

Codes

這段程式是說每個 action 有個 id, 奇數 id 會報 exception.

Exception without catch when join

先是不要 catch 的情況

public class ForkJoinPoolErrorMain {

    static final CountDownLatch latch = new CountDownLatch(1);

    public static void main(String[] params) throws InterruptedException {
        ForkJoinPool pool = new ForkJoinPool(2);
        ScheduledExecutorService s = Executors.newScheduledThreadPool(1);
        s.scheduleAtFixedRate(() -> System.out.println(pool), 0, 1, TimeUnit.SECONDS);
        pool.submit(new ErrorAction(MAIN_TASK_ID));
        latch.await(10, TimeUnit.SECONDS);
        pool.shutdown();
        s.shutdown();
    }

    static class ErrorAction extends RecursiveAction {

        public static final int MAIN_TASK_ID = -1;
        private final int id;

        ErrorAction(int id) {
            this.id = id;
        }

        private boolean isSubTask() {
            return id >= 0;
        }

        @Override
        protected void compute() {
            if (isSubTask()) {
                if (id % 2 == 1) {
                    throw new IllegalStateException("Error when id is odd number");
                }
            } else {
                List<ErrorAction> actions = IntStream.range(0,10).mapToObj(idx -> new ErrorAction(idx)).collect(Collectors.toList());
                actions.forEach(ErrorAction::fork);
                actions.forEach(action -> {
                    action.join();
                });
                latch.countDown();
            }
            System.out.println("compute done. id=" + id);
        }
    }

}

輸出如下, 不過一個 exception 也沒有.
main action 也沒有完成.(main action id = -1)

Exception with catch statement

有 catch 的程式如下

public class ForkJoinPoolErrorMain {

    static final CountDownLatch latch = new CountDownLatch(1);

    public static void main(String[] params) throws InterruptedException {
        ForkJoinPool pool = new ForkJoinPool(2);
        ScheduledExecutorService s = Executors.newScheduledThreadPool(1);
        s.scheduleAtFixedRate(() -> System.out.println(pool), 0, 1, TimeUnit.SECONDS);
        pool.submit(new ErrorAction(MAIN_TASK_ID));
        latch.await(10, TimeUnit.SECONDS);
        pool.shutdown();
        s.shutdown();
    }

    static class ErrorAction extends RecursiveAction {

        public static final int MAIN_TASK_ID = -1;
        private final int id;

        ErrorAction(int id) {
            this.id = id;
        }

        private boolean isSubTask() {
            return id >= 0;
        }

        @Override
        protected void compute() {
            if (isSubTask()) {
                if (id % 2 == 1) {
                    throw new IllegalStateException("Error when id is odd number");
                }
            } else {
                List<ErrorAction> actions = IntStream.range(0,10).mapToObj(idx -> new ErrorAction(idx)).collect(Collectors.toList());
                actions.forEach(ErrorAction::fork);
                actions.forEach(action -> {
                    try {
                        action.join();
                    } catch (Exception ex) {
                        System.err.println(ex + ", id=" + action.id);
                    }
                });
                latch.countDown();
            }
            System.out.println("compute done. id=" + id);
        }
    }

}

這個程式就可以印 error, 而且 maintask 可以執行完成

Exception with ExecutorService

如果是 ExecutorService, 就算是丟 RuntimeException.

ExecutorService 還是會把 exception 印到 console 上.

public class ExecutorServiceWithExceptionMain {

    public static void main(String[] params) {
        ExecutorService ex = Executors.newSingleThreadExecutor();
        ex.execute(() -> {
            throw new IllegalStateException("test throw illegalState");
        });
        ex.shutdown();
    }


}

即使是直接丟 exception 也會印到 console.