ScheduledExecutorService Exception handling

Question

I use ScheduledExecutorService to execute a method periodically.

p-code:

ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
ScheduledFuture<?> handle =
        scheduler.scheduleWithFixedDelay(new Runnable() {
             public void run() { 
                 //Do business logic, may Exception occurs
             }
        }, 1, 10, TimeUnit.SECONDS);

My question:

How to continue the scheduler, if run() throws Exception? Should I try-catch all Exception in method run()? Or any built-in callback method to handle the Exception? Thanks!

Answer 1

tl;dr

Any exception escaping your run method halts all further work, without notice.

Always use a try-catch within your run method. Try to recover if you want scheduled activity to continue.

@Override
public void run ()
{
    try {
        doChore();
    } catch ( Exception e ) { 
        logger.error( "Caught exception in ScheduledExecutorService. StackTrace:\n" + t.getStackTrace() );
    }
}

The Problem

The question refers to the critical trick with a ScheduledExecutorService: Any thrown exception or error reaching the executor causes the executor to halt. No more invocations on the Runnable, no more work done. This work stoppage happens silently, you'll not be informed. This naughty-language blog posting entertainingly narrates the hard way to learn about this behavior.

The Solution

The answer by yegor256 and the answer by arun_suresh both seem to be basically correct. Two issues with those answers:

• Catch errors as well as exceptions
• A bit complicated

Errors and Exceptions ?

In Java we normally catch only exceptions, not errors. But in this special case of ScheduledExecutorService, failing to catch either will mean a work stoppage. So you may want to catch both. I'm not 100% sure about this, not knowing fully the implications of catching all errors. Please correct me if needed.

One reason to catch errors as well as exceptions might involve the use of libraries within your task. See the comment by jannis.

One way to catch both exceptions and errors is to catch their superclass, Throwable for an example.

} catch ( Throwable t ) {

…rather than…

} catch ( Exception e ) {

Simplest Approach: Just Add a Try-Catch

But both answers are a bit complicated. Just for the record, I'll show the simplest solution:

Always wrap your Runnable's code in a Try-Catch to catch any and all exceptions and errors.

Lambda Syntax

With a lambda (in Java 8 and later).

final Runnable someChoreRunnable = () -> {
    try {
        doChore();
    } catch ( Throwable t ) {  // Catch Throwable rather than Exception (a subclass).
        logger.error( "Caught exception in ScheduledExecutorService. StackTrace:\n" + t.getStackTrace() );
    }
};

Old-Fashioned Syntax

The old-fashioned way, before lambdas.

final Runnable someChoreRunnable = new Runnable()
{
    @Override
    public void run ()
    {
        try {
            doChore();
        } catch ( Throwable t ) {  // Catch Throwable rather than Exception (a subclass).
            logger.error( "Caught exception in ScheduledExecutorService. StackTrace:\n" + t.getStackTrace() );
        }
    }
};

In Every Runnable/Callable

Regardless of a ScheduledExecutorService, it seems sensible to me to always use a general try-catch( Exception† e ) in any run method of a Runnable. Ditto for any call method of a Callable.

---

Complete example code

In real work, I would likely define the Runnable separately rather than nested. But this makes for neat all-in-one example.

package com.basilbourque.example;

import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

/**
 *  Demo `ScheduledExecutorService`
 */
public class App {
    public static void main ( String[] args ) {
        App app = new App();
        app.doIt();
    }

    private void doIt () {

        // Demonstrate a working scheduled executor service.
        // Run, and watch the console for 20 seconds.
        System.out.println( "BASIL - Start." );

        ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
        ScheduledFuture < ? > handle =
                scheduler.scheduleWithFixedDelay( new Runnable() {
                    public void run () {
                        try {
                            // doChore ;   // Do business logic.
                            System.out.println( "Now: " + ZonedDateTime.now( ZoneId.systemDefault() ) );  // Report current moment.
                        } catch ( Exception e ) {
                            // … handle exception/error. Trap any unexpected exception here rather to stop it reaching and shutting-down the scheduled executor service.
                            // logger.error( "Caught exception in ScheduledExecutorService. StackTrace:\n" + e.getStackTrace() );
                        }   // End of try-catch.
                    }   // End of `run` method.
                } , 0 , 2 , TimeUnit.SECONDS );


        // Wait a long moment, for background thread to do some work.
        try {
            Thread.sleep( TimeUnit.SECONDS.toMillis( 20 ) );
        } catch ( InterruptedException e ) {
            e.printStackTrace();
        }

        // Time is up. Kill the executor service and its thread pool.
        scheduler.shutdown();

        System.out.println( "BASIL - Done." );

    }
}

When run.

BASIL - Start.

Now: 2018-04-10T16:46:01.423286-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:03.449178-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:05.450107-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:07.450586-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:09.456076-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:11.456872-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:13.461944-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:15.463837-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:17.469218-07:00[America/Los_Angeles]

Now: 2018-04-10T16:46:19.473935-07:00[America/Los_Angeles]

BASIL - Done.

---

† Or perhaps Throwable instead of Exception to catch Error objects too.