Java - LinkedList : Weird NullPointerException

Question

I have a little problem with a NullPointerException that I can't really understand.

My code runs 24/7 and works pretty well but I have that exception that pops randomly from 1 day to 1 week after the application is started.

Here is the stacktrace :

java.lang.NullPointerException
at java.util.LinkedList.get(LinkedList.java:477)
at com.ch4process.acquisition.ScenarioWorker.eventHandling(ScenarioWorker.java:97)
at com.ch4process.acquisition.ScenarioWorker.call(ScenarioWorker.java:79)
at com.ch4process.acquisition.ScenarioWorker.call(ScenarioWorker.java:1)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

As you can see this exception is rised in a thread.

Here is the code (simplified a bit) :

public class ScenarioWorker implement Callable<Integer>
{
List<SignalValueEvent> eventList = new LinkedList<>();

boolean busy = false;

@Override
public Integer call() throws Exception
{
    try
    {
        while (true)
        {
            eventHandling();
            Thread.sleep(1000);
        }
    }
    catch (Exception ex)
    {
        // Redirects the exception to a custom class
    }
}

private void eventHandling()
{
    if (! busy)
    {
        while (eventList.size() > 0)
        {       
            SignalValueEvent event = eventList.get(0); // NullPointerException here according to stacktrace

            if(event.isTriggered()))
            {
                busy = true;
                doScenario(event);
            }
            deleteEvent();
        }
    }
}


private void deleteEvent()
{
    try
    {
        eventList.remove(0);
    }
    catch (Exception ex)
    {
        // Redirects the exception to custom class
    }
    finally
    {
        busy = false;
    }
}


@Override
public void SignalValueChanged(SignalValueEvent event)
{
    if (event.isValid())
    {
        eventList.add(event);
    }
}

}

[Edit : the line 97 in the stacktrace is the line saying SignalValueEvent event = eventList.get(0); ]

This class implements an interface which allows another class to notify it by calling the SignalValueChanged method.

So basically my LinkedList is created at the initialization of the class, filled by an external class whenever an event needs to be put in the list, and the call method loops on the list to see if there's anything in it. If there is, it's treated and the event is deleted.

I've tested this and the only reason I should have a NPException is if my list is equal to null... but I don't do that anywhere in my code...

As I said this code is working 24/7 and I had this bug almost one week and a half after I started the application. Is it something obvious I am missing ?

Thank you so much for reading this and if you can help me I'll be glad :)

1) Which line is line 97? ScenarioWorker.java:97 2) Is the code failing at every run now, or intermittently? 3) If intermittently, then this suggests a concurrency problem. Are you sure that your code is thread safe? — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Jan 2, 2017 at 13:50
@HovercraftFullOfEels : The line 97 is SignalValueEvent event = eventList.get(0); I will edit my post to reflect that. The problem is intermitent, and I thought that LinkedList was thread safe ?.. — Caerbannog
– Caerbannog, Commented Jan 2, 2017 at 13:52
"Java Concurrency in Practice" amazon.co.uk/d/Books/Java-Concurrency-Practice-Brian-Goetz/… - get hold of this book and study it at great length. If you don't you will be living in a world of pain forever. — mike rodent
– mike rodent, Commented Jan 2, 2017 at 13:56
Note that the NPE isn't because your list is null -- if it were, the root of your exception would be on ScenarioWorker.java line 97. Instead the code is making it into the internals of LinkedList, which indicates that something internal to the LinkedList implementation is getting screwed up -- a huge red flag for concurrency problems. As for why, probably a race in one thread calling SignalValueChanged and another calling eventHandling at the same time. — jacobm
– jacobm, Commented Jan 2, 2017 at 14:06
@KevinEsche: this is most definitely not a duplicate of the question you've posted as the NPE has nothing to do with a variable being null but rather a concurrency issue. The usual techniques of NPE debugging will not work here. Please see jacobm's comment to see why. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Jan 2, 2017 at 14:21

jacobm · Accepted Answer · 2017-01-02 15:20:26Z

4

The NPE isn't because your list is null -- if it were, the root of your exception would be on ScenarioWorker.java line 97. Instead the code is making it into the internals of LinkedList, which indicates that something internal to the LinkedList implementation is getting screwed up -- a huge red flag for concurrency problems. As for why, probably a race in one thread calling SignalValueChanged and another calling eventHandling at the same time.

You can solve it by synchronizing all access to eventList. The easiest way is probably just to mark the methods SignalValueChanged and eventHandling as synchronized.

edited Jan 2, 2017 at 15:20

answered Jan 2, 2017 at 15:11

jacobm

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1 Comment

mike rodent Over a year ago

Yup... well spotted. And as I mentioned, line 477 (stack trace) also shows (if you look at the source for LinkedList, Java 8, _091) that the "index check" passed... i.e. it is not a question of an out-of-bounds due to an empty list (which would give a different Exception anyway, of course).

AhmadWabbi · Accepted Answer · 2017-01-02 13:56:28Z

3

I think this is related to threads synchronization too. Solution: Encapsulate eventList in an object with synchronized methods. Access eventList from all threads by calling these synchronized methods only.

answered Jan 2, 2017 at 13:56

AhmadWabbi

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Comments

Community · Accepted Answer · 2020-06-20 09:12:55Z

2

Just read LinkedLists JavaDoc:

Note that this implementation is not synchronized. If multiple threads access a linked list concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be "wrapped" using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:

List list = Collections.synchronizedList(new LinkedList(...));

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 2, 2017 at 14:57

Markus Mitterauer

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Comments

shiri · Accepted Answer · 2017-01-02 14:26:56Z

1

As Hovercraft Full Of Eels suggests above, I'm guessing that this is a concurrency issue.

Most likely, at the time that you're calling eventList.get(0), despite having "just" tested that the list is not empty, either the list became null or the list became empty, via another concurrent thread.

Edit: As discussed in the comments to the question, the NullPointerException is being thrown from the inner workings of LinkedList, meaning it's most definitely a concurrency issue.

Make your objects thread-safe, and you will solve the problem.

edited Jan 2, 2017 at 14:26

answered Jan 2, 2017 at 13:52

shiri

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4 Comments

Caerbannog Over a year ago

I should have an IndexOutOfBound exception if the list is empty, not a NullPointerException. And I never ever destroy that list or set it to null so I don't know why it would be that way.

Code-Apprentice Over a year ago

@Caerbannog Except that you are using a LinkedList not an ArrayList.

shiri Over a year ago

@Code-Apprentice, strictly speaking @Caerbannog is correct. LinkedList also throws an IndexOutOfBounds exception. docs.oracle.com/javase/7/docs/api/java/util/… Which means that the NPE is more likely to be due to the list itself being set to null, but that's unclear why it would happen. If possible, I'd add a test for a null list there to see if that condition flips in runtime. I still suspect concurrency.

mike rodent Over a year ago

No! As jacobm points out in the other comments, the Exception is being thrown by the internals of LinkedList, not by eventList.get(...)!!!

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