Why writeObject method in LinkedList does not checking for ConcurrentModificationException

Question

Why overloaded method writeObject in LinkedList does not checking for ConcurrentModificationException where as ArrayList does.
I seen java code of LinkedList

private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        // Write out any hidden serialization magic
        s.defaultWriteObject();

        // Write out size
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (Node<E> x = first; x != null; x = x.next)
            s.writeObject(x.item);
    }

And java code of ArrayList

private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

What can be possible reasons.

Stephen C · Accepted Answer · 2015-07-21 13:45:45Z

1

First observation: the javadocs for ArrayList and LinkedList don't say if concurrent modification checks are performed while serializing. Therefore, it is consistent with the respective API contracts to do the checks, or not.

So why might they be different?

Without examining the Java source code history all the way back to Java 1.1, we can only guess. It could possibly be an oversight, or an early bug fix that was made in one class and no the other. It could also be that the inconsistency was noticed, but it was not fixed because of concerns that a fix would break customer code.

I couldn't find any related bug reports in the Java Bug Database.

answered Jul 21, 2015 at 13:45

Stephen C

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2 Comments

Pydi Over a year ago

Thank u Stephen,i have two questions 1. If it is a bug and not yet reported how can we report it, and 2. "fix would break customer code", how it will break? because if we didn't handle it will cause some problem like Little Santi said.

Stephen C Over a year ago

1. It is not a bug. 2. Someone's code might depend on concurrent modifications while a linked list is being serialized not throwing an exception.

Little Santi · Accepted Answer · 2015-07-21 14:16:00Z

1

After analyzing the code, I guess it is clearly a bug in LinkedList:

Notice that the first thing it does is writting on the stream the size of the list, and then its nodes. But, if some node is added/removed while serializing the list, the value of the size already written isn't coherent anymore with the number of nodes actually serialized.

A client that serializes a LinkedList would end up with a ObjectStream that declares 5 elements but has only 4. (And that would eventually produce an exception in readObject.)

answered Jul 21, 2015 at 14:16

Little Santi

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2 Comments

Pydi Over a year ago

Thanks Little Santi, this is exactly my question was ,what happens when added/removed while serializing.so it was a bug?

Little Santi Over a year ago

If, according to my reasoning, the algorithm of LinkedList.writeObject might produce incoherent and bogus information in the serialization stream, I have to conclude: Yes, it is a bug.

user4948585 · Accepted Answer · 2015-07-21 13:51:13Z

0

I think one reason can be that in LinkList throws IndexOutOfBoundException during write ( You can not concurrently write on it) I am not sure and need to test this.

answered Jul 21, 2015 at 13:51

user4948585

2 Comments

Pydi Over a year ago

Thanks Amit, but how it can throw IndexOutOfBoundException, because it is not opearting on index it is operating on next address.

user4948585 Over a year ago

Correct but the entry method of LinkedList throws this, you basically insert object to some index location within LinkedList (add, addAll etc call entry method which throws IndexOutOfBoundException)

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