NPE in java.util.LinkedList addBefore method.

Question

Somehow, I'm getting a null pointer exception here with JDK 1.6.0_14:

HttpSession session = request.getSession(true);
LinkedList<MyObject> list = (LinkedList<MyObject>) session.getAttribute(MY_LIST_KEY);
....
list.addFirst( new MyObject(str1, str2, map) );

Then, I get this:

 at java.util.LinkedList.addBefore(LinkedList.java:779)

Here's the method:

private Entry<E> addBefore(E e, Entry<E> entry) {
    Entry<E> newEntry = new Entry<E>(e, entry, entry.previous);
    newEntry.previous.next = newEntry;//this line NPEs
    newEntry.next.previous = newEntry;
    size++;
    modCount++;
    return newEntry;
}

which is called by

public void addFirst(E e) {
    addBefore(e, header.next);
}

Is there any weird way the list can be serialized/deserialized to break the header entry to cause this to happen? I don't see how this could possibly be failing.

Here's the serialization methods for 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 (Entry e = header.next; e != header; e = e.next)
        s.writeObject(e.element);
}

private void readObject(java.io.ObjectInputStream s)
    throws java.io.IOException, ClassNotFoundException {
    // Read in any hidden serialization magic
    s.defaultReadObject();

    // Read in size
    int size = s.readInt();

    // Initialize header
    header = new Entry<E>(null, null, null);
    header.next = header.previous = header;

    // Read in all elements in the proper order.
    for (int i=0; i<size; i++)
        addBefore((E)s.readObject(), header);
}

Answer 1

Most probably, (LinkedList<MyObject>) session.getAttribute(MY_LIST_KEY) returned null.
Update: As correctly pointed out in comments, null can not be returned by getAttribute() here, otherwise stacktrace wouldn't point into internals of LinkedList. So, my suggestion is wrong.

Answer 2

My guess would be improper sharing of the List across multiple threads. I would guess the list is being simultaneously modified by two different threads.