Question

I have the following (simplified) code periodically run by a Thread in the class A (once per second):

Socket s = new Socket(IP,PORT);
ObjectOutputStream oos = new ObjectOutputStream(s.getOutputStream());
synchronized(this) {
   oos.writeObject(this);   //Exception HERE
   oos.flush();
}
...

The object (this) to send through the socket has an object of the class B as instance variable and B has a LinkedList<Long> as instance variable.

The application throws ConcurrentModificationException:

E/AndroidRuntime(681): FATAL EXCEPTION: Thread-10
E/AndroidRuntime(681): java.util.ConcurrentModificationException
E/AndroidRuntime(681): at java.util.LinkedList$LinkIterator.next(LinkedList.java:124)
E/AndroidRuntime(681): at java.util.LinkedList.writeObject(LinkedList.java:973)
E/AndroidRuntime(681): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(681): at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1219)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1575)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeFieldValues(ObjectOutputStream.java:1143)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:413)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1241)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1575)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeFieldValues(ObjectOutputStream.java:1143)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:413)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1241)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1575)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
E/AndroidRuntime(681): at java.util.LinkedList.writeObject(LinkedList.java:973)
E/AndroidRuntime(681): at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime(681): at java.lang.reflect.Method.invoke(Method.java:507)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1219)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1575)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeFieldValues(ObjectOutputStream.java:1143)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:413)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1241)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1575)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1847)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1689)
E/AndroidRuntime(681): at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1653)
E/AndroidRuntime(681): at qoe.Application.connectUpdate(Application.java:194)
E/AndroidRuntime(681): at qoe.Application.access$0(Application.java:183)
E/AndroidRuntime(681): at qoe.Application$AutoUpdate.run(Application.java:217)

I run this app in Android with Eclipse and AVD, Windows 7 x64. Thanks in advance.

Edit: After many tests I think that the method that could cause problem is the following:

/* Instance variables */
private LinkedList<Long> mylist;
private long value;
/* The incriminated method */
public synchronized void myBadMethod() {
   this.mylist.add(this.value);
}

Answer 1

Solved in 2 step.

1) I’ve used CopyOnWriteArrayList (a thread-safe variant of ArrayList). But in this way I had another issue: the operation remove by Iterator is not supported.

2) To resolve this issue I referred to this question:

LinkedList<Long> to_remove = new LinkedList<Long>();
    for(int i = 0; i < this.mylist.size(); i++) {

        Long current = this.mylist.get(i);
        if(conditionToRemove)
            to_remove.add(current);
    }
    this.mylist.removeAll(to_remove);

The drawback is that this is an expensive operation.