Dfyjkt

I'm getting a java.util.ConcurrentModificationException on the line where the for-loop starts (see comment in code).

Why am i getting ConcurrentModificationException on this unmodifiableSet?

final Set<Port> portSet = Collections.unmodifiableSet(node.getOpenPorts());
if (!portSet.isEmpty()) 
 StringBuilder tmpSb = new StringBuilder();
 for (Port pp : portSet) // <------- exception happening here
 tmpSb.append(pp.getNum()).append(" ");
 

I've never witnessed this, but I'm getting crash reports from Google.

Something must be modifying the underlying set; i.e. the set returned by node.getOpenPorts().

Instead of wrapping the set with an "unmodifiable" wrapper, you could copy it.

 final Set<Port> portSet = new HashSet<>(node.getOpenPorts());

But as a commenter (@Slaw) pointed out, that just moves the iteration inside the constructor and you would still get CCMEs.

The only real solutions are:

• Change the implementation of the node class to use a concurrent set class for the port list that won't throw CCMEs if the collection is mutated while you are iterating it.




• Change the implementation of the node class to return a copy of the port list. Deal with the updates-while-copying race condition with some internal locking.




• Put a try / catch around the code and repeat the operation if you get a CCME

I've never witnessed this, but I'm getting crash reports from Google.

Yes. The problem only occurs if this code is executed while the open port list is changing.