ThreadLocal & static

[Pho Tek]

Hi,

I have a Util class with a static method process which will accept a tree data structure (Composite Pattern). The problem is I want to prevent the possibility of a StackOveflow if the user of this method is stupid enough to introduce loops within the tree.

What I want to do is to keep a list of nodes which I have already visited. So during the navigation down the tree, I will add the visited nodes to this list.

I want to use a ThreadLocal variable to keep the list of visited nodes.
The issue here is that the variable has to be declared static.

I don't understand the semantics of static ThreadLocal variable.
Will different threads that call the process method be using the
same ThreadLocal variable ? Are there any huge design flaws ? Thanks all

class Util
{
    ...snip...
 
    private static ThreadLocalList list = new ThreadLocalList();
    
    /**
     * This method is recursive
     */
    public static Result process( TreeData tree )
    {
        ...snip...
 
        /**
         * Check the node in the threadlocal
         */
        List lst = list.get().getList();
        if (lst.contains( tree.id)
        {
            //abort
            return Result.NULL;
        }  
        else
        {
            // continue to process its children
            lst.add( tree.id );
        }
 
        for ( TreeData o : tree.getChildren() )
        {
            process( o );
        }    
    }
 
    private static class ThreadLocalList extends ThreadLocal<ArrayList<Long>>
    {
        @Override
        protected ArrayList<Long> initialValue()
        {
            return new ArrayList<Long>(16);
        }
        
        public List<Long> getList() 
        { 
            return super.get();
        }
 
        public void zeroOutList()
        {
            super.set( initialValue() );
        }
    }
}

[Peter Chase]

All threads will be using the same ThreadLocalList object. But the whole point of ThreadLocal is that each thread gets its own value to work with. Therefore, the different threads will not see or corrupt each others' lists.