我的ThreadLocal包含并始终返回null

Question

I am wondering how the threadlocal.set() has no data stored when I set it to a collection of 32 elements. ThreadLocal.get() returns null always; and the correspoding FutureTask object has an outcome property = NullPointerException. Any idea why ThreadLocal is unable to store collection items?

public class MyCallable<T> implements Callable<Collection<T>> {

    public MyCallable( Collection<T> items ){
        tLocal = new ThreadLocal<Collection<T>>();
        tLocal.set( items );                    //SETS NULL ALTHOUGH the PARAMETER CONTAINS 32 ITEMS
    }

    @Override
    @SuppressWarnings("unchecked")
    public Collection<T> call() throws Exception {
        synchronized( lock ){
            ArrayList<T> _items = new ArrayList<T>();
            ArrayList<T> _e = ( ArrayList<T> ) tLocal.get();   //RETURNS NULL
            for( T item : _e ){
                _items = getPValue( item ));
            }
            return _items ;
        }
    }

    private ThreadLocal<Collection<T>> tLocal;

    private final Object lock = new Object();
}

usage snippet:

List<Future<Collection<T>>> futures = new ArrayList<Future<Collection<T>>>();
ExecutorService pool = Executors.newFixedThreadPool( 8 );

        for( int x = 0; x < numBatches; ++x ){
            List<T> items = retrieveNext32Items( x );
            futures.add( pool.submit( new MyCallable<T>( items ));
        }

        pool.shutdown();

        for( Future<Collection<T>> future : futures ) {
            _items.addAll( future.get() );                  //future.outcome = NullPointerException 
        }

        return _items
}

Answer 1

You create object of type MyCallable in the main thread then submit them to a thread pool. So constructors of MyCallable are called in one thread and methods call in another. Thread local keeps a separate data for each thread so no wonder you get nulls.

I don't understand why you use thread local. items should be a simple field in MyCallable. If you modify the collection maybe it would be better to copy it to a new collection.

Answer 2

The value stored in a thread local is stored for the particular thread . So if you store a value in a thread local for a particular thread t1 and try to obtain the value from another thread t2 using the same thread local...you would not get the value but would get null. Check whether the thread in which the value is set in the thread local is the same thread from which the value is being retrieved from the thread local