为什么从 Futures 获取值时会收到 NullPointerException？

Question

I'm using and ExecutorService to concurrently run some Callables . Here's a simplified version of my code:

ArrayList<Book> objResults = new ArrayList<Book>();
ExecutorService esrExecutor = Executors.newFixedThreadPool(2);
Set<Callable<ArrayList<Book>>> setCallables = new HashSet<Callable<ArrayList<Book>>>();

setCallables.add(new Callable<ArrayList<Book>>() {

    public ArrayList<Book> call() throws Exception {    
        ArrayList<Book> objAmazonResults = new ArrayList<Book>();

        try {    
            Amazon objAmazon = new Amazon();
            objAmazonResults = objAmazon.doSearch(strQuery);    
        } catch (Exception e) {
            e.printStackTrace();
        }    
        return objAmazonResults;    
    }

});


List<Future<ArrayList<Book>>> lstFutures;
try {
    lstFutures = esrExecutor.invokeAll(setCallables);

    for(Future<ArrayList<Book>> futFuture : lstFutures){
        if (futFuture.get().isEmpty() == false) //NullPointerException occurs here
            objResults.addAll(futFuture.get());
    }
} catch (InterruptedException e) {
    e.printStackTrace();
} catch (ExecutionException e) {
    e.printStackTrace();
}

esrExecutor.shutdown();

I get a NullPointerException on this bit of code if (futFuture.get().isEmpty() == false) . I can't seem to figure out why or how this bit of code can possibly be null.

If you look at my Callable you'll see that I'm trapping all exceptions and simply printing the stacktrace and I always return new ArrayList<Book>() .

I'm not much for helping other debug my code soup on SO but sometimes everyone hits a roadblock but this kind of exceptions has taught me one thing — when chaining methods together, it's always harder to debug.

Answer 1

May be objAmazon.doSearch(strQuery) returns null? Insert a check for this into the Callable .

The Future itself should never be null, and Future.get() will not return null, if the computation terminated with an exception (so catching all exceptions inside the Callable is not necessary).