具有多线程的Collection.sort（）中的NullpointerException

Question

I have a class that should test my Fibonacci function using multithreading

public class PerformanceTesterImpl implements PerformanceTester{
    public static List<Long> executionTimesList = new ArrayList();
    public static List<Runnable> tasksList = new ArrayList();
    public int fib;
    public PerformanceTestResult performanceTestResult;

    @Override
    public PerformanceTestResult runPerformanceTest(Runnable task, int calculationCount, int threadPoolSize) {

        for(int i=0; i<calculationCount; i++){
            tasksList.add(createTask(fib));
        }

        ExecutorService executor = Executors.newFixedThreadPool(threadPoolSize);
        for(Runnable r : tasksList){
            executor.execute(r);
        }
        executor.shutdown();  

        try {
            executor.awaitTermination(1, TimeUnit.MINUTES);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        // Here all threads should complete all work. Is it OK?
        mapValues();
        return performanceTestResult;
    }

    private PerformanceTestResult mapValues(){
        Collections.sort(executionTimesList);
        performanceTestResult = new PerformanceTestResult(getSum(executionTimesList), (Long)executionTimesList.get(0), (Long)executionTimesList.get(executionTimesList.size()-1));
        return performanceTestResult;
    }


    public  Runnable createTask (final int n) {
        fib = n;
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                long startTime = System.currentTimeMillis();
                FibCalc fibCalc = new FibCalcImpl();
                fibCalc.fib(n);
                long executionTime = System.currentTimeMillis() - startTime;
                executionTimesList.add(executionTime);
            }
        };
        return runnable;
    }


    private static long getSum(List<Long> executionTimes){
        long sum = 0;
        for(long l : executionTimes){
            sum += l;
        }
        return sum;
    }
}

but from time to time NULL appears in my collection and when I'm trying to sort executionTimeList I receive NullpointerException. I think there is a problem with executing threads. What should I do to correct this exception?

Answer 1

ArrayList is not thread safe.

From the Javadoc :

 * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * <i>must</i> be synchronized externally.  (A structural modification is
 * any operation that adds or deletes one or more elements, or explicitly
 * resizes the backing array; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.

 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new ArrayList(...));</pre>

When you call add from multiple threads, the ArrayList may be in inconsistent state.You should synchronize access to it.

Try :

        public void run() {
            long startTime = System.currentTimeMillis();
            FibCalc fibCalc = new FibCalcImpl();
            fibCalc.fib(n);
            long executionTime = System.currentTimeMillis() - startTime;
            synchronized (this) {
                executionTimesList.add(executionTime);
            }
        }

Answer 2

executionTimesList is shared among all threads. They run in concurrence in your code with no synchronization. So it's logical that any inconsistency state appears if a thread works on the list and has not finished its job and the cpu gives the priority to another thread which works on the list too, the first thread will be in a inconsistent state when it will be restarted. You must synchronize the access for the static field executionTimesList