A behavior mismatch in AbstractCollection.toArray(T[] )

[Sean Chou]

Hi ,

    Is it possible to change the spec ? I found it is defined in
java.utils.Collection
interface. It would be easy for AbstractCollection to state that it is not
designed
for concurrent operations, and its subclass should take care of them.

    However, I think the simplest way may be modifying toArray(T[]) method
for
an additional check, which would work for most subclasses of
AbstractCollection...
Is that ok ?

On Tue, Dec 13, 2011 at 3:41 PM, David Holmes <david.holmes at oracle.com>wrote:

> Hi Sean,
>
>
> On 13/12/2011 5:21 PM, Sean Chou wrote:
>
>> When I was reading the code of AbstractCollection.toArray(T[] ), I
>> found its behavior maybe different from the spec in multithread
>> environment. The spec says "If the collection fits in the specified
>> array, it is returned therein. Otherwise, a new array is allocated
>> with the runtime type of the specified array and the size of this
>> collection."  However, in multithread environment, it is not easy to
>> tell if the collection fits in the specified array, because the
>> items may be removed when toArray is copying.
>>
>
> Right. The problem is that AbstractCollection doesn't address
> thread-safety or any other concurrency issues so doesn't account for the
> collection growing or shrinking while the toArray snapshot is being taken.
> Really the collection implementations that are designed to support multiple
> threads should override toArray to make it clear how it should behave. As
> it stands, in my opinion, it is more a "quality of implementation" issue as
> to whether AbstractCollection expends effort after creating the array to
> see if the array is actually full or not; or whether after creating an
> array it turns out it could have fit in the original.
>
> For a concurrent collection  I would write the spec for toArray something
> like:
>
> "The current size of the collection is examined and if the collection fits
> in the specified array it will be the target array, else a new array is
> allocated based on that current size and it becomes the target array. If
> the collection grows such that the target array no longer fits then extra
> elements will not be copied into the target array. If the collection
> shrinks then the target array will contain null elements."
>
> Or for the last part "then the target array will be copied to a new array
> that exactly fits the number of elements returned".
>
> David Holmes
> ------------
>
>
>
>>     Here is a testcase:
>> //////////////////////////////**//////////////////////////////**
>> //////////////
>>
>> import java.util.Map;
>> import java.util.concurrent.**ConcurrentHashMap;
>>
>> public class CollectionToArrayTest {
>>
>>     static volatile Map<String, String>  map = new
>> TConcurrentHashMap<String, String>();
>>     static volatile boolean gosleep = true;
>>
>>     static class TConcurrentHashMap<K, V>  extends ConcurrentHashMap<K,
>> V>  {
>>         public int size() {
>>             int oldresult = super.size();
>>             System.out.println("map size before concurrent remove is "
>>                     + oldresult);
>>             while (gosleep) {
>>                 try {
>>                     // Make sure the map is modified during toArray is
>> called,
>>                     // between getsize and being iterated.
>>                     Thread.sleep(1000);
>>                     // System.out.println("size called, size is " +
>> oldresult +
>>                     // " take a sleep to make sure the element is deleted
>> before size is returned.");
>>                 } catch (Exception e) {
>>                 }
>>             }
>>             return oldresult;
>>         }
>>     }
>>
>>     static class ToArrayThread implements Runnable {
>>         public void run() {
>>             for (int i = 0; i<  5; i++) {
>>                 String str = Integer.toString(i);
>>                 map.put(str, str);
>>             }
>>             String[] buffer = new String[4];
>>             String[] strings = map.values().toArray(buffer);
>>             // System.out.println("length is " + strings.length);
>>             if (strings.length<= buffer.length) {
>>                 System.out.println("given array size is "
>>                                 + buffer.length
>>                                 + " \nreturned array size is "
>>                                 + strings.length
>>                                 + ", \nbuffer should be used according to
>> spec. Is buffer used : "
>>                                 + (strings == buffer));
>>             }
>>         }
>>     }
>>
>>     static class RemoveThread implements Runnable {
>>         public void run() {
>>             String str = Integer.toString(0);
>>             map.remove(str);
>>             gosleep = false;
>>         }
>>     }
>>
>>     public static void main(String args[]) {
>>         CollectionToArrayTest app = new CollectionToArrayTest();
>>         app.test_concurrentRemove();
>>     }
>>
>>     public void test_concurrentRemove() {
>>
>> System.out.println("//////////**//////////////////////////////**//////\n"
>> +
>>                 "The spec says if the given array is large\n "  +
>>                         "enough to hold all elements, the given array\n" +
>>                         "should be returned by toArray. This \n" +
>>                         "testcase checks this case. \n" +
>>                         "/////////////////////////////**
>> /////////////////");
>>
>>         Thread[] threads = new Thread[2];
>>         threads[0] = new Thread(new ToArrayThread());
>>         threads[1] = new Thread(new RemoveThread());
>>
>>         threads[0].start();
>>
>>         try {
>>             // Take a sleep to make sure toArray is already called.
>>             Thread.sleep(1200);
>>         } catch (Exception e) {
>>         }
>>
>>         threads[1].start();
>>     }
>> }
>>
>> //////////////////////////////**//////////////////////////////**
>> //////////////
>>
>> TConcurrentHashMap is used to make sure the collection is modified
>> during toArray is invoked. So the returned array fits in the specified
>> array, but a new array is used because toArray checks the size
>> before copying.
>>
>> Is this a bug ?
>>
>>
>>


-- 
Best Regards,
Sean Chou