同步对象以确保完成所有任务

我应该使用哪个Java同步对象来确保完成任意大量的任务？限制是:

每项任务都需要花费大量的时间来完成,并行执行任务是合适的.

有太多的任务要适应内存(即我不能将Future每个任务放入a Collection然后调用get所有期货).

我不知道将会有多少任务(即我不能使用a CountDownLatch).

该ExecutorService可共享的,所以我不能用awaitTermination( long, TimeUnit )

例如,在Grand Central Dispatch中,我可能会这样做:

let workQueue = dispatch_get_global_queue( QOS_CLASS_BACKGROUND, 0 )
let latch = dispatch_group_create()
let startTime = NSDate()
var itemsProcessed = 0
let countUpdateQueue = dispatch_queue_create( "countUpdateQueue", DISPATCH_QUEUE_SERIAL )
for item in fetchItems() // generator returns too many items to store in memory
{
    dispatch_group_enter( latch )
    dispatch_async( workQueue )
    {
        self.processItem( item ) // method takes a non-trivial amount of time to run
        dispatch_async( countUpdateQueue )
        {
            itemsProcessed++
        }
        dispatch_group_leave( latch )
    }
}
dispatch_group_wait( latch, DISPATCH_TIME_FOREVER )
let endTime = NSDate()
let totalTime = endTime.timeIntervalSinceDate( startTime )
print( "Processed \(itemsProcessed) items in \(totalTime) seconds." )

它产生的输出看起来像这样(128项): Processed 128 items in 1.846794962883 seconds.

我试过类似的东西Phaser:

final Executor executor = new ThreadPoolExecutor( 64, 64, 1l, MINUTES, new LinkedBlockingQueue( 8 ), new CallerRunsPolicy() );
final Phaser latch = new Phaser( 0 );
final long startTime = currentTimeMillis();
final AtomicInteger itemsProcessed = new AtomicInteger( 0 );
for( final String item : fetchItems() ) // iterator returns too many items to store in memory
{
    latch.register();
    final Runnable task = new Runnable() {
        public void run() {
            processItem( item ); // method takes a non-trivial amount of time to run
            itemsProcessed.incrementAndGet();
            latch.arrive();
        }
    };
    executor.execute( task );
}
latch.awaitAdvance( 0 );
final long endTime = currentTimeMillis();
out.println( "Processed " + itemsProcessed.get() + " items in " + ( endTime - startTime ) / 1000.0 + " seconds." );

任务并不总是在最后一个print语句之前完成,我可能会得到如下所示的输出(对于128项):Processed 121 items in 5.296 seconds.是否Phaser使用正确的对象？文档表明它只支持65,535个参与方,因此我需要批处理要处理的项目或引入某种Phaser分层.