Executor Service with LIFO ordering

Question

I wrote a lazy image downloader for my app using an ExecutorService. It gives me great control about how many downloads are running in parallel at what time and so on.

Answer 1

You can do it in two or three simple steps:

1. Create a LifoBlockingDeque class:

   public class LifoBlockingDeque  extends LinkedBlockingDeque {
   
   @Override
   public boolean offer(E e) { 
       // Override to put objects at the front of the list
       return super.offerFirst(e);
   }
   
   @Override
   public boolean offer(E e,long timeout, TimeUnit unit) throws InterruptedException { 
       // Override to put objects at the front of the list
       return super.offerFirst(e,timeout, unit);
   }
   
   
   @Override
   public boolean add(E e) { 
       // Override to put objects at the front of the list
       return super.offerFirst(e);
   }
   
   @Override
   public void put(E e) throws InterruptedException { 
       //Override to put objects at the front of the list
       super.putFirst(e);
       }
   }
   

2. Create the executor:

   mThreadPool = new ThreadPoolExecutor(THREAD_POOL_SIZE, 
                                        THREAD_POOL_SIZE, 0L, 
                                        TimeUnit.MILLISECONDS, 
                                        new LifoBlockingDeque());
   

3. LinkedBlockingDeque is supported only from API Level 9. To use it on earlier versions do the following:

   Use the Java 1.6 implementation - download it from here.

   Then change

   implements BlockingDeque
   

   to

   implements BlockingQueue
   

   To make it compile on Android. BlockingDeque is subtype of BlockingQueue, so no harm done.

And you're done!